Laser-powered phosphor display (LPD) is a large-format display technology similar to the cathode ray tube (CRT). Prysm, Inc., a video wall designer and manufacturer in Silicon Valley, California, invented and patented[1] the LPD technology.[2][3] The key components of the LPD technology are its TD2 tiles, its image processor, and its backing frame that supports LPD tile arrays.[4] The company unveiled the LPD in January 2010.[4][5][6]
https://en.wikipedia.org/wiki/Laser-powered_phosphor_display
A surface-conduction electron-emitter display (SED) is a display technology for flat panel displays developed by a number of companies. SEDs use nanoscopic-scale electron emitters to energize colored phosphors and produce an image. In a general sense, a SED consists of a matrix of tiny cathode-ray tubes, each "tube" forming a single sub-pixel on the screen, grouped in threes to form red-green-blue (RGB) pixels. SEDs combine the advantages of CRTs, namely their high contrast ratios, wide viewing angles, and very fast response times, with the packaging advantages of LCD and other flat panel displays. They also use much less power than an LCD television of the same size.
After considerable time and effort in the early and mid-2000s, SED efforts started winding down in 2009 as LCD became the dominant technology. In August 2010, Canon announced they were shutting down their joint effort to develop SEDs commercially, signaling the end of development efforts.[1] SEDs are closely related to another developing display technology, the field emission display, or FED, differing primarily in the details of the electron emitters. Sony, the main backer of FED, has similarly backed off from their development efforts.[2]
https://en.wikipedia.org/wiki/Surface-conduction_electron-emitter_display
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen.[2] The images may represent electrical waveforms (oscilloscope), pictures (television set, computer monitor), radar targets, or other phenomena. A CRT on a television set is commonly called a picture tube. CRTs have also been used as memory devices, in which case the screen is not intended to be visible to an observer. The term cathode ray was used to describe electron beams when they were first discovered, before it was understood that what was emitted from the cathode was a beam of electrons.
In CRT television sets and computer monitors, the entire front area of the tube is scanned repeatedly and systematically in a fixed pattern called a raster. In color devices, an image is produced by controlling the intensity of each of three electron beams, one for each additive primary color (red, green, and blue) with a video signal as a reference.[3] In modern CRT monitors and televisions the beams are bent by magnetic deflection, using a deflection yoke. Electrostatic deflection is commonly used in oscilloscopes.[3]
A CRT is a glass envelope which is deep (i.e., long from front screen face to rear end), heavy, and fragile. The interior is evacuated to 0.01 pascals (1×10−7 atm)[4] to 0.1 micropascals (1×10−12 atm) or less,[5] to facilitate the free flight of electrons from the gun(s) to the tube's face without scattering due to collisions with air molecules. As such, handling a CRT carries the risk of violent implosion that can hurl glass at great velocity. The face is typically made of thick lead glass or special barium-strontium glass to be shatter-resistant and to block most X-ray emissions. CRTs make up most of the weight of CRT TVs and computer monitors.[6][7]
Since the mid-late 2000's, CRTs have been superseded by flat-panel display technologies such as LCD, plasma display, and OLED displays which are cheaper to manufacture and run, as well as significantly lighter and less bulky. Flat-panel displays can also be made in very large sizes whereas 40 in (100 cm) to 45 in (110 cm)[8] was about the largest size of a CRT.[9]
A CRT works by electrically heating a tungsten coil[10] which in turn heats a cathode in the rear of the CRT, causing it to emit electrons which are modulated and focused by electrodes. The electrons are steered by deflection coils or plates, and an anode accelerates them towards the phosphor-coated screen, which generates light when hit by the electrons.[11][12][13]
History
Discoveries
Cathode rays were discovered by Julius Plücker and Johann Wilhelm Hittorf.[14] Hittorf observed that some unknown rays were emitted from the cathode (negative electrode) which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines. In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, and William Crookes showed they could be deflected by magnetic fields. In 1897, J. J. Thomson succeeded in measuring the charge-mass-ratio of cathode rays, showing that they consisted of negatively charged particles smaller than atoms, the first "subatomic particles", which had already been named electrons by Irish physicist George Johnstone Stoney in 1891. The earliest version of the CRT was known as the "Braun tube", invented by the German physicist Ferdinand Braun in 1897.[15] It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen. Braun was the first to conceive the use of a CRT as a display device.[16]
In 1908, Alan Archibald Campbell-Swinton, fellow of the Royal Society (UK), published a letter in the scientific journal Nature, in which he described how "distant electric vision" could be achieved by using a cathode-ray tube (or "Braun" tube) as both a transmitting and receiving device.[17] He expanded on his vision in a speech given in London in 1911 and reported in The Times[18] and the Journal of the Röntgen Society.[19][20]
The first cathode-ray tube to use a hot cathode was developed by John Bertrand Johnson (who gave his name to the term Johnson noise) and Harry Weiner Weinhart of Western Electric, and became a commercial product in 1922.[21] The introduction of hot cathodes allowed for lower acceleration anode voltages and higher electron beam currents, since the anode now only accelerated the electrons emitted by the hot cathode, and no longer had to have a very high voltage to induce electron emission from the cold cathode.[22]
Development
In 1926, Kenjiro Takayanagi demonstrated a CRT television receiver with a mechanical video camera that received images with a 40-line resolution.[23] By 1927, he improved the resolution to 100 lines, which was unrivaled until 1931.[24] By 1928, he was the first to transmit human faces in half-tones on a CRT display.[25] In 1927, Philo Farnsworth created a television prototype.[26][27][28][29][30] The CRT was named in 1929 by inventor Vladimir K. Zworykin.[25]: 84 RCA was granted a trademark for the term (for its cathode-ray tube) in 1932; it voluntarily released the term to the public domain in 1950.[31]
In the 1930s, Allen B. DuMont made the first CRTs to last 1,000 hours of use, which was one of the factors that led to the widespread adoption of television.[32]
The first commercially made electronic television sets with cathode-ray tubes were manufactured by Telefunken in Germany in 1934.[33][34]
In 1947, the cathode-ray tube amusement device, the earliest known interactive electronic game as well as the first to incorporate a cathode-ray tube screen, was created.[35]
From 1949 to the early 1960s, there was a shift from circular CRTs to rectangular CRTs, although the first rectangular CRTs were made in 1938 by Telefunken.[36][22][37][38][39][40] While circular CRTs were the norm, European TV sets often blocked portions of the screen to make it appear somewhat rectangular while American sets often left the entire front of the CRT exposed or only blocked the upper and lower portions of the CRT.[41][42]
In 1954, RCA produced some of the first color CRTs, the 15GP22 CRTs used in the CT-100,[43] the first color TV set to be mass-produced.[44] The first rectangular color CRTs were also made in 1954.[45][46] However, the first rectangular color CRTs to be offered to the public were made in 1963. One of the challenges that had to be solved to produce the rectangular color CRT was convergence at the corners of the CRT.[39][38] In 1965, brighter rare earth phosphors began replacing dimmer and cadmium-containing red and green phosphors. Eventually blue phosphors were replaced as well.[47][48][49][50][51][52]
The size of CRTs increased over time, from 20 inches in 1938,[53] to 21 inches in 1955,[54][55] 35 inches by 1985,[56] and 43 inches by 1989.[57] However, experimental 31 inch CRTs were made as far back as 1938.[58]
In 1960, the Aiken tube was invented. It was a CRT in a flat-panel display format with a single electron gun.[59][60] Deflection was electrostatic and magnetic, but due to patent problems, it was never put into production. It was also envisioned as a head-up display in aircraft.[61] By the time patent issues were solved, RCA had already invested heavily in conventional CRTs.[62]
1968 marks the release of Sony Trinitron brand with the model KV-1310, which was based on Aperture Grille technology. It was acclaimed to have improved the output brightness. The Trinitron screen was identical with its upright cylindrical shape due to its unique triple cathode single gun construction.
In 1987, flat-screen CRTs were developed by Zenith for computer monitors, reducing reflections and helping increase image contrast and brightness.[63][64] Such CRTs were expensive, which limited their use to computer monitors.[65] Attempts were made to produce flat-screen CRTs using inexpensive and widely available float glass.[66]
In 1990, the first CRTs with HD resolution were released to the market by Sony.[67]
In the mid-1990s, some 160 million CRTs were made per year.[68]
In the mid-2000s, Canon and Sony presented the surface-conduction electron-emitter display and field-emission displays, respectively. They both were flat-panel displays that had one (SED) or several (FED) electron emitters per subpixel in place of electron guns. The electron emitters were placed on a sheet of glass and the electrons were accelerated to a nearby sheet of glass with phosphors using an anode voltage. The electrons were not focused, making each subpixel essentially a flood beam CRT. They were never put into mass production as LCD technology was significantly cheaper, eliminating the market for such displays.[69]
The last large-scale manufacturer of (in this case, recycled)[70] CRTs, Videocon, ceased in 2015.[71][72] CRT TVs stopped being made around the same time.[73]
In 2015, several CRT manufacturers were convicted in the US for price fixing. The same occurred in Canada in 2018.[74][75]
Worldwide sales of CRT computer monitors peaked in 2000, at 90 million units, while those of CRT TVs peaked in 2005 at 130 million units.[76]
Decline
Beginning in the late 90s to the early 2000s, CRTs began to be replaced with LCDs, starting first with computer monitors smaller than 15 inches in size,[77] largely because of their lower bulk.[78] Among the first manufacturers to stop CRT production was Hitachi in 2001,[79][80] followed by Sony in Japan in 2004,[81] Flat-panel displays dropped in price and started significantly displacing cathode-ray tubes in the 2000s. LCD monitor sales began exceeding those of CRTs in 2003–2004[82][83][84] and LCD TV sales started exceeding those of CRTs in some markets in 2005.[85]
Despite being a mainstay of display technology for decades, CRT-based computer monitors and televisions are now virtually a dead technology. Demand for CRT screens dropped in the late 2000s.[86] Despite efforts from Samsung and LG to make CRTs competitive with their LCD and plasma counterparts, offering slimmer and cheaper models to compete with similarly sized and more expensive LCDs,[87][88][89][90][91] CRTs eventually became obsolete and were relegated to developing markets once LCDs fell in price, with their lower bulk, weight and ability to be wall mounted coming as pluses.
Some industries still use CRTs because it is either too much effort, downtime, and/or cost to replace them, or there is no substitute available; a notable example is the airline industry. Planes such as the Boeing 747-400 and the Airbus A320 used CRT instruments in their glass cockpits instead of mechanical instruments.[92] Airlines such as Lufthansa still use CRT technology, which also uses floppy disks for navigation updates.[93] They are also used in some military equipment for similar reasons.
As of 2022, at least one company manufactures new CRTs for these markets.[94]
A popular consumer usage of CRTs is for retrogaming. Some games are impossible to play without CRT display hardware, and some games play better. Reasons for this include:
- CRTs refresh faster than LCDs, because they use interlaced lines.
- CRTs are able to correctly display certain oddball resolutions, such as the 256x224 resolution of the Nintendo Entertainment System (NES).[95]
- Light guns only work on CRTs because they depend on the progressive timing properties of CRTs.
Construction
Body
The body of a CRT is usually made up of three parts: A screen/faceplate/panel, a cone/funnel, and a neck.[96][97][98][99][100] The joined screen, funnel and neck are known as the bulb or envelope.[38]
The neck is made from a glass tube[101] while the funnel and screen are made by pouring and then pressing glass into a mold.[102][103][104][105][106] The glass, known as CRT glass[107][108] or TV glass,[109] needs special properties to shield against x-rays while providing adequate light transmission in the screen or being very electrically insulating in the funnel and neck. The formulation that gives the glass its properties is also known as the melt. The glass is of very high quality, being almost contaminant and defect free. Most of the costs associated with glass production come from the energy used to melt the raw materials into glass. Glass furnaces for CRT glass production have several taps to allow molds to be replaced without stopping the furnace, to allow production of CRTs of several sizes. Only the glass used on the screen needs to have precise optical properties. The optical properties of the glass used on the screen affects color reproduction and purity in Color CRTs. Transmittance, or how transparent the glass is, may be adjusted to be more transparent to certain colors (wavelengths) of light. Transmittance is measured at the center of the screen with a 546 nm wavelength light, and a 10.16mm thick screen. Transmittance goes down with increasing thickness. Standard transmittances for Color CRT screens are 86%, 73%, 57%, 46%, 42% and 30%. Lower transmittances are used to improve image contrast but they put more stress on the electron gun, requiring more power on the electron gun for a higher electron beam power to light the phosphors more brightly to compensate for the reduced transmittance.[65][110] The transmittance must be uniform across the screen to ensure color purity. The radius (curvature) of screens has increased (grown less curved) over time, from 30 to 68 inches, ultimately evolving into completely flat screens, reducing reflections. The thickness of both curved[111] and flat screens gradually increases from the center outwards, and with it, transmittance is gradually reduced. This means that flat-screen CRTs may not be completely flat on the inside.[111][112] The glass used in CRTs arrives from the glass factory to the CRT factory as either separate screens and funnels with fused necks, for Color CRTs, or as bulbs made up of a fused screen, funnel and neck. There were several glass formulations for different types of CRTs, that were classified using codes specific to each glass manufacturer. The compositions of the melts were also specific to each manufacturer.[113] Those optimized for high color purity and contrast were doped with Neodymium, while those for monochrome CRTs were tinted to differing levels, depending on the formulation used and had transmittances of 42% or 30%.[114] Purity is ensuring that the correct colors are activated (for example, ensuring that red is displayed uniformly across the screen) while convergence ensures that images are not distorted. Convergence may be modified using a cross hatch pattern.[115][116][117]
CRT glass used to be made by dedicated companies[118] such as AGC Inc.,[119][120][121] O-I Glass,[122] Samsung Corning Precision Materials,[123] Corning Inc.,[124][125] and Nippon Electric Glass;[126] others such as Videocon, Sony for the US market and Thomson made their own glass.[127][128][129][130][131]
The funnel and the neck are made of leaded potash-soda glass or lead silicate glass[7] formulation to shield against x-rays generated by high voltage electrons as they decelerate after striking a target, such as the phosphor screen or shadow mask of a color CRT. The velocity of the electrons depends on the anode voltage of the CRT; the higher the voltage, the higher the speed.[132] The amount of x-rays emitted by a CRT can also lowered by reducing the brightness of the image.[133][134][135][99] Leaded glass is used because it is inexpensive, while also shielding heavily against x-rays, although some funnels may also contain barium.[136][137][138][114] The screen is usually instead made out of a special lead-free silicate[7] glass formulation with barium and strontium to shield against x-rays. Another glass formulation uses 2-3% of lead on the screen.[99] Monochrome CRTs may have a tinted barium-lead glass formulation in both the screen and funnel, with a potash-soda lead glass in the neck; the potash-soda and barium-lead formulations have different thermal expansion coefficients. The glass used in the neck must be an excellent electrical insulator to contain the voltages used in the electron optics of the electron gun, such as focusing lenses. The lead in the glass causes it to brown (darken) with use due to x-rays, usually the CRT cathode wears out due to cathode poisoning before browning becomes apparent. The glass formulation determines the highest possible anode voltage and hence the maximum possible CRT screen size. For color, maximum voltages are often 24 to 32 kV, while for monochrome it is usually 21 or 24.5 kV,[139] limiting the size of monochrome CRTs to 21 inches, or approx. 1 kV per inch. The voltage needed depends on the size and type of CRT.[140] Since the formulations are different, they must be compatible with one another, having similar thermal expansion coefficients.[114] The screen may also have an anti-glare or anti-reflective coating,[141][110][142] or be ground to prevent reflections.[143] CRTs may also have an anti-static coating.[110][144][65]
The leaded glass in the funnels of CRTs may contain 21 to 25% of lead oxide (PbO),[145][146][113] The neck may contain 30 to 40% of lead oxide,[147][148] and the screen may contain 12% of barium oxide, and 12% of strontium oxide.[7] A typical CRT contains several kilograms of lead as lead oxide in the glass[100] depending on its size; 12 inch CRTs contain 0.5 kg of lead in total while 32 inch CRTs contain up to 3 kg.[7] Strontium oxide began being used in CRTs, its major application, in the 1970s.[149][150][151]
Some early CRTs used a metal funnel insulated with polyethylene instead of glass with conductive material.[54] Others had ceramic or blown pyrex instead of pressed glass funnels.[152][153][40][154][155] Early CRTs did not have a dedicated anode cap connection; the funnel was the anode connection, so it was live during operation.[156]
The funnel is coated on the inside and outside with a conductive coating,[157][158] making the funnel a capacitor, helping stabilize and filter the anode voltage of the CRT, and significantly reducing the amount of time needed to turn on a CRT. The stability provided by the coating solved problems inherent to early power supply designs, as they used vacuum tubes. Because the funnel is used as a capacitor, the glass used in the funnel must be an excellent electrical insulator (dielectric). The inner coating has a positive voltage (the anode voltage that can be several kV) while the outer coating is connected to ground. CRTs powered by more modern power supplies do not need to be connected to ground, due to the more robust design of modern power supplies. The value of the capacitor formed by the funnel is .005-.01uF, although at the voltage the anode is normally supplied with. The capacitor formed by the funnel can also suffer from dielectric absorption, similarly to other types of capacitors.[159][139][160][161][157][114] Because of this CRTs have to be discharged[162] before handling to prevent injury.
The depth of a CRT is related to its screen size.[163] Usual deflection angles were 90° for computer monitor CRTs and small CRTs and 110° which was the standard in larger TV CRTs, with 120 or 125° being used in slim CRTs made since 2001–2005 in an attempt to compete with LCD TVs. [164][110][90][98][165] Over time, deflection angles increased as they became practical, from 50° in 1938 to 110° in 1959,[22] and 125° in the 2000s. 140° deflection CRTs were researched but never commercialized, as convergence problems were never resolved.[166]
A monochrome CRT with 110° deflection
A monochrome CRT with 90° deflection
Size and weight
The size of the screen of a CRT is measured in two ways: the size of the screen or the face diagonal, and the viewable image size/area or viewable screen diagonal, which is the part of the screen with phosphor. The size of the screen is the viewable image size plus its black edges which are not coated with phosphor.[167][158][168] The viewable image may be perfectly square or rectangular while the edges of the CRT are black and have a curvature (such as in black stripe CRTs) or the edges may be black and truly flat (such as in Flatron CRTs),[111][131][169] or the edges of the image may follow the curvature of the edges of the CRT, which may be the case in CRTs without and with black edges and curved edges.[170][171][172] Black stripe CRTs were first made by Toshiba in 1972.[131]
Small CRTs below 3 inches were made for handheld televisions such as the MTV-1 and viewfinders in camcorders. In these, there may be no black edges, that are however truly flat.[173][160][174][175][176]
Most of the weight of a CRT comes from the thick glass screen, which comprises 65% of the total weight of a CRT. The funnel and neck glass comprise the remaining 30% and 5% respectively. The glass in the funnel is thinner than on the screen.[7][6] Chemically or thermally tempered glass may be used to reduce the weight of the CRT glass.[177][178][179][180]
Anode
The outer conductive coating is connected to ground while the inner conductive coating is connected using the anode button/cap through a series of capacitors and diodes (a Cockcroft–Walton generator) to the high voltage flyback transformer; the inner coating is the anode of the CRT,[181] which, together with an electrode in the electron gun, is also known as the final anode.[182][183] The inner coating is connected to the electrode using springs. The electrode forms part of a bipotential lens.[183][184] The capacitors and diodes serve as a voltage multiplier for the current delivered by the flyback.
For the inner funnel coating, monochrome CRTs use aluminum while color CRTs use aquadag;[114] Some CRTs may use iron oxide on the inside.[7] On the outside, most CRTs (but not all)[185] use aquadag.[186] Aquadag is an electrically conductive graphite-based paint. In color CRTs, the aquadag is sprayed onto the interior of the funnel[187][114] whereas historically aquadag was painted into the interior of monochrome CRTs.[22]
The anode is used to accelerate the electrons towards the screen and also collects the secondary electrons that are emitted by the phosphor particles in the vacuum of the CRT.[188][189][190][191][22]
The anode cap connection in modern CRTs must be able to handle up to 55–60 kV depending on the size and brightness of the CRT. Higher voltages allow for larger CRTs, higher image brightness, or a tradeoff between the two.[192][140] It consists of a metal clip that expands on the inside of an anode button that is embedded on the funnel glass of the CRT.[193][194] The connection is insulated by a silicone suction cup, possibly also using silicone grease to prevent corona discharge.[195][196]
The anode button must be specially shaped to establish a hermetic seal between the button and funnel. X-rays may leak through the anode button, although that may not be the case in newer CRTs starting from the late 1970s to early 1980s, thanks to a new button and clip design.[140] The button may consist of a set of 3 nested cups, with the outermost cup being made of a Nickel–Chromium–Iron alloy containing 40 to 49% of Nickel and 3 to 6% of Chromium to make the button easy to fuse to the funnel glass, with a first inner cup made of thick inexpensive iron to shield against x-rays, and with the second innermost cup also being made of iron or any other electrically conductive metal to connect to the clip. The cups must be heat resistant enough and have similar thermal expansion coefficients similar to that of the funnel glass to withstand being fused to the funnel glass. The inner side of the button is connected to the inner conductive coating of the CRT.[189] The anode button may be attached to the funnel while its being pressed into shape in a mold.[197][198][140] Alternatively, the x-ray shielding may instead be built into the clip.[199]
The flyback transformer is also known as an IHVT (Integrated High Voltage Transformer) if it includes a voltage multiplier. The flyback uses a ceramic or powdered iron core to enable efficient operation at high frequencies. The flyback contains one primary and many secondary windings that provide several different voltages. The main secondary winding supplies the voltage multiplier with voltage pulses to ultimately supply the CRT with the high anode voltage it uses, while the remaining windings supply the CRT's filament voltage, keying pulses, focus voltage and voltages derived from the scan raster. When the transformer is turned off, the flyback's magnetic field quickly collapses which induces high voltage in its windings. The speed at which the magnetic field collapses determines the voltage that is induced, so the voltage increases alongside its speed. A capacitor (Retrace Timing Capacitor) or series of capacitors (to provide redundancy) is used to slow the collapse of the magnetic field.[200][201]
The design of the high voltage power supply in a product using a CRT has an influence in the amount of x-rays emitted by the CRT. The amount of emitted x-rays increases with both higher voltages and currents. If the product such as a TV set uses an unregulated high voltage power supply, meaning that anode and focus voltage go down with increasing electron current when displaying a bright image, the amount of emitted x-rays is as its highest when the CRT is displaying a moderately bright images, since when displaying dark or bright images, the higher anode voltage counteracts the lower electron beam current and vice versa respectively. The high voltage regulator and rectifier vacuum tubes in some old CRT TV sets may also emit x-rays.[202]
Electron gun
The electron gun emits the electrons that ultimately hit the phosphors on the screen of the CRT. The electron gun contains a heater, which heats a cathode, which generates electrons that, using grids, are focused and ultimately accelerated into the screen of the CRT. The acceleration occurs in conjunction with the inner aluminum or aquadag coating of the CRT. The electron gun is positioned so that it aims at the center of the screen.[183] It is inside the neck of the CRT, and it is held together and mounted to the neck using glass beads or glass support rods, which are the glass strips on the electron gun.[22][183][203] The electron gun is made separately and then placed inside the neck through a process called "winding", or sealing.[66][204][205][206][207][208] The electron gun has a glass wafer that is fused to the neck of the CRT. The connections to the electron gun penetrate the glass wafer.[205][209] Once the electron gun is inside the neck, its metal parts (grids) are arced between each other using high voltage to smooth any rough edges in a process called spot knocking, to prevent the rough edges in the grids from generating secondary electrons.[210][211][212]
Construction and method of operation
It has a hot cathode that is heated by a tungsten filament heating element; the heater may draw 0.5 to 2 A of current depending on the CRT. The voltage applied to the heater can affect the life of the CRT.[213][214] Heating the cathode energizes the electrons in it, aiding electron emission,[215] while at the same time current is supplied to the cathode; typically anywhere from 140 mA at 1.5 V to 600 mA at 6.3 V.[216] The cathode creates an electron cloud (emits electrons) whose electrons are extracted, accelerated and focused into an electron beam.[22] Color CRTs have three cathodes: one for red, green and blue. The heater sits inside the cathode but does not touch it; the cathode has its own separate electrical connection. The cathode is coated onto a piece of nickel which provides the electrical connection and structural support; the heater sits inside this piece without touching it.[181][217][218][219]
There are several shortcircuits that can occur in a CRT electron gun. One is a heater-to-cathode short, that causes the cathode to permanently emit electrons which may cause an image with a bright red, green or blue tint with retrace lines, depending on the cathode (s) affected. Alternatively, the cathode may short to the control grid, possibly causing similar effects, or, the control grid and screen grid (G2)[220] can short causing a very dark image or no image at all. The cathode may be surrounded by a shield to prevent sputtering.[221][222]
The cathode is a layer of barium oxide which is coated on a piece of nickel for electrical and mechanical support.[223][139] The barium oxide must be activated by heating to enable it to release electrons. Activation is necessary because barium oxide is not stable in air, so it is applied to the cathode as barium carbonate, which cannot emit electrons. Activation heats the barium carbonate to decompose it into barium oxide and carbon dioxide while forming a thin layer of metallic barium on the cathode.[224][223] Activation occurs during evacuation of (at the same time a vacuum is formed in) the CRT. After activation the oxide can become damaged by several common gases such as water vapor, carbon dioxide, and oxygen.[225] Alternatively, barium strontium calcium carbonate may be used instead of barium carbonate, yielding barium, strontium and calcium oxides after activation.[226][22] During operation, the barium oxide is heated to 800-1000°C, at which point it starts shedding electrons.[227][139][215]
Since it is a hot cathode, it is prone to cathode poisoning, which is the formation of a positive ion layer that prevents the cathode from emitting electrons, reducing image brightness significantly or completely and causing focus and intensity to be affected by the frequency of the video signal preventing detailed images from being displayed by the CRT. The positive ions come from leftover air molecules inside the CRT or from the cathode itself[22] that react over time with the surface of the hot cathode.[228][222] Reducing metals such as manganese, zirconium, magnesium, aluminum or titanium may be added to the piece of nickel to lengthen the life of the cathode, as during activation, the reducing metals diffuse into the barium oxide, improving its lifespan, especially at high electron beam currents.[229] In color CRTs with red, green and blue cathodes, one or more cathodes may be affected independently of the others, causing total or partial loss of one or more colors.[222] CRTs can wear or burn out due to cathode poisoning. Cathode poisoning is accelerated by increased cathode current (overdriving).[230] In color CRTs, since there are three cathodes, one for red, green and blue, a single or more poisoned cathode may cause the partial or complete loss of one or more colors, tinting the image.[222] The layer may also act as a capacitor in series with the cathode, inducing thermal lag. The cathode may instead be made of scandium oxide or incorporate it as a dopant, to delay cathode poisoning, extending the life of the cathode by up to 15%.[231][139][232]
The amount of electrons generated by the cathodes is related to their surface area. A cathode with more surface area creates more electrons, in a larger electron cloud, which makes focusing the electron cloud into an electron beam more difficult.[230] Normally, only a part of the cathode emits electrons unless the CRT displays images with parts that are at full image brightness; only the parts at full brightness cause all of the cathode to emit electrons. The area of the cathode that emits electrons grows from the center outwards as brightness increases, so cathode wear may be uneven. When only the center of the cathode is worn, the CRT may light brightly those parts of images that have full image brightness but not show darker parts of images at all, in such a case the CRT displays a poor gamma characteristic.[222]
The second (screen) grid of the gun (G2) accelerates the electrons towards the screen using several hundred DC volts. A negative current[233] is applied to the first (control) grid (G1) to converge the electron beam. G1 in practice is a Wehnelt cylinder.[216][234] The brightness of the screen is not controlled by varying the anode voltage nor the electron beam current (they are never varied) despite them having an influence on image brightness, rather image brightness is controlled by varying the difference in voltage between the cathode and the G1 control grid. A third grid (G3) electrostatically focuses the electron beam before it is deflected and accelerated by the anode voltage onto the screen.[235] Electrostatic focusing of the electron beam may be accomplished using an Einzel lens energized at up to 600 volts.[236][224] Before electrostatic focusing, focusing the electron beam required a large, heavy and complex mechanical focusing system placed outside the electron gun.[156]
However, electrostatic focusing cannot be accomplished near the final anode of the CRT due to its high voltage in the dozens of Kilovolts, so a high voltage (≈600[237] to 8000 volt) electrode, together with an electrode at the final anode voltage of the CRT, may be used for focusing instead. Such an arrangement is called a bipotential lens, which also offers higher performance than an Einzel lens, or, focusing may be accomplished using a magnetic focusing coil together with a high anode voltage of dozens of kilovolts. However, magnetic focusing is expensive to implement, so it is rarely used in practice.[181][224][238][239] Some CRTs may use two grids and lenses to focus the electron beam.[231] The focus voltage is generated in the flyback using a subset of the flyback's high voltage winding in conjunction with a resistive voltage divider. The focus electrode is connected alongside the other connections that are in the neck of the CRT.[240]
There is a voltage called cutoff voltage which is the voltage that creates black on the screen since it causes the image on the screen created by the electron beam to disappear, the voltage is applied to G1. In a color CRT with three guns, the guns have different cutoff voltages. Many CRTs share grid G1 and G2 across all three guns, increasing image brightness and simplifying adjustment since on such CRTs there is a single cutoff voltage for all three guns (since G1 is shared across all guns).[183] but placing additional stress on the video amplifier used to feed video into the electron gun's cathodes, since the cutoff voltage becomes higher. Monochrome CRTs do not suffer from this problem. In monochrome CRTs video is fed to the gun by varying the voltage on the first control grid.[241][156]
During retracing of the electron beam, the preamplifier that feeds the video amplifier is disabled and the video amplifier is biased to a voltage higher than the cutoff voltage to prevent retrace lines from showing, or G1 can have a large negative voltage applied to it to prevent electrons from getting out of the cathode.[22] This is known as blanking. (see Vertical blanking interval and Horizontal blanking interval.) Incorrect biasing can lead to visible retrace lines on one or more colors, creating retrace lines that are tinted or white (for example, tinted red if the red color is affected, tinted magenta if the red and blue colors are affected, and white if all colors are affected).[242][243][244] Alternatively, the amplifier may be driven by a video processor that also introduces an OSD (On Screen Display) into the video stream that is fed into the amplifier, using a fast blanking signal.[245] TV sets and computer monitors that incorporate CRTs need a DC restoration circuit to provide a video signal to the CRT with a DC component, restoring the original brightness of different parts of the image.[246]
The electron beam may be affected by the earth's magnetic field, causing it to normally enter the focusing lens off-center; this can be corrected using astigmation controls. Astigmation controls are both magnetic and electronic (dynamic); magnetic does most of the work while electronic is used for fine adjustments.[247] One of the ends of the electron gun has a glass disk, the edges of which are fused with the edge of the neck of the CRT, possibly using frit;[248] the metal leads that connect the electron gun to the outside pass through the disk.[249]
Some electron guns have a quadrupole lens with dynamic focus to alter the shape and adjust the focus of the electron beam, varying the focus voltage depending on the position of the electron beam to maintain image sharpness across the entire screen, specially at the corners.[110][250][251][252][253] They may also have a bleeder resistor to derive voltages for the grids from the final anode voltage.[254][255][256]
After the CRTs were manufactured, they were aged to allow cathode emission to stabilize.[257][258]
The electron guns in color CRTs are driven by a video amplifier which takes a signal per color channel and amplifies it to 40-170v per channel, to be fed into the electron gun's cathodes;[244] each electron gun has its own channel (one per color) and all channels may be driven by the same amplifier, which internally has three separate channels.[259] The amplifier's capabilities limit the resolution, refresh rate and contrast ratio of the CRT, as the amplifier needs to provide high bandwidth and voltage variations at the same time; higher resolutions and refresh rates need higher bandwidths (speed at which voltage can be varied and thus switching between black and white) and higher contrast ratios need higher voltage variations or amplitude for lower black and higher white levels. 30Mhz of bandwidth can usually provide 720p or 1080i resolution, while 20Mhz usually provides around 600 (horizontal, from top to bottom) lines of resolution, for example.[260][244] The difference in voltage between the cathode and the control grid is what modulates the electron beam, modulating its current and thus the brightness of the image.[222] The phosphors used in color CRTs produce different amounts of light for a given amount of energy, so to produce white on a color CRT, all three guns must output differing amounts of energy. The gun that outputs the most energy is the red gun since the red phosphor emits the least amount of light.[244]
Gamma
CRTs have a pronounced triode characteristic, which results in significant gamma (a nonlinear relationship in an electron gun between applied video voltage and beam intensity).[261]
Deflection
There are two types of deflection: magnetic and electrostatic. Magnetic is usually used in TVs and monitors as it allows for higher deflection angles (and hence shallower CRTs) and deflection power (which allows for higher electron beam current and hence brighter images)[262] while avoiding the need for high voltages for deflection of up to 2000 volts,[165] while oscilloscopes often use electrostatic deflection since the raw waveforms captured by the oscilloscope can be applied directly (after amplification) to the vertical electrostatic deflection plates inside the CRT.[263]
Magnetic deflection
Those that use magnetic deflection may use a yoke that has two pairs of deflection coils; one pair for vertical, and another for horizontal deflection.[264] The yoke can be bonded (be integral) or removable. Those that were bonded used glue[265] or a plastic[266] to bond the yoke to the area between the neck and the funnel of the CRT while those with removable yokes are clamped.[267][116] The yoke generates heat whose removal is essential since the conductivity of glass goes up with increasing temperature, the glass needs to be insulating for the CRT to remain usable as a capacitor. The temperature of the glass below the yoke is thus checked during the design of a new yoke.[139] The yoke contains the deflection and convergence coils with a ferrite core to reduce loss of magnetic force[268][264] as well as the magnetized rings used to align or adjust the electron beams in color CRTs (The color purity and convergence rings, for example)[269] and monochrome CRTs.[270][271] The yoke may be connected using a connector, the order in which the deflection coils of the yoke are connected determines the orientation of the image displayed by the CRT.[162] The deflection coils may be held in place using polyurethane glue.[265]
The deflection coils are driven by sawtooth signals[272][273][244] that may be delivered through VGA as horizontal and vertical sync signals.[274] A CRT needs two deflection circuits: a horizontal and a vertical circuit, which are similar except that the horizontal circuit runs at a much higher frequency (a Horizontal scan rate) of 15 to 240 kHz depending on the refresh rate of the CRT and the number of horizontal lines to be drawn (the vertical resolution of the CRT). The higher frequency makes it more susceptible to interference, so an automatic frequency control (AFC) circuit may be used to lock the phase of the horizontal deflection signal to that of a sync signal, to prevent the image from becoming distorted diagonally. The vertical frequency varies according to the refresh rate of the CRT. So a CRT with a 60 Hz refresh rate has a vertical deflection circuit running at 60 Hz. The horizontal and vertical deflection signals may be generated using two circuits that work differently; the horizontal deflection signal may be generated using a voltage controlled oscillator (VCO) while the vertical signal may be generated using a triggered relaxation oscillator. In many TVs, the frequencies at which the deflection coils run is in part determined by the inductance value of the coils.[275][244] CRTs had differing deflection angles; the higher the deflection angle, the shallower the CRT[276] for a given screen size, but at the cost of more deflection power and lower optical performance.[139][277]
Higher deflection power means more current[278] is sent to the deflection coils to bend the electron beam at a higher angle,[110] which in turn may generate more heat or require electronics that can handle the increased power.[277] Heat is generated due to resistive and core losses.[279] The deflection power is measured in mA per inch.[244] The vertical deflection coils may require approximately 24 volts while the horizontal deflection coils require approx. 120 volts to operate.
The deflection coils are driven by deflection amplifiers.[280] The horizontal deflection coils may also be driven in part by the horizontal output stage of a TV set. The stage contains a capacitor that is in series with the horizontal deflection coils that performs several functions, among them are: shaping the sawtooth deflection signal to match the curvature of the CRT and centering the image by preventing a DC bias from developing on the coil. At the beginning of retrace, the magnetic field of the coil collapses, causing the electron beam to return to the center of the screen, while at the same time the coil returns energy into capacitors, the energy of which is then used to force the electron beam to go to the left of the screen. [200]
Due to the high frequency at which the horizontal deflection coils operate, the energy in the deflection coils must be recycled to reduce heat dissipation. Recycling is done by transferring the energy in the deflection coils' magnetic field to a set of capacitors.[200] The voltage on the horizontal deflection coils is negative when the electron beam is on the left side of the screen and positive when the electron beam is on the right side of the screen. The energy required for deflection is dependent on the energy of the electrons.[281] Higher energy (voltage and/or current) electron beams need more energy to be deflected,[132] and are used to achieve higher image brightness.[282][283][192]
Electrostatic deflection
Mostly used in oscilloscopes. Deflection is carried out by applying a voltage across two pairs of plates, one for horizontal, and the other for vertical deflection. The electron beam is steered by varying the voltage difference across plates in a pair; For example, applying a voltage to the upper plate of the vertical deflection pair, while keeping the voltage in the bottom plate at 0 volts, will cause the electron beam to be deflected towards the upper part of the screen; increasing the voltage in the upper plate while keeping the bottom plate at 0 will cause the electron beam to be deflected to a higher point in the screen (will cause the beam to be deflected at a higher deflection angle). The same applies with the horizontal deflection plates. Increasing the length and proximity between plates in a pair can also increase the deflection angle.[284]
Burn-in
Burn-in is when images are physically "burned" into the screen of the CRT; this occurs due to degradation of the phosphors due to prolonged electron bombardment of the phosphors, and happens when a fixed image or logo is left for too long on the screen, causing it to appear as a "ghost" image or, in severe cases, also when the CRT is off. To counter this, screensavers were used in computers to minimize burn-in.[285] Burn-in is not exclusive to CRTs, as it also happens to plasma displays and OLED displays.
Evacuation
CRTs are evacuated or exhausted (a vacuum is formed) inside an oven at approx. 375–475 °C, in a process called baking or bake-out.[286] The evacuation process also outgasses any materials inside the CRT, while decomposing others such as the polyvinyl alcohol used to apply the phosphors.[287] The heating and cooling are done gradually to avoid inducing stress, stiffening and possibly cracking the glass; the oven heats the gases inside the CRT, increasing the speed of the gas molecules which increases the chances of them getting drawn out by the vacuum pump. The temperature of the CRT is kept to below that of the oven, and the oven starts to cool just after the CRT reaches 400 °C, or, the CRT was kept at a temperature higher than 400 °C for up to 15–55 minutes. The CRT was heated during or after evacuation, and the heat may have been used simultaneously to melt the frit in the CRT, joining the screen and funnel.[288][289][290] The pump used is a turbomolecular pump or a diffusion pump.[291][292][293][294] Formerly mercury vacuum pumps were also used.[295][296] After baking, the CRT is disconnected ("sealed or tipped off") from the vacuum pump.[297][298][299] The getter is then fired using an RF (induction) coil. The getter is usually in the funnel or in the neck of the CRT.[300][301] The getter material which is often barium-based, catches any remaining gas particles as it evaporates due to heating induced by the RF coil (that may be combined with exothermic heating within the material); the vapor fills the CRT, trapping any gas molecules that it encounters and condenses on the inside of the CRT forming a layer that contains trapped gas molecules. Hydrogen may be present in the material to help distribute the barium vapor. The material is heated to temperatures above 1000 °C, causing it to evaporate.[302][303][225] Partial loss of vacuum in a CRT can result in a hazy image, blue glowing in the neck of the CRT, flashovers, loss of cathode emission or focusing problems.[156] The vacuum inside of a CRT causes atmospheric pressure to exert (in a 27-inch CRT) a pressure of 5,800 pounds (2,600 kg) in total.[304]
Rebuilding
CRTs used to be rebuilt; repaired or refurbished. The rebuilding process included the disassembly of the CRT, the disassembly and repair or replacement of the electron gun(s), the removal and redeposition of phosphors and aquadag, etc. Rebuilding was popular until the 1960s because CRTs were expensive and wore out quickly, making repair worth it.[300] The last CRT rebuilder in the US closed in 2010,[305] and the last in Europe, RACS, which was located in France, closed in 2013.[306]
Reactivation
Also known as rejuvenation, the goal is to temporarily restore the brightness of a worn CRT. This is often done by carefully increasing the voltage on the cathode heater and the current and voltage on the control grids of the electron gun manually[citation needed]. Some rejuvenators can also fix heater-to-cathode shorts by running a capacitive discharge through the short.[222]
Phosphors
Phosphors in CRTs emit secondary electrons due to them being inside the vacuum of the CRT. The secondary electrons are collected by the anode of the CRT.[191] Secondary electrons generated by phosphors need to be collected to prevent charges from developing in the screen, which would lead to reduced image brightness[22] since the charge would repel the electron beam.
The phosphors used in CRTs often contain rare earth metals,[307][308][285] replacing earlier dimmer phosphors. Early red and green phosphors contained Cadmium,[309] and some black and white CRT phosphors also contained beryllium in the form of Zinc beryllium silicate,[50] although white phosphors containing cadmium, zinc and magnesium with silver, copper or manganese as dopants were also used.[22] The rare earth phosphors used in CRTs are more efficient (produce more light) than earlier phosphors.[310] The phosphors adhere to the screen because of Van der Waals and electrostatic forces. Phosphors composed of smaller particles adhere more strongly to the screen. The phosphors together with the carbon used to prevent light bleeding (in color CRTs) can be easily removed by scratching.[136][311]
Several dozen types of phosphors were available for CRTs.[312] Phosphors were classified according to color, persistence, luminance rise and fall curves, color depending on anode voltage (for phosphors used in penetration CRTs), Intended use, chemical composition, safety, sensitivity to burn-in, and secondary emission properties.[313] Examples of rare earth phosphors are yittrium oxide for red[314] and yittrium silicide for blue,[citation needed] while examples of earlier phosphors are copper cadmium sulfide for red,
SMPTE-C phosphors have properties defined by the SMPTE-C standard, which defines a color space of the same name. The standard prioritizes accurate color reproduction, which was made difficult by the different phosphors and color spaces used in the NTSC and PAL color systems. PAL TV sets have subjectively better color reproduction due to the use of saturated green phosphors, which have relatively long decay times that are tolerated in PAL since there is more time in PAL for phosphors to decay, due to its lower framerate. SMPTE-C phosphors were used in professional video monitors.[315][316]
The phosphor coating on monochrome and color CRTs may have an aluminum coating on its rear side used to reflect light forward, provide protection against ions to prevent ion burn by negative ions on the phosphor, manage heat generated by electrons colliding against the phosphor,[317] prevent static build up that could repel electrons from the screen, form part of the anode and collect the secondary electrons generated by the phosphors in the screen after being hit by the electron beam, providing the electrons with a return path.[318][139][319][317][22] The electron beam passes through the aluminum coating before hitting the phosphors on the screen; the aluminum attenuates the electron beam voltage by about 1 kv.[320][22][313] A film or lacquer may be applied to the phosphors to reduce the surface roughness of the surface formed by the phosphors to allow the aluminum coating to have a uniform surface and prevent it from touching the glass of the screen.[321][322] This is known as filming.[172] The lacquer contains solvents that are later evaporated; the lacquer may be chemically roughened to cause an aluminum coating with holes to be created to allow the solvents to escape.[322]
Phosphor persistence
Various phosphors are available depending upon the needs of the measurement or display application. The brightness, color, and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistences ranging from less than one microsecond to several seconds.[323] For visual observation of brief transient events, a long persistence phosphor may be desirable. For events which are fast and repetitive, or high frequency, a short-persistence phosphor is generally preferable.[324] The phosphor persistence must be low enough to avoid smearing or ghosting artifacts at high refresh rates.[110]
Limitations and workarounds
Blooming
Variations in anode voltage can lead to variations in brightness in parts or all of the image, in addition to blooming, shrinkage or the image getting zoomed in or out. Lower voltages lead to blooming and zooming in, while higher voltages do the opposite.[325][326] Some blooming is unavoidable, which can be seen as bright areas of an image that expand, distorting or pushing aside surrounding darker areas of the same image. Blooming occurs because bright areas have a higher electron beam current from the electron gun, making the beam wider and harder to focus. Poor voltage regulation causes focus and anode voltage to go down with increasing electron beam current.[202]
Doming
Doming is a phenomenon found on some CRT televisions in which parts of the shadow mask become heated. In televisions that exhibit this behavior, it tends to occur in high-contrast scenes in which there is a largely dark scene with one or more localized bright spots. As the electron beam hits the shadow mask in these areas it heats unevenly. The shadow mask warps due to the heat differences, which causes the electron gun to hit the wrong colored phosphors and incorrect colors to be displayed in the affected area.[327] Thermal expansion causes the shadow mask to expand by around 100 microns.[328][329][330][331]
During normal operation, the shadow mask is heated to around 80–90 °C.[332] Bright areas of images heat the shadow mask more than dark areas, leading to uneven heating of the shadow mask and warping (blooming) due to thermal expansion caused by heating by increased electron beam current.[333][334] The shadow mask is usually made of steel but it can be made of Invar[115] (a low-thermal expansion Nickel-Iron alloy) as it withstands two to three times more current than conventional masks without noticeable warping,[110][335][64] while making higher resolution CRTs easier to achieve.[336] Coatings that dissipate heat may be applied on the shadow mask to limit blooming[337][338] in a process called blackening.[339][340]
Bimetal springs may be used in CRTs used in TVs to compensate for warping that occurs as the electron beam heats the shadow mask, causing thermal expansion.[63] The shadow mask is installed to the screen using metal pieces[341] or a rail or frame[342][343][344] that is fused to the funnel or the screen glass respectively,[251] holding the shadow mask in tension to minimize warping (if the mask is flat, used in flat-screen CRT computer monitors) and allowing for higher image brightness and contrast.
Aperture grille screens are brighter since they allow more electrons through, but they require support wires. They are also more resistant to warping.[110] Color CRTs need higher anode voltages than monochrome CRTs to achieve the same brightness since the shadow mask blocks most of the electron beam. Slot masks[51] and specially Aperture grilles do not block as many electrons resulting in a brighter image for a given anode voltage, but aperture grille CRTs are heavier.[115] Shadow masks block[345] 80–85%[333][332] of the electron beam while Aperture grilles allow more electrons to pass through.[346]
High voltage
Image brightness is related to the anode voltage and to the CRTs size, so higher voltages are needed for both larger screens[347] and higher image brightness. Image brightness is also controlled by the current of the electron beam.[230] Higher anode voltages and electron beam currents also mean higher amounts of x-rays and heat generation since the electrons have a higher speed and energy.[202] Leaded glass and special barium-strontium glass are used to block most x-ray emissions.
Size
Size is limited by anode voltage, as it would require a higher dielectric strength to prevent arcing (corona discharge) and the electrical losses and ozone generation it causes, without sacrificing image brightness. The weight of the CRT, which originates from the thick glass needed to safely sustain a vacuum, imposes a practical limit on the size of a CRT.[348] The 43-inch Sony PVM-4300 CRT monitor weighs 440 pounds (200 kg).[349] Smaller CRTs weigh significantly less, as an example, 32-inch CRTs weigh up to 163 pounds (74 kg) and 19-inch CRTs weigh up to 60 pounds (27 kg). For comparison, a 32-inch flat panel TV only weighs approx. 18 pounds (8.2 kg) and a 19-inch flat panel TV weighs 6.5 pounds (2.9 kg).[350]
Shadow masks become more difficult to make with increasing resolution and size.[336]
Limits imposed by deflection
At high deflection angles, resolutions and refresh rates (since higher resolutions and refresh rates require significantly higher frequencies to be applied to the horizontal deflection coils), the deflection yoke starts to produce large amounts of heat, due to the need to move the electron beam at a higher angle, which in turn requires exponentially larger amounts of power. As an example, to increase the deflection angle from 90 to 120°, power consumption of the yoke must also go up from 40 watts to 80 watts, and to increase it further from 120 to 150°, deflection power must again go up from 80 watts to 160 watts. This normally makes CRTs that go beyond certain deflection angles, resolutions and refresh rates impractical, since the coils would generate too much heat due to resistance caused by the skin effect, surface and eddy current losses, and/or possibly causing the glass underneath the coil to become conductive (as the electrical conductivity of glass decreases with increasing temperature). Some deflection yokes are designed to dissipate the heat that comes from their operation.[114][351][279][352][353][354] Higher deflection angles in color CRTs directly affect convergence at the corners of the screen which requires additional compensation circuitry to handle electron beam power and shape, leading to higher costs and power consumption.[355][356] Higher deflection angles allow a CRT of a given size to be slimmer, however they also impose more stress on the CRT envelope, specially on the panel, the seal between the panel and funnel and on the funnel. The funnel needs to be long enough to minimize stress, as a longer funnel can be better shaped to have lower stress.[98][357]
Comparison with other technologies
- LCD advantages over CRT: Lower bulk, power consumption and heat generation, higher refresh rates (up to 360 Hz),[358] higher contrast ratios
- CRT advantages over LCD: Better color reproduction, no motion blur, multisyncing available in many monitors, no input lag[359]
- OLED advantages over CRT: Lower bulk, similar color reproduction,[359] higher contrast ratios, similar refresh rates (over 60 Hz, up to 120 Hz)[360][361][362] except for computer monitors.[363]
On CRTs, refresh rate depends on resolution, both of which are ultimately limited by the maximum horizontal scanning frequency of the CRT. Motion blur also depends on the decay time of the phosphors. Phosphors that decay too slowly for a given refresh rate may cause smearing or motion blur on the image. In practice, CRTs are limited to a refresh rate of 160 Hz.[364] LCDs that can compete with OLED (Dual Layer, and mini-LED LCDs) are not available in high refresh rates, although quantum dot LCDs (QLEDs) are available in high refresh rates (up to 144 Hz)[365] and are competitive in color reproduction with OLEDs.[366]
CRT monitors can still outperform LCD and OLED monitors in input lag, as there is no signal processing between the CRT and the display connector of the monitor, since CRT monitors often use VGA which provides an analog signal that can be fed to a CRT directly. Video cards designed for use with CRTs may have a RAMDAC to generate the analog signals needed by the CRT.[367][11] Also, CRT monitors are often capable of displaying sharp images at several resolutions, an ability known as multisyncing.[368] Due to these reasons, CRTs are sometimes preferred by PC gamers in spite of their bulk, weight and heat generation.[369][359]
CRTs tend to be more durable than their flat panel counterparts,[11] though specialised LCDs that have similar durability also exist.
Types
CRTs were produced in two major categories, picture tubes and display tubes.[68] Picture tubes were used in TVs while display tubes were used in computer monitors. Display tubes had no overscan and were of higher resolution. Picture tube CRTs have overscan, meaning the actual edges of the image are not shown; this is deliberate to allow for adjustment variations between CRT TVs, preventing the ragged edges (due to blooming) of the image from being shown on screen. The shadow mask may have grooves that reflect away the electrons that do not hit the screen due to overscan.[370][110] Color picture tubes used in TVs were also known as CPTs.[371] CRTs are also sometimes called Braun tubes.[372][373]
Monochrome CRTs
If the CRT is a black and white (B&W or monochrome) CRT, there is a single electron gun in the neck and the funnel is coated on the inside with aluminum that has been applied by evaporation; the aluminum is evaporated in a vacuum and allowed to condense on the inside of the CRT.[172] Aluminum eliminates the need for ion traps, necessary to prevent ion burn on the phosphor, while also reflecting light generated by the phosphor towards the screen, managing heat and absorbing electrons providing a return path for them; previously funnels were coated on the inside with aquadag, used because it can be applied like paint;[161] the phosphors were left uncoated.[22] Aluminum started being applied to CRTs in the 1950s, coating the inside of the CRT including the phosphors, which also increased image brightness since the aluminum reflected light (that would otherwise be lost inside the CRT) towards the outside of the CRT.[22][374][375][376] In aluminized monochrome CRTs, Aquadag is used on the outside. There is a single aluminum coating covering the funnel and the screen.[172]
The screen, funnel and neck are fused together into a single envelope, possibly using lead enamel seals, a hole is made in the funnel onto which the anode cap is installed and the phosphor, aquadag and aluminum are applied afterwards.[66] Previously monochrome CRTs used ion traps that required magnets; the magnet was used to deflect the electrons away from the more difficult to deflect ions, letting the electrons through while letting the ions collide into a sheet of metal inside the electron gun.[377][156][317] Ion burn results in premature wear of the phosphor. Since ions are harder to deflect than electrons, ion burn leaves a black dot in the center of the screen.[156][317]
The interior aquadag or aluminum coating was the anode and served to accelerate the electrons towards the screen, collect them after hitting the screen while serving as a capacitor together with the outer aquadag coating. The screen has a single uniform phosphor coating and no shadow mask, technically having no resolution limit.[378][163][379]
Monochrome CRTs may use ring magnets to adjust the centering of the electron beam and magnets around the deflection yoke to adjust the geometry of the image.[271][380]
![Older monochrome CRT[381] without aluminum, only aquadag](https://upload.wikimedia.org/wikipedia/commons/thumb/4/40/Osziroehre.jpg/430px-Osziroehre.jpg)
Older monochrome CRT[381] without aluminum, only aquadag
The electron gun of a monochrome CRT
![Older monochrome CRT[381] without aluminum, only aquadag](https://en.wikipedia.org/wiki/File:Osziroehre.jpg)
Color CRTs
Color CRTs use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in aperture grille designs) or clusters called "triads" (as in shadow mask CRTs).[382][383]
Color CRTs have three electron guns, one for each primary color, (red, green and blue) arranged either in a straight line (in-line) or in an equilateral triangular configuration (the guns are usually constructed as a single unit).[183][264][384][385][386] (The triangular configuration is often called "delta-gun", based on its relation to the shape of the Greek letter delta Δ.) The arrangement of the phosphors is the same as that of the electron guns.[183][387] A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor.[388]
A shadow mask tube uses a metal plate with tiny holes, typically in a delta configuration, placed so that the electron beam only illuminates the correct phosphors on the face of the tube;[382] blocking all other electrons.[99] Shadow masks that use slots instead of holes are known as slot masks.[11] The holes or slots are tapered[389][390] so that the electrons that strike the inside of any hole will be reflected back, if they are not absorbed (e.g. due to local charge accumulation), instead of bouncing through the hole to strike a random (wrong) spot on the screen. Another type of color CRT (Trinitron) uses an aperture grille of tensioned vertical wires to achieve the same result.[388] The shadow mask has a single hole for each triad.[183] The shadow mask is usually 1/2 inch behind the screen.[115]
Trinitron CRTs were different from other color CRTs in that they had a single electron gun with three cathodes, an aperture grille which lets more electrons through, increasing image brightness (since the aperture grille does not block as many electrons), and a vertically cylindrical screen, rather than a curved screen.[391]
The three electron guns are in the neck (except for Trinitrons) and the red, green and blue phosphors on the screen may be separated by a black grid or matrix (called black stripe by Toshiba).[65]
The funnel is coated with aquadag on both sides while the screen has a separate aluminum coating applied in a vacuum.[183][114] The aluminum coating protects the phosphor from ions, absorbs secondary electrons, providing them with a return path, preventing them from electrostatically charging the screen which would then repel electrons and reduce image brightness, reflects the light from the phosphors forwards and helps manage heat. It also serves as the anode of the CRT together with the inner aquadag coating. The inner coating is electrically connected to an electrode of the electron gun using springs, forming the final anode.[184][183] The outer aquadag coating is connected to ground, possibly using a series of springs or a harness that makes contact with the aquadag.[392][393]
Shadow mask
The shadow mask absorbs or reflects electrons that would otherwise strike the wrong phosphor dots,[379] causing color purity issues (discoloration of images); in other words, when set up correctly, the shadow mask helps ensure color purity.[183] When the electrons strike the shadow mask, they release their energy as heat and x-rays. If the electrons have too much energy due to an anode voltage that is too high for example, the shadow mask can warp due to the heat, which can also happen during the Lehr baking at approx. 435 °C of the frit seal between the faceplate and the funnel of the CRT.[345][394]
Shadow masks were replaced in TVs by slot masks in the 1970s, since slot masks let more electrons through, increasing image brightness. Shadow masks may be connected electrically to the anode of the CRT.[395][51][396][397] Trinitron used a single electron gun with three cathodes instead of three complete guns. CRT PC monitors usually use shadow masks, except for Sony's Trinitron, Mitsubishi's Diamondtron and NEC's Cromaclear; Trinitron and Diamondtron use aperture grilles while Cromaclear uses a slot mask. Some shadow mask CRTs have color phosphors that are smaller in diameter than the electron beams used to light them,[398] with the intention being to cover the entire phosphor, increasing image brightness.[399] Shadow masks may be pressed into a curved shape.[400][401][402]
Screen manufacture
Early color CRTs did not have a black matrix, which was introduced by Zenith in 1969, and Panasonic in 1970.[399][403][131] The black matrix eliminates light leaking from one phosphor to another since the black matrix isolates the phosphor dots from one another, so part of the electron beam touches the black matrix. This is also made necessary by warping of the shadow mask.[65][398] Light bleeding may still occur due to stray electrons striking the wrong phosphor dots. At high resolutions and refresh rates, phosphors only receive a very small amount of energy, limiting image brightness.[336]
Several methods were used to create the black matrix. One method coated the screen in photoresist such as dichromate-sensitized polyvinyl alcohol photoresist which was then dried and exposed; the unexposed areas were removed and the entire screen was coated in colloidal graphite to create a carbon film, and then hydrogen peroxide was used to remove the remaining photoresist alongside the carbon that was on top of it, creating holes that in turn created the black matrix. The photoresist had to be of the correct thickness to ensure sufficient adhesion to the screen, while the exposure step had to be controlled to avoid holes that were too small or large with ragged edges caused by light diffraction, ultimately limiting the maximum resolution of large color CRTs.[398] The holes were then filled with phosphor using the method described above. Another method used phosphors suspended in an aromatic diazonium salt that adhered to the screen when exposed to light; the phosphors were applied, then exposed to cause them to adhere to the screen, repeating the process once for each color. Then carbon was applied to the remaining areas of the screen while exposing the entire screen to light to create the black matrix, and a fixing process using an aqueous polymer solution was applied to the screen to make the phosphors and black matrix resistant to water.[403] Black chromium may be used instead of carbon in the black matrix.[398] Other methods were also used.[404][405][406][407]
The phosphors are applied using photolithography. The inner side of the screen is coated with phosphor particles suspended in PVA photoresist slurry,[408][409] which is then dried using infrared light,[410] exposed, and developed. The exposure is done using a "lighthouse" that uses an ultraviolet light source with a corrector lens to allow the CRT to achieve color purity. Removable shadow masks with spring-loaded clips are used as photomasks. The process is repeated with all colors. Usually the green phosphor is the first to be applied.[183][411][412][413] After phosphor application, the screen is baked to eliminate any organic chemicals (such as the PVA that was used to deposit the phosphor) that may remain on the screen.[403][414] Alternatively, the phosphors may be applied in a vacuum chamber by evaporating them and allowing them to condense on the screen, creating a very uniform coating.[231] Early color CRTs had their phosphors deposited using silkscreen printing.[43] Phosphors may have color filters over them (facing the viewer), contain pigment of the color emitted by the phosphor,[415][308] or be encapsulated in color filters to improve color purity and reproduction while reducing glare.[412][397] Poor exposure due to insufficient light leads to poor phosphor adhesion to the screen, which limits the maximum resolution of a CRT, as the smaller phosphor dots required for higher resolutions cannot receive as much light due to their smaller size.[416]
After the screen is coated with phosphor and aluminum and the shadow mask installed onto it the screen is bonded to the funnel using a glass frit that may contain 65 to 88% of lead oxide by weight. The lead oxide is necessary for the glass frit to have a low melting temperature. Boron oxide (III) may also present to stabilize the frit, with alumina powder as filler powder to control the thermal expansion of the frit.[417][145][7] The frit may be applied as a paste consisting of frit particles suspended in amyl acetate or in a polymer with an alkyl methacrylate monomer together with an organic solvent to dissolve the polymer and monomer.[418][419] The CRT is then baked in an oven in what is called a Lehr bake, to cure the frit, sealing the funnel and screen together. The frit contains a large quantity of lead, causing color CRTs to contain more lead than their monochrome counterparts. Monochrome CRTs on the other hand do not require frit; the funnel can be fused directly to the glass[99] by melting and joining the edges of the funnel and screen using gas flames. Frit is used in color CRTs to prevent deformation of the shadow mask and screen during the fusing process. The edges of the screen and funnel of the CRT are never melted.[183] A primer may be applied on the edges of the funnel and screen before the frit paste is applied to improve adhesion.[420] The Lehr bake consists of several successive steps that heat and then cool the CRT gradually until it reaches a temperature of 435 to 475 °C[418] (other sources may state different temperatures, such as 440 °C)[421] After the Lehr bake, the CRT is flushed with air or nitrogen to remove contaminants, the electron gun is inserted and sealed into the neck of the CRT, and a vacuum is formed on the CRT.[422][206]
Convergence and purity in color CRTs
Due to limitations in the dimensional precision with which CRTs can be manufactured economically, it has not been practically possible to build color CRTs in which three electron beams could be aligned to hit phosphors of respective color in acceptable coordination, solely on the basis of the geometric configuration of the electron gun axes and gun aperture positions, shadow mask apertures, etc. The shadow mask ensures that one beam will only hit spots of certain colors of phosphors, but minute variations in physical alignment of the internal parts among individual CRTs will cause variations in the exact alignment of the beams through the shadow mask, allowing some electrons from, for example, the red beam to hit, say, blue phosphors, unless some individual compensation is made for the variance among individual tubes.
Color convergence and color purity are two aspects of this single problem. Firstly, for correct color rendering it is necessary that regardless of where the beams are deflected on the screen, all three hit the same spot (and nominally pass through the same hole or slot) on the shadow mask.[clarification needed] This is called convergence.[423] More specifically, the convergence at the center of the screen (with no deflection field applied by the yoke) is called static convergence, and the convergence over the rest of the screen area (specially at the edges and corners) is called dynamic convergence.[116] The beams may converge at the center of the screen and yet stray from each other as they are deflected toward the edges; such a CRT would be said to have good static convergence but poor dynamic convergence. Secondly, each beam must only strike the phosphors of the color it is intended to strike and no others. This is called purity. Like convergence, there is static purity and dynamic purity, with the same meanings of "static" and "dynamic" as for convergence. Convergence and purity are distinct parameters; a CRT could have good purity but poor convergence, or vice versa. Poor convergence causes color "shadows" or "ghosts" along displayed edges and contours, as if the image on the screen were intaglio printed with poor registration. Poor purity causes objects on the screen to appear off-color while their edges remain sharp. Purity and convergence problems can occur at the same time, in the same or different areas of the screen or both over the whole screen, and either uniformly or to greater or lesser degrees over different parts of the screen.
The solution to the static convergence and purity problems is a set of color alignment ring magnets installed around the neck of the CRT.[424] These movable weak permanent magnets are usually mounted on the back end of the deflection yoke assembly and are set at the factory to compensate for any static purity and convergence errors that are intrinsic to the unadjusted tube. Typically there are two or three pairs of two magnets in the form of rings made of plastic impregnated with a magnetic material, with their magnetic fields parallel to the planes of the magnets, which are perpendicular to the electron gun axes. Often, one ring has two poles, another has 4, and the remaining ring has 6 poles.[425] Each pair of magnetic rings forms a single effective magnet whose field vector can be fully and freely adjusted (in both direction and magnitude). By rotating a pair of magnets relative to each other, their relative field alignment can be varied, adjusting the effective field strength of the pair. (As they rotate relative to each other, each magnet's field can be considered to have two opposing components at right angles, and these four components [two each for two magnets] form two pairs, one pair reinforcing each other and the other pair opposing and canceling each other. Rotating away from alignment, the magnets' mutually reinforcing field components decrease as they are traded for increasing opposed, mutually cancelling components.) By rotating a pair of magnets together, preserving the relative angle between them, the direction of their collective magnetic field can be varied. Overall, adjusting all of the convergence/purity magnets allows a finely tuned slight electron beam deflection or lateral offset to be applied, which compensates for minor static convergence and purity errors intrinsic to the uncalibrated tube. Once set, these magnets are usually glued in place, but normally they can be freed and readjusted in the field (e.g. by a TV repair shop) if necessary.
On some CRTs, additional fixed adjustable magnets are added for dynamic convergence or dynamic purity at specific points on the screen, typically near the corners or edges. Further adjustment of dynamic convergence and purity typically cannot be done passively, but requires active compensation circuits, one to correct convergence horizontally and another to correct it vertically. The deflection yoke contains convergence coils, a set of two per color, wound on the same core, to which the convergence signals are applied. That means 6 convergence coils in groups of 3, with 2 coils per group, with one coil for horizontal convergence correction and another for vertical convergence correction, with each group sharing a core. The groups are separated 120° from one another. Dynamic convergence is necessary because the front of the CRT and the shadow mask are not spherical, compensating for electron beam defocusing and astigmatism. The fact that the CRT screen is not spherical[426] leads to geometry problems which may be corrected using a circuit.[427] The signals used for convergence are parabolic waveforms derived from three signals coming from a vertical output circuit. The parabolic signal is fed into the convergence coils, while the other two are sawtooth signals that, when mixed with the parabolic signals, create the necessary signal for convergence. A resistor and diode are used to lock the convergence signal to the center of the screen to prevent it from being affected by the static convergence. The horizontal and vertical convergence circuits are similar. Each circuit has two resonators, one usually tuned to 15,625 Hz and the other to 31,250 Hz, which set the frequency of the signal sent to the convergence coils.[428] Dynamic convergence may be accomplished using electrostatic quadrupole fields in the electron gun.[429] Dynamic convergence means that the electron beam does not travel in a perfectly straight line between the deflection coils and the screen, since the convergence coils cause it to become curved to conform to the screen.
The convergence signal may instead be a sawtooth signal with a slight sine wave appearance, the sine wave part is created using a capacitor in series with each deflection coil. In this case, the convergence signal is used to drive the deflection coils. The sine wave part of the signal causes the electron beam to move more slowly near the edges of the screen. The capacitors used to create the convergence signal are known as the s-capacitors. This type of convergence is necessary due to the high deflection angles and flat screens of many CRT computer monitors. The value of the s-capacitors must be chosen based on the scan rate of the CRT, so multi-syncing monitors must have different sets of s-capacitors, one for each refresh rate.[110]
Dynamic convergence may instead be accomplished in some CRTs using only the ring magnets, magnets glued to the CRT, and by varying the position of the deflection yoke, whose position may be maintained using set screws, a clamp and rubber wedges.[116][430] 90° deflection angle CRTs may use "self-convergence" without dynamic convergence, which together with the in-line triad arrangement, eliminates the need for separate convergence coils and related circuitry, reducing costs. complexity and CRT depth by 10 millimeters. Self-convergence works by means of "nonuniform" magnetic fields. Dynamic convergence is necessary in 110° deflection angle CRTs, and quadrupole windings on the deflection yoke at a certain frequency may also be used for dynamic convergence.[431]
Dynamic color convergence and purity are one of the main reasons why until late in their history, CRTs were long-necked (deep) and had biaxially curved faces; these geometric design characteristics are necessary for intrinsic passive dynamic color convergence and purity. Only starting around the 1990s did sophisticated active dynamic convergence compensation circuits become available that made short-necked and flat-faced CRTs workable. These active compensation circuits use the deflection yoke to finely adjust beam deflection according to the beam target location. The same techniques (and major circuit components) also make possible the adjustment of display image rotation, skew, and other complex raster geometry parameters through electronics under user control.[110]
The guns are aligned with one another (converged) using convergence rings placed right outside the neck; there is one ring per gun. The rings have north and south poles. There are 4 sets of rings, one to adjust RGB convergence, a second to adjust Red and Blue convergence, a third to adjust vertical raster shift, and a fourth to adjust purity. The vertical raster shift adjusts the straightness of the scan line. CRTs may also employ dynamic convergence circuits, which ensure correct convergence at the edges of the CRT. Permalloy magnets may also be used to correct the convergence at the edges. Convergence is carried out with the help of a crosshatch (grid) pattern.[432][433] Other CRTs may instead use magnets that are pushed in and out instead of rings.[393] In early color CRTs, the holes in the shadow mask became progressively smaller as they extended outwards from the center of the screen, to aid in convergence.[399]
Magnetic shielding and degaussing
If the shadow mask or aperture grille becomes magnetized, its magnetic field alters the paths of the electron beams. This causes errors of "color purity" as the electrons no longer follow only their intended paths, and some will hit some phosphors of colors other than the one intended. For example, some electrons from the red beam may hit blue or green phosphors, imposing a magenta or yellow tint to parts of the image that are supposed to be pure red. (This effect is localized to a specific area of the screen if the magnetization is localized.) Therefore, it is important that the shadow mask or aperture grille not be magnetized. The earth's magnetic field may have an effect on the color purity of the CRT.[432] Because of this, some CRTs have external magnetic shields over their funnels. The magnetic shield may be made of soft iron or mild steel and contain a degaussing coil.[434] The magnetic shield and shadow mask may be permanently magnetized by the earth's magnetic field, adversely affecting color purity when the CRT is moved. This problem is solved with a built-in degaussing coil, found in many TVs and computer monitors. Degaussing may be automatic, occurring whenever the CRT is turned on.[435][183] The magnetic shield may also be internal, being on the inside of the funnel of the CRT.[436][437][110][438][439][440]
Color CRT displays in television sets and computer monitors often have a built-in degaussing (demagnetizing) coil mounted around the perimeter of the CRT face. Upon power-up of the CRT display, the degaussing circuit produces a brief, alternating current through the coil which fades to zero over a few seconds, producing a decaying alternating magnetic field from the coil. This degaussing field is strong enough to remove shadow mask magnetization in most cases, maintaining color purity.[441][442] In unusual cases of strong magnetization where the internal degaussing field is not sufficient, the shadow mask may be degaussed externally with a stronger portable degausser or demagnetizer. However, an excessively strong magnetic field, whether alternating or constant, may mechanically deform (bend) the shadow mask, causing a permanent color distortion on the display which looks very similar to a magnetization effect.
Resolution
Dot pitch defines the maximum resolution of the display, assuming delta-gun CRTs. In these, as the scanned resolution approaches the dot pitch resolution, moiré appears, as the detail being displayed is finer than what the shadow mask can render.[443] Aperture grille monitors do not suffer from vertical moiré, however, because their phosphor stripes have no vertical detail. In smaller CRTs, these strips maintain position by themselves, but larger aperture-grille CRTs require one or two crosswise (horizontal) support strips; one for smaller CRTs, and two for larger ones. The support wires block electrons, causing the wires to be visible.[444] In aperture grille CRTs, dot pitch is replaced by stripe pitch. Hitachi developed the Enhanced Dot Pitch (EDP) shadow mask, which uses oval holes instead of circular ones, with respective oval phosphor dots.[397] Moiré is reduced in shadow mask CRTs by arranging the holes in the shadow mask in a honeycomb-like pattern.[110]
Projection CRTs
Projection CRTs were used in CRT projectors and CRT rear-projection televisions, and are usually small (being 7 to 9 inches across);[260] have a phosphor that generates either red, green or blue light, thus making them monochrome CRTs;[445] and are similar in construction to other monochrome CRTs. Larger projection CRTs in general lasted longer, and were able to provide higher brightness levels and resolution, but were also more expensive.[446][447] Projection CRTs have an unusually high anode voltage for their size (such as 27 or 25 kV for a 5 or 7-inch projection CRT respectively),[448][449] and a specially made tungsten/barium cathode (instead of the pure barium oxide normally used) that consists of barium atoms embedded in 20% porous tungsten or barium and calcium aluminates or of barium, calcium and aluminum oxides coated on porous tungsten; the barium diffuses through the tungsten to emit electrons.[450] The special cathode can deliver 2mA of current instead of the 0.3mA of normal cathodes,[451][450][224][163] which makes them bright enough to be used as light sources for projection. The high anode voltage and the specially made cathode increase the voltage and current, respectively, of the electron beam, which increases the light emitted by the phosphors, and also the amount of heat generated during operation; this means that projector CRTs need cooling. The screen is usually cooled using a container (the screen forms part of the container) with glycol; the glycol may itself be dyed,[452] or colorless glycol may be used inside a container which may be colored (forming a lens known as a c-element). Colored lenses or glycol are used for improving color reproduction at the cost of brightness, and are only used on red and green CRTs.[453][454] Each CRT has its own glycol, which has access to an air bubble to allow the glycol to shrink and expand as it cools and warms. Projector CRTs may have adjustment rings just like color CRTs to adjust astigmatism,[455] which is flaring of the electron beam (stray light similar to shadows).[456] They have three adjustment rings; one with two poles, one with four poles, and another with 6 poles. When correctly adjusted, the projector can display perfectly round dots without flaring.[457] The screens used in projection CRTs were more transparent than usual, with 90% transmittance.[114] The first projection CRTs were made in 1933.[458]
Projector CRTs were available with electrostatic and electromagnetic focusing, the latter being more expensive. Electrostatic focusing used electronics to focus the electron beam, together with focusing magnets around the neck of the CRT for fine focusing adjustments. This type of focusing degraded over time. Electromagnetic focusing was introduced in the early 1990s and included an electromagnetic focusing coil in addition to the already existing focusing magnets. Electromagnetic focusing was much more stable over the lifetime of the CRT, retaining 95% of its sharpness by the end of life of the CRT.[459]
Beam-index tube
Beam-index tubes, also known as Uniray, Apple CRT or Indextron,[460] was an attempt in the 1950s by Philco to create a color CRT without a shadow mask, eliminating convergence and purity problems, and allowing for shallower CRTs with higher deflection angles.[461] It also required a lower voltage power supply for the final anode since it did not use a shadow mask, which normally blocks around 80% of the electrons generated by the electron gun. The lack of a shadow mask also made it immune to the earth's magnetic field while also making degaussing unnecessary and increasing image brightness.[462] It was constructed similarly to a monochrome CRT, with an aquadag outer coating, an aluminum inner coating, and a single electron gun but with a screen with an alternating pattern of red, green, blue and UV (index) phosphor stripes (similarly to a Trinitron) with a side mounted photomultiplier tube[463][462] or photodiode pointed towards the rear of the screen and mounted on the funnel of CRT, to track the electron beam to activate the phosphors separately from one another using the same electron beam. Only the index phosphor stripe was used for tracking, and it was the only phosphor that was not covered by an aluminum layer.[320] It was shelved because of the precision required to produce it.[464][465] It was revived by Sony in the 1980s as the Indextron but its adoption was limited, at least in part due to the development of LCD displays. Beam-index CRTs also suffered from poor contrast ratios of only around 50:1 since some light emission by the phosphors was required at all times by the photodiodes to track the electron beam. It allowed for single CRT color CRT projectors due to a lack of shadow mask; normally CRT projectors use three CRTs, one for each color,[466] since a lot of heat is generated due to the high anode voltage and beam current, making a shadow mask impractical and inefficient since it would warp under the heat produced (shadow masks absorb most of the electron beam, and, hence, most of the energy carried by the relativistic electrons); the three CRTs meant that an involved calibration and adjustment procedure[467] had to be carried out during installation of the projector, and moving the projector would require it to be recalibrated. A single CRT meant the need for calibration was eliminated, but brightness was decreased since the CRT screen had to be used for three colors instead of each color having its own CRT screen.[460] A stripe pattern also imposes a horizontal resolution limit; in contrast, three-screen CRT projectors have no theoretical resolution limit, due to them having single, uniform phosphor coatings.
Flat CRTs
Flat CRTs are those with a flat screen. Despite having a flat screen, they may not be completely flat, especially on the inside, instead having a greatly increased curvature. A notable exception is the LG Flatron (made by LG.Philips Displays, later LP Displays) which is truly flat on the outside and inside, but has a bonded glass pane on the screen with a tensioned rim band to provide implosion protection. Such completely flat CRTs were first introduced by Zenith in 1986, and used flat tensioned shadow masks, where the shadow mask is held under tension, providing increased resistance to blooming.[468][469][470][251][342][471] Flat CRTs have a number of challenges, like deflection. Vertical deflection boosters are required to increase the amount of current that is sent to the vertical deflection coils to compensate for the reduced curvature.[278] The CRTs used in the Sinclair TV80, and in many Sony Watchmans were flat in that they were not deep and their front screens were flat, but their electron guns were put to a side of the screen.[472][473] The TV80 used electrostatic deflection[474] while the Watchman used magnetic deflection with a phosphor screen that was curved inwards. Similar CRTs were used in video door bells.[475]
The side of a Sony Watchman monochrome CRT. One of the pairs of deflection coils is easily noticeable.
Radar CRTs
Radar CRTs such as the 7JP4 had a circular screen and scanned the beam from the center outwards. The deflection yoke rotated, causing the beam to rotate in a circular fashion.[476] The screen often had two colors, often a bright short persistence color that only appeared as the beam scanned the display and a long persistence phosphor afterglow. When the beam strikes the phosphor, the phosphor brightly illuminates, and when the beam leaves, the dimmer long persistence afterglow would remain lit where the beam struck the phosphor, alongside the radar targets that were "written" by the beam, until the beam re-struck the phosphor.[477][478]
Oscilloscope CRTs
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In oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs. The beam is deflected horizontally by applying an electric field between a pair of plates to its left and right, and vertically by applying an electric field to plates above and below. Televisions use magnetic rather than electrostatic deflection because the deflection plates obstruct the beam when the deflection angle is as large as is required for tubes that are relatively short for their size. Some Oscilloscope CRTs incorporate post deflection anodes (PDAs) that are spiral-shaped to ensure even anode potential across the CRT and operate at up to 15,000 volts. In PDA CRTs the electron beam is deflected before it is accelerated, improving sensitivity and legibility, specially when analyzing voltage pulses with short duty cycles.[479][155][480]
Microchannel plate
When displaying fast one-shot events, the electron beam must deflect very quickly, with few electrons impinging on the screen, leading to a faint or invisible image on the display. Oscilloscope CRTs designed for very fast signals can give a brighter display by passing the electron beam through a micro-channel plate just before it reaches the screen. Through the phenomenon of secondary emission, this plate multiplies the number of electrons reaching the phosphor screen, giving a significant improvement in writing rate (brightness) and improved sensitivity and spot size as well.[481][482]
Graticules
Most oscilloscopes have a graticule as part of the visual display, to facilitate measurements. The graticule may be permanently marked inside the face of the CRT, or it may be a transparent external plate made of glass or acrylic plastic. An internal graticule eliminates parallax error, but cannot be changed to accommodate different types of measurements.[483] Oscilloscopes commonly provide a means for the graticule to be illuminated from the side, which improves its visibility.[484]
Image storage tubes
These are found in analog phosphor storage oscilloscopes. These are distinct from digital storage oscilloscopes which rely on solid state digital memory to store the image.
Where a single brief event is monitored by an oscilloscope, such an event will be displayed by a conventional tube only while it actually occurs. The use of a long persistence phosphor may allow the image to be observed after the event, but only for a few seconds at best. This limitation can be overcome by the use of a direct view storage cathode-ray tube (storage tube). A storage tube will continue to display the event after it has occurred until such time as it is erased. A storage tube is similar to a conventional tube except that it is equipped with a metal grid coated with a dielectric layer located immediately behind the phosphor screen. An externally applied voltage to the mesh initially ensures that the whole mesh is at a constant potential. This mesh is constantly exposed to a low velocity electron beam from a 'flood gun' which operates independently of the main gun. This flood gun is not deflected like the main gun but constantly 'illuminates' the whole of the storage mesh. The initial charge on the storage mesh is such as to repel the electrons from the flood gun which are prevented from striking the phosphor screen.
When the main electron gun writes an image to the screen, the energy in the main beam is sufficient to create a 'potential relief' on the storage mesh. The areas where this relief is created no longer repel the electrons from the flood gun which now pass through the mesh and illuminate the phosphor screen. Consequently, the image that was briefly traced out by the main gun continues to be displayed after it has occurred. The image can be 'erased' by resupplying the external voltage to the mesh restoring its constant potential. The time for which the image can be displayed was limited because, in practice, the flood gun slowly neutralises the charge on the storage mesh. One way of allowing the image to be retained for longer is temporarily to turn off the flood gun. It is then possible for the image to be retained for several days. The majority of storage tubes allow for a lower voltage to be applied to the storage mesh which slowly restores the initial charge state. By varying this voltage a variable persistence is obtained. Turning off the flood gun and the voltage supply to the storage mesh allows such a tube to operate as a conventional oscilloscope tube.[485]
Vector monitors
Vector monitors were used in early computer aided design systems[486] and are in some late-1970s to mid-1980s arcade games such as Asteroids.[487] They draw graphics point-to-point, rather than scanning a raster. Either monochrome or color CRTs can be used in vector displays, and the essential principles of CRT design and operation are the same for either type of display; the main difference is in the beam deflection patterns and circuits.
Data storage tubes
The Williams tube or Williams-Kilburn tube was a cathode-ray tube used to electronically store binary data. It was used in computers of the 1940s as a random-access digital storage device. In contrast to other CRTs in this article, the Williams tube was not a display device, and in fact could not be viewed since a metal plate covered its screen.
Cat's eye
In some vacuum tube radio sets, a "Magic Eye" or "Tuning Eye" tube was provided to assist in tuning the receiver. Tuning would be adjusted until the width of a radial shadow was minimized. This was used instead of a more expensive electromechanical meter, which later came to be used on higher-end tuners when transistor sets lacked the high voltage required to drive the device.[488] The same type of device was used with tape recorders as a recording level meter, and for various other applications including electrical test equipment.
Charactrons
Some displays for early computers (those that needed to display more text than was practical using vectors, or that required high speed for photographic output) used Charactron CRTs. These incorporate a perforated metal character mask (stencil), which shapes a wide electron beam to form a character on the screen. The system selects a character on the mask using one set of deflection circuits, but that causes the extruded beam to be aimed off-axis, so a second set of deflection plates has to re-aim the beam so it is headed toward the center of the screen. A third set of plates places the character wherever required. The beam is unblanked (turned on) briefly to draw the character at that position. Graphics could be drawn by selecting the position on the mask corresponding to the code for a space (in practice, they were simply not drawn), which had a small round hole in the center; this effectively disabled the character mask, and the system reverted to regular vector behavior. Charactrons had exceptionally long necks, because of the need for three deflection systems.[489][490]
Nimo
Nimo was the trademark of a family of small specialised CRTs manufactured by Industrial Electronic Engineers. These had 10 electron guns which produced electron beams in the form of digits in a manner similar to that of the charactron. The tubes were either simple single-digit displays or more complex 4- or 6- digit displays produced by means of a suitable magnetic deflection system. Having little of the complexities of a standard CRT, the tube required a relatively simple driving circuit, and as the image was projected on the glass face, it provided a much wider viewing angle than competitive types (e.g., nixie tubes).[491] However, their requirement for several voltages and their high voltage made them uncommon.
Flood-beam CRT
Flood-beam CRTs are small tubes that are arranged as pixels for large video walls like Jumbotrons. The first screen using this technology (called Diamond Vision by Mitsubishi Electric) was introduced by Mitsubishi Electric for the 1980 Major League Baseball All-Star Game. It differs from a normal CRT in that the electron gun within does not produce a focused controllable beam. Instead, electrons are sprayed in a wide cone across the entire front of the phosphor screen, basically making each unit act as a single light bulb.[492] Each one is coated with a red, green or blue phosphor, to make up the color sub-pixels. This technology has largely been replaced with light-emitting diode displays. Unfocused and undeflected CRTs were used as grid-controlled stroboscope lamps since 1958.[493] Electron-stimulated luminescence (ESL) lamps, which use the same operating principle, were released in 2011.[494]
Print-head CRT
CRTs with an unphosphored front glass but with fine wires embedded in it were used as electrostatic print heads in the 1960s. The wires would pass the electron beam current through the glass onto a sheet of paper where the desired content was therefore deposited as an electrical charge pattern. The paper was then passed near a pool of liquid ink with the opposite charge. The charged areas of the paper attract the ink and thus form the image.[495][496]
Zeus – thin CRT display
In the late 1990s and early 2000s Philips Research Laboratories experimented with a type of thin CRT known as the Zeus display, which contained CRT-like functionality in a flat-panel display.[497][498][499][500][501] The devices were demonstrated but never marketed.
Slimmer CRT
Some CRT manufacturers, both LG.Philips Displays (later LP Displays) and Samsung SDI, innovated CRT technology by creating a slimmer tube. Slimmer CRT had the trade names Superslim,[502] Ultraslim,[503] Vixlim (by Samsung)[504] and Cybertube and Cybertube+ (both by LG Philips displays).[505][506] A 21-inch (53 cm) flat CRT has a 447.2-millimetre (17.61 in) depth. The depth of Superslim was 352 millimetres (13.86 in)[507] and Ultraslim was 295.7 millimetres (11.64 in).[508]
Health concerns
Ionizing radiation
CRTs can emit a small amount of X-ray radiation; this is a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors, which produces bremsstrahlung (braking radiation) as the high-energy electrons are decelerated. The amount of radiation escaping the front of the monitor is widely considered to be not harmful. The Food and Drug Administration regulations in 21 CFR 1020.10 are used to strictly limit, for instance, television receivers to 0.5 milliroentgens per hour at a distance of 5 cm (2 in) from any external surface; since 2007, most CRTs have emissions that fall well below this limit.[509] Note that the roentgen is an outdated unit and does not account for dose absorption. The conversion rate is about .877 roentgen per rem.[510] Assuming that the viewer absorbed the entire dose (which is unlikely), and that they watched TV for 2 hours a day, a .5 milliroentgen hourly dose would increase the viewers yearly dose by 320 millirem. For comparison, the average background radiation in the United States is 310 millirem a year. Negative effects of chronic radiation are not generally noticeable until doses over 20,000 millirem.[511]
The density of the x-rays that would be generated by a CRT is low because the raster scan of a typical CRT distributes the energy of the electron beam across the entire screen. Voltages above 15,000 volts are enough to generate "soft" x-rays. However, since CRTs may stay on for several hours at a time, the amount of x-rays generated by the CRT may become significant, hence the importance of using materials to shield against x-rays, such as the thick leaded glass and barium-strontium glass used in CRTs.[135]
Concerns about x-rays emitted by CRTs began in 1967 when it was found that TV sets made by General Electric were emitting "X-radiation in excess of desirable levels". It was later found that TV sets from all manufacturers were also emitting radiation. This caused television industry representatives to be brought before a U.S. congressional committee, which later proposed a federal radiation regulation bill, which became the 1968 Radiation Control for Health and Safety Act. It was recommended to TV set owners to always be at a distance of at least 6 feet from the screen of the TV set, and to avoid "prolonged exposure" at the sides, rear or underneath a TV set. It was discovered that most of the radiation was directed downwards. Owners were also told to not modify their set's internals to avoid exposure to radiation. Headlines about "radioactive" TV sets continued until the end of the 1960s. There once was a proposal by two New York congressmen that would have forced TV set manufacturers to "go into homes to test all of the nation's 15 million color sets and to install radiation devices in them". The FDA eventually began regulating radiation emissions from all electronic products in the US.[512]
Toxicity
Older color and monochrome CRTs may have been manufactured with toxic substances, such as cadmium, in the phosphors.[50][513][514][515] The rear glass tube of modern CRTs may be made from leaded glass, which represent an environmental hazard if disposed of improperly.[516] Since 1970, glass in the front panel (the viewable portion of the CRT) used strontium oxide rather than lead, though the rear of the CRT was still produced from leaded glass. Monochrome CRTs typically do not contain enough leaded glass to fail EPA TCLP tests. While the TCLP process grinds the glass into fine particles in order to expose them to weak acids to test for leachate, intact CRT glass does not leach (The lead is vitrified, contained inside the glass itself, similar to leaded glass crystalware).
Flicker
At low refresh rates (60 Hz and below), the periodic scanning of the display may produce a flicker that some people perceive more easily than others, especially when viewed with peripheral vision. Flicker is commonly associated with CRT as most televisions run at 50 Hz (PAL) or 60 Hz (NTSC), although there are some 100 Hz PAL televisions that are flicker-free. Typically only low-end monitors run at such low frequencies, with most computer monitors supporting at least 75 Hz and high-end monitors capable of 100 Hz or more to eliminate any perception of flicker.[517] Though the 100 Hz PAL was often achieved using interleaved scanning, dividing the circuit and scan into two beams of 50 Hz. Non-computer CRTs or CRT for sonar or radar may have long persistence phosphor and are thus flicker free. If the persistence is too long on a video display, moving images will be blurred.
High-frequency audible noise
50 Hz/60 Hz CRTs used for television operate with horizontal scanning frequencies of 15,750 and 15,734.25 Hz (for NTSC systems) or 15,625 Hz (for PAL systems).[518] These frequencies are at the upper range of human hearing and are inaudible to many people; however, some people (especially children) will perceive a high-pitched tone near an operating CRT television.[519] The sound is due to magnetostriction in the magnetic core and periodic movement of windings of the flyback transformer[520] but the sound can also be created by movement of the deflection coils, yoke or ferrite beads.[521]
This problem does not occur on 100/120 Hz TVs and on non-CGA (Color Graphics Adapter) computer displays, because they use much higher horizontal scanning frequencies that produce sound which is inaudible to humans (22 kHz to over 100 kHz).
Implosion
High vacuum inside glass-walled cathode-ray tubes permits electron beams to fly freely—without colliding into molecules of air or other gas. If the glass is damaged, atmospheric pressure can collapse the vacuum tube into dangerous fragments which accelerate inward and then spray at high speed in all directions. Although modern cathode-ray tubes used in televisions and computer displays have epoxy-bonded face-plates or other measures to prevent shattering of the envelope, CRTs must be handled carefully to avoid personal injury.[522]
Implosion protection
Early CRTs had a glass plate over the screen that was bonded to it using glue,[139] creating a laminated glass screen: initially the glue was polyvinyl acetate (PVA),[523] while later versions such as the LG Flatron used a resin, perhaps a UV-curable resin.[524][342] The PVA degrades over time creating a "cataract", a ring of degraded glue around the edges of the CRT that does not allow light from the screen to pass through.[523] Later CRTs instead use a tensioned metal rim band mounted around the perimeter that also provides mounting points for the CRT to be mounted to a housing.[372] In a 19-inch CRT, the tensile stress in the rim band is 70 kg/cm².[525] Older CRTs were mounted to the TV set using a frame. The band is tensioned by heating it, then mounting it on the CRT; the band cools afterwards, shrinking in size and putting the glass under compression,[526][139][527] which strengthens the glass and reduces the necessary thickness (and hence weight) of the glass. This makes the band an integral component that should never be removed from an intact CRT that still has a vacuum; attempting to remove it may cause the CRT to implode.[317] The rim band prevents the CRT from imploding should the screen be broken. The rim band may be glued to the perimeter of the CRT using epoxy, preventing cracks from spreading beyond the screen and into the funnel.[528][527]
Electric shock
To accelerate the electrons from the cathode to the screen with enough energy[529] to achieve sufficient image brightness, a very high voltage (EHT or extra-high tension) is required,[530] from a few thousand volts for a small oscilloscope CRT to tens of thousands for a larger screen color TV. This is many times greater than household power supply voltage. Even after the power supply is turned off, some associated capacitors and the CRT itself may retain a charge for some time and therefore dissipate that charge suddenly through a ground such as an inattentive human grounding a capacitor discharge lead. An average monochrome CRT may use 1 to 1.5 kV of anode voltage per inch.[531][271]
Security concerns
Under some circumstances, the signal radiated from the electron guns, scanning circuitry, and associated wiring of a CRT can be captured remotely and used to reconstruct what is shown on the CRT using a process called Van Eck phreaking.[532] Special TEMPEST shielding can mitigate this effect. Such radiation of a potentially exploitable signal, however, occurs also with other display technologies[533] and with electronics in general.[citation needed]
Recycling
Due to the toxins contained in CRT monitors the United States Environmental Protection Agency created rules (in October 2001) stating that CRTs must be brought to special e-waste recycling facilities. In November 2002, the EPA began fining companies that disposed of CRTs through landfills or incineration. Regulatory agencies, local and statewide, monitor the disposal of CRTs and other computer equipment.[534]
As electronic waste, CRTs are considered one of the hardest types to recycle.[535] CRTs have relatively high concentration of lead and phosphors, both of which are necessary for the display. There are several companies in the United States that charge a small fee to collect CRTs, then subsidize their labor by selling the harvested copper, wire, and printed circuit boards. The United States Environmental Protection Agency (EPA) includes discarded CRT monitors in its category of "hazardous household waste"[536] but considers CRTs that have been set aside for testing to be commodities if they are not discarded, speculatively accumulated, or left unprotected from weather and other damage.[537]
Various states participate in the recycling of CRTs, each with their reporting requirements for collectors and recycling facilities. For example, in California the recycling of CRTs is governed by CALRecycle, the California Department of Resources Recycling and Recovery through their Payment System.[538] Recycling facilities that accept CRT devices from business and residential sector must obtain contact information such as address and phone number to ensure the CRTs come from a California source in order to participate in the CRT Recycling Payment System.
In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive.[539]
Multiple methods have been proposed for the recycling of CRT glass. The methods involve thermal, mechanical and chemical processes.[540][541][542][543] All proposed methods remove the lead oxide content from the glass. Some companies operated furnaces to separate the lead from the glass.[544] A coalition called the Recytube project was once formed by several European companies to devise a method to recycle CRTs.[6] The phosphors used in CRTs often contain rare earth metals.[545][546][547][307] A CRT contains about 7g of phosphor.[548]
The funnel can be separated from the screen of the CRT using laser cutting, diamond saws or wires or using a resistively heated nichrome wire.[549][550][551][552][553]
Leaded CRT glass was sold to be remelted into other CRTs,[76] or even broken down and used in road construction or used in tiles,[554][555] concrete, concrete and cement bricks,[556] fiberglass insulation or used as flux in metals smelting.[557][558]
A considerable portion of CRT glass is landfilled, where it can pollute the surrounding environment.[6] It is more common for CRT glass to be disposed of than being recycled.[559]
See also
Applying CRT in different display-purpose:
Historical aspects:
- Direct-view bistable storage tube
- Flat-panel display
- Geer tube
- History of display technology
- Image dissector
- LCD television, LED-backlit LCD, LED display
- Penetron
- Surface-conduction electron-emitter display
- Trinitron
Safety and precautions:
References
Evidence for the existence of "cathode-rays" was first found by Plücker and Hittorf ...
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24:TV viewers of the 1970s will see their programs on sets quite different from today's, if designs now being worked out are developed. At the Home Furnishings Market in Chicago, Illinois, on June 21, 1961, a thin TV screen is a feature of this design model. Another feature is an automatic timing device which would record TV programs during the viewers' absence to be played back later. The 32x22-inch color screen is four inches thick.
- Mehmet Ali Recai Önal (1 December 2018). "Recovering rare earths from old TVs and computer screens". Solvomet.
- "California CRT glass heads to disposal sites amid downstream challenges". 22 September 2016.
Selected patents
- U.S. Patent 1,691,324: Zworykin Television System
External links
- "CRTs". Virtual Valve Museum. Archived from the original on 10 October 2011. Retrieved 31 December 2006.
- Goldwasser, Samuel M. (28 February 2006). "TV and Monitor CRT (Picture Tube) Information". repairfaq.org. Archived from the original on 26 September 2006.
- "The Cathode Ray Tube site". crtsite.com.
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https://en.wikipedia.org/wiki/Cathode-ray_tube
A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Plasma televisions were the first large (over 32 inches diagonal) flat panel displays to be released to the public.
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Until about 2007, plasma displays were commonly used in large televisions. By 2013, they had lost nearly all market share due to competition from low-cost LCDs and more expensive but high-contrast OLED flat-panel displays. Manufacturing of plasma displays for the United States retail market ended in 2014,[1][2] and manufacturing for the Chinese market ended in 2016.[3][4] Plasma displays are obsolete, having been superseded in most if not all aspects by OLED displays.[5]
https://en.wikipedia.org/wiki/Plasma_display
An LED-backlit LCD is a liquid-crystal display that uses LEDs for backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting.[1] LED-backlit displays use the same TFT LCD (thin-film-transistor liquid-crystal display) technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.
While not an LED display, a television using such a combination of an LED backlight with an LCD panel is advertised as an LED TV by some manufacturers and suppliers.[1][2]
https://en.wikipedia.org/wiki/LED-backlit_LCD
A thin-film-transistor liquid-crystal display (TFT LCD) is a variant of a liquid-crystal display that uses thin-film-transistor technology[1] to improve image qualities such as addressability and contrast. A TFT LCD is an active matrix LCD, in contrast to passive matrix LCDs or simple, direct-driven (i.e. with segments directly connected to electronics outside the LCD) LCDs with a few segments.
TFT LCDs are used in appliances including television sets, computer monitors, mobile phones, handheld devices, video game systems, personal digital assistants, navigation systems, projectors,[2] and dashboards in automobiles.
https://en.wikipedia.org/wiki/TFT_LCD
Lamps
Cold-cathode lamps include cold-cathode fluorescent lamps (CCFLs) and neon lamps. Neon lamps primarily rely on excitation of gas molecules to emit light; CCFLs use a discharge in mercury vapor to develop ultraviolet light, which in turn causes a fluorescent coating on the inside of the lamp to emit visible light.
Cold-cathode fluorescent lamps were used for backlighting of LCDs, for example computer monitors and television screens.
In the lighting industry, “cold cathode” historically refers to luminous tubing larger than 20 mm in diameter and operating on a current of 120 to 240 milliamperes. This larger-diameter tubing is often used for interior alcove and general lighting.[3][4] The term "neon lamp" refers to tubing that is smaller than 15 mm in diameter[citation needed] and typically operates at approximately 40 milliamperes. These lamps are commonly used for neon signs.
https://en.wikipedia.org/wiki/Cold_cathode#Lamps
Details
The cathode is the negative electrode. Any gas-discharge lamp has a positive (anode) and a negative electrode. Both electrodes alternate between acting as an anode and a cathode when these devices run with alternating current.
A cold cathode is distinguished from a hot cathode that is heated to induce thermionic emission of electrons. Discharge tubes with hot cathodes have an envelope filled with low-pressure gas and containing two electrodes. Hot cathode devices include common vacuum tubes, fluorescent lamps, high-pressure discharge lamps and vacuum fluorescent displays.
https://en.wikipedia.org/wiki/Cold_cathode#Lamps
A vacuum fluorescent display (VFD) is a display device once commonly used on consumer electronics equipment such as video cassette recorders, car radios, and microwave ovens.
A VFD operates on the principle of cathodoluminescence, roughly similar to a cathode ray tube, but operating at much lower voltages. Each tube in a VFD has a phosphor-coated carbon anode that is bombarded by electrons emitted from the cathode filament.[1][2] In fact, each tube in a VFD is a triode vacuum tube because it also has a mesh control grid.[3]
Unlike liquid crystal displays, a VFD emits very bright light with high contrast and can support display elements of various colors. Standard illumination figures for VFDs are around 640 cd/m2 with high-brightness VFDs operating at 4,000 cd/m2, and experimental units as high as 35,000 cd/m2 depending on the drive voltage and its timing.[3] The choice of color (which determines the nature of the phosphor) and display brightness significantly affect the lifetime of the tubes, which can range from as low as 1,500 hours for a vivid red VFD to 30,000 hours for the more common green ones.[3] Cadmium was commonly used in the phosphors of VFDs in the past, but the current RoHS-compliant VFDs have eliminated this metal from their construction, using instead phosphors consisting of a matrix of alkaline earth and very small amounts of group III metals, doped with very small amounts of rare earth metals.[4]
VFDs can display seven-segment numerals, multi-segment alpha-numeric characters or can be made in a dot-matrix to display different alphanumeric characters and symbols. In practice, there is little limit to the shape of the image that can be displayed: it depends solely on the shape of phosphor on the anode(s).
The first VFD was the single indication DM160 by Philips in 1959.[5] The first multi-segment VFD was a 1967 Japanese single-digit, seven-segment device. The displays became common on calculators and other consumer electronics devices.[6] In the late 1980s hundreds of millions of units were made yearly.[7]
Design
The device consists of a hot cathode (filaments), grids and anodes (phosphor) encased in a glass envelope under a high vacuum condition. The cathode is made up of fine tungsten wires, coated by alkaline earth metal oxides (barium,[2] strontium and calcium oxides[8][9]), which emit electrons when heated to 650 °C[2] by an electric current. These electrons are controlled and diffused by the grids (made using Photochemical machining), which are made up of thin (50 micron thick) stainless steel.[2] If electrons impinge on the phosphor-coated anode plates, they fluoresce, emitting light. Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much lower temperatures, and are therefore essentially invisible.[10] The anode consists of a glass plate with electrically conductive traces (each trace is connected to a single indicator segment), which is coated with an insulator, which is then partially etched to create holes which are then filled with a conductor like graphite, which in turn is coated with phosphor. This transfers energy from the trace to the segment. The shape of the phosphor will determine the shape of the VFD's segments. The most widely used phosphor is Zinc-doped copper-activated Zinc oxide,[2] which generates light at a peak wavelength of 505 nm.
The cathode wire to which the oxides are applied is made of tungsten or ruthenium-tungsten alloy. The oxides in the cathodes are not stable in air, so they are applied to the cathode as carbonates, the cathodes are assembled into the VFD, and the cathodes are heated by passing a current through them while inside the vacuum of the VFD to convert the carbonates into oxides.[2][9]
The principle of operation is identical to that of a vacuum tube triode. Electrons can only reach (and "illuminate") a given plate element if both the grid and the plate are at a positive potential with respect to the cathode.[11] This allows the displays to be organized as multiplexed displays where the multiple grids and plates form a matrix, minimizing the number of signal pins required. In the example of the VCR display shown to the right, the grids are arranged so that only one digit is illuminated at a time. All of the similar plates in all of the digits (for example, all of the lower-left plates in all of the digits) are connected in parallel. One by one, the microprocessor driving the display enables a digit by placing a positive voltage on that digit's grid and then placing a positive voltage on the appropriate plates. Electrons flow through that digit's grid and strike those plates that are at a positive potential. The microprocessor cycles through illuminating the digits in this way at a rate high enough to create the illusion of all digits glowing at once via persistence of vision.
The extra indicators (in our example, "VCR", "Hi-Fi", "STEREO", "SAP", etc.) are arranged as if they were segments of an additional digit or two or extra segments of existing digits and are scanned using the same multiplexed strategy as the real digits. Some of these extra indicators may use a phosphor that emits a different color of light, for example, orange.
The light emitted by most VFDs contains many colors and can often be filtered to enhance the color saturation providing a deep green or deep blue, depending on the whims of the product's designers. Phosphors used in VFDs are different from those in cathode-ray displays since they must emit acceptable brightness with only around 50 volts of electron energy, compared to several thousand volts in a CRT.[12] The insulating layer in a VFD is normally black, however it can be removed or made transparent to allow the display to be transparent. AMVFD displays that incorporate a driver IC are available for applications that require high image brightness and an increased number of pixels. Phosphors of different colors can be stacked on top of each other for achieving gradations and various color combinations. Hybrid VFDs include both fixed display segments and a graphic VFD in the same unit. VFDs may have display segments, grids and related circuitry on their front and rear plass panels, using a central cathode for both panels, allowing for increased segment density. The segments can also be placed exclusively on the front instead of on the back, improving viewing angles and brightness.[13][14][15][16][17][18][19][20][21]
The Russian IV-15 VFD tube is very similar to the DM160. The DM160, DM70/DM71 and Russian IV-15 can (like a VFD panel) be used as triodes. The DM160 is thus the smallest VFD and smallest triode valve. The IV-15 is slightly different shape (see photo of DM160 and IV-15 for comparison).
https://en.wikipedia.org/wiki/Vacuum_fluorescent_display
Cold-cathode fluorescent lamps
Most fluorescent lamps use electrodes that emit electrons into the tube by heat, known as hot cathodes. However, cold cathode tubes have cathodes that emit electrons only due to the large voltage between the electrodes. The cathodes will be warmed by current flowing through them, but are not hot enough for significant thermionic emission. Because cold cathode lamps have no thermionic emission coating to wear out, they can have much longer lives than hot cathode tubes. This makes them desirable for long-life applications (such as backlights in liquid crystal displays). Sputtering of the electrode may still occur, but electrodes can be shaped (e.g. into an internal cylinder) to capture most of the sputtered material so it is not lost from the electrode.
Cold cathode lamps are generally less efficient than thermionic emission lamps because the cathode fall voltage is much higher. Power dissipated due to cathode fall voltage does not contribute to light output. However, this is less significant with longer tubes. The increased power dissipation at tube ends also usually means cold cathode tubes have to be run at a lower loading than their thermionic emission equivalents. Given the higher tube voltage required anyway, these tubes can easily be made long, and even run as series strings. They are better suited for bending into special shapes for lettering and signage, and can also be instantly switched on or off.
Starting
The gas used in the fluorescent tube must be ionized before the arc can "strike" . For small lamps, it does not take much voltage to strike the arc and starting the lamp presents no problem, but larger tubes require a substantial voltage (in the range of a thousand volts). Many different starting circuits have been used. The choice of circuit is based on cost, AC voltage, tube length, instant versus non-instant starting, temperature ranges and parts availability.
Preheating
Preheating, also called switchstart, uses a combination filament–cathode at each end of the lamp in conjunction with a mechanical or automatic (bi-metallic) switch (see circuit diagram to the right) that initially connect the filaments in series with the ballast to preheat them; after a short preheating time the starting switch opens. If timed correctly relative to the phase of the supply AC, this causes the ballast to induce a voltage over the tube high enough to initiate the starting arc.[35] These systems are standard equipment in 200–240 V countries (and in the United States lamps up to about 30 watts).
Before the 1960s, four-pin thermal starters and manual switches were used.[citation needed] A glow switch starter automatically preheats the lamp cathodes. It consists of a normally open bi-metallic switch in a small sealed gas-discharge lamp containing inert gas (neon or argon). The glow switch will cyclically warm the filaments and initiate a pulse voltage to strike the arc; the process repeats until the lamp is lit. Once the tube strikes, the impinging main discharge keeps the cathodes hot, permitting continued electron emission. The starter switch does not close again because the voltage across the lit tube is insufficient to start a glow discharge in the starter.[35]
With glow switch starters a failing tube will cycle repeatedly. Some starter systems used a thermal over-current trip to detect repeated starting attempts and disable the circuit until manually reset.
A power factor correction (PFC) capacitor draws leading current from the mains to compensate for the lagging current drawn by the lamp circuit.[35]
Electronic starters use a different method to preheat the cathodes.[36] They may be plug-in interchangeable with glow starters. They use a semiconductor switch and "soft start" the lamp by preheating the cathodes before applying a starting pulse which strikes the lamp first time without flickering; this dislodges a minimal amount of material from the cathodes during starting, giving longer lamp life.[35] This is claimed to prolong lamp life by a factor of typically 3 to 4 times for a lamp frequently switched on as in domestic use,[37] and to reduce the blackening of the ends of the lamp typical of fluorescent tubes. While the circuit is complex, the complexity is built into an integrated circuit chip. Electronic starters may be optimized for fast starting (typical start time of 0.3 seconds),[37][38] or for most reliable starting even at low temperatures and with low supply voltages, with a startup time of 2–4 seconds.[39] The faster-start units may produce audible noise during start-up.[40]
Electronic starters only attempt to start a lamp for a short time when power is initially applied, and do not repeatedly attempt to restrike a lamp that is dead and unable to sustain an arc; some automatically stop trying to start a failed lamp.[36] This eliminates the re-striking of a lamp and the continuous flashing of a failing lamp with a glow starter. Electronic starters are not subject to wear and do not need replacing periodically, although they may fail like any other electronic circuit. Manufacturers typically quote lives of 20 years, or as long as the light fitting.[38][39]
Instant start
Instant start fluorescent tubes were invented in 1944. Instant start simply uses a high enough voltage to break down the gas column and thereby start arc conduction. Once the high-voltage spark "strikes" the arc, the current is boosted until a glow discharge forms. As the lamp warms and pressure increases, the current continues to rise and both resistance and voltage falls, until mains or line-voltage takes over and the discharge becomes an arc. These tubes have no filaments and can be identified by a single pin at each end of the tube (for common lamps; compact cold-cathode lamps may also have a single pin, but operate from a transformer rather than a ballast). The lamp holders have a "disconnect" socket at the low-voltage end which disconnects the ballast when the tube is removed, to prevent electric shock. Instant-start lamps are slightly more energy efficient than rapid start, because they do not constantly send a heating current to the cathodes during operation, but the cold cathodes starting increases sputter, and they take much longer to transition from a glow discharge to an arc during warm up, thus the lifespan is typically about half of those seen in comparable rapid-start lamps.[41]
Rapid start
Because the formation of an arc requires the thermionic emission of large quantities of electrons from the cathode, rapid start ballast designs provide windings within the ballast that continuously warm the cathode filaments. Usually operating at a lower arc voltage than the instant start design; no inductive voltage spike is produced for starting, so the lamps must be mounted near a grounded (earthed) reflector to allow the glow discharge to propagate through the tube and initiate the arc discharge via capacitive coupling. In some lamps a grounded "starting aid" strip is attached to the outside of the lamp glass. This ballast type is incompatible with the European energy saver T8 fluorescent lamps because these lamps requires a higher starting voltage than that of the open circuit voltage of rapid start ballasts.
Quick-start
Quick-start ballasts use a small auto-transformer to heat the filaments when power is first applied. When an arc strikes, the filament heating power is reduced and the tube will start within half a second. The auto-transformer is either combined with the ballast or may be a separate unit. Tubes need to be mounted near an earthed metal reflector in order for them to strike. Quick-start ballasts are more common in commercial installations because of lower maintenance costs. A quick-start ballast eliminates the need for a starter switch, a common source of lamp failures. Nonetheless, Quick-start ballasts are also used in domestic (residential) installations because of the desirable feature that a Quick-start ballast light turns on nearly immediately after power is applied (when a switch is turned on). Quick-start ballasts are used only on 240 V circuits and are designed for use with the older, less efficient T12 tubes.
Semi-resonant start
The semi-resonant start circuit was invented by Thorn Lighting for use with T12 fluorescent tubes. This method uses a double wound transformer and a capacitor. With no arc current, the transformer and capacitor resonate at line frequency and generate about twice the supply voltage across the tube, and a small electrode heating current.[42] This tube voltage is too low to strike the arc with cold electrodes, but as the electrodes heat up to thermionic emission temperature, the tube striking voltage falls below that of the ringing voltage, and the arc strikes. As the electrodes heat, the lamp slowly, over three to five seconds, reaches full brightness. As the arc current increases and tube voltage drops, the circuit provides current limiting.
Semi-resonant start circuits are mainly restricted to use in commercial installations because of the higher initial cost of circuit components. However, there are no starter switches to be replaced and cathode damage is reduced during starting making lamps last longer, reducing maintenance costs. Because of the high open circuit tube voltage, this starting method is particularly good for starting tubes in cold locations. Additionally, the circuit power factor is almost 1.0, and no additional power factor correction is needed in the lighting installation. As the design requires that twice the supply voltage must be lower than the cold-cathode striking voltage (or the tubes would erroneously instant-start), this design cannot be used with 240 volt AC power unless the tubes are at least 1.2 m (3 ft 11 in) length. Semi-resonant start fixtures are generally incompatible with energy saving T8 retrofit tubes, because such tubes have a higher starting voltage than T12 lamps and may not start reliably, especially in low temperatures. Recent proposals in some countries to phase out T12 tubes will reduce the application of this starting method.
Electronic ballasts
Electronic ballasts employ transistors to change the supply frequency into high-frequency AC while regulating the current flow in the lamp. These ballasts take advantage of the higher efficacy of lamps, which rises by almost 10% at 10 kHz, compared to efficacy at normal power frequency. When the AC period is shorter than the relaxation time to de-ionize mercury atoms in the discharge column, the discharge stays closer to optimum operating condition.[43] Electronic ballasts convert supply frequency AC power to variable frequency AC. The conversion can reduce lamp brightness modulation at twice the power supply frequency.
Low cost ballasts contain only a simple oscillator and series resonant LC circuit. This principle is called the current resonant inverter circuit. After a short time the voltage across the lamp reaches about 1 kV and the lamp instant-starts in cold cathode mode. The cathode filaments are still used for protection of the ballast from overheating if the lamp does not ignite. A few manufacturers use positive temperature coefficient (PTC) thermistors to disable instant starting and give some time to preheat the filaments.
More complex electronic ballasts use programmed start. The output frequency is started above the resonance frequency of the output circuit of the ballast; and after the filaments are heated, the frequency is rapidly decreased. If the frequency approaches the resonant frequency of the ballast, the output voltage will increase so much that the lamp will ignite. If the lamp does not ignite, an electronic circuit stops the operation of the ballast.
Many electronic ballasts are controlled by a microcontroller, and these are sometimes called digital ballasts. Digital ballasts can apply quite complex logic to lamp starting and operation. This enables functions such as testing for broken electrodes and missing tubes before attempting to start, detection of tube replacement, and detection of tube type, such that a single ballast can be used with several different tubes. Features such as dimming can be included in the embedded microcontroller software, and can be found in various manufacturers' products.
Since introduction in the 1990s, high-frequency ballasts have been used in general lighting fixtures with either rapid start or pre-heat lamps. These ballasts convert the incoming power to an output frequency in excess of 20 kHz. This increases lamp efficiency.[44] These ballasts operate with voltages that can be almost 600 volts, requiring some consideration in housing design, and can cause a minor limitation in the length of the wire leads from the ballast to the lamp ends.
End of life
The life expectancy of a fluorescent lamp is primarily limited by the life of the cathode electrodes. To sustain an adequate current level, the electrodes are coated with an emission mixture of metal oxides. Every time the lamp is started, and during operation, some small amount of the cathode coating is sputtered off the electrodes by the impact of electrons and heavy ions within the tube. The sputtered material collects on the walls of the tube, darkening it. The starting method and frequency affect cathode sputtering. A filament may also break, disabling the lamp.
Low-mercury designs of lamps may fail when mercury is absorbed by the glass tube, phosphor, and internal components, and is no longer available to vaporize in the fill gas. Loss of mercury initially causes an extended warm-up time to full light output, and finally causes the lamp to glow a dim pink when the argon gas takes over as the primary discharge.[45]
Subjecting the tube to asymmetric current flow, effectively operates it under a DC bias, and causes asymmetric distribution of mercury ions along the tube. The localized depletion of mercury vapor pressure manifests itself as pink luminescence of the base gas in the vicinity of one of the electrodes, and the operating lifetime of the lamp may be dramatically shortened. This can be an issue with some poorly designed inverters.[46]
The phosphors lining the lamp degrade with time as well, until a lamp no longer produces an acceptable fraction of its initial light output.
Failure of the integral electronic ballast of a compact fluorescent bulb will also end its usable life.
Phosphors and the spectrum of emitted light
The spectrum of light emitted from a fluorescent lamp is the combination of light directly emitted by the mercury vapor, and light emitted by the phosphorescent coating. The spectral lines from the mercury emission and the phosphorescence effect give a combined spectral distribution of light that is different from those produced by incandescent sources. The relative intensity of light emitted in each narrow band of wavelengths over the visible spectrum is in different proportions compared to that of an incandescent source. Colored objects are perceived differently under light sources with differing spectral distributions. For example, some people find the color rendition produced by some fluorescent lamps to be harsh and displeasing. A healthy person can sometimes appear to have an unhealthy skin tone under fluorescent lighting. The extent to which this phenomenon occurs is related to the light's spectral composition, and may be gauged by its color rendering index (CRI).
Color temperature
Correlated color temperature (CCT) is a measure of the "shade" of whiteness of a light source compared with a blackbody. Typical incandescent lighting is 2700 K, which is yellowish-white.[47] Halogen lighting is 3000 K.[48] Fluorescent lamps are manufactured to a chosen CCT by altering the mixture of phosphors inside the tube. Warm-white fluorescents have CCT of 2700 K and are popular for residential lighting. Neutral-white fluorescents have a CCT of 3000 K or 3500 K. Cool-white fluorescents have a CCT of 4100 K and are popular for office lighting. Daylight fluorescents have a CCT of 6500 K, which is bluish-white.
Color rendering index
Color rendering index (CRI) is a measure of how well colors can be perceived using light from a source, relative to light from a reference source such as daylight or a blackbody of the same color temperature. By definition, an incandescent lamp has a CRI of 100. Real-life fluorescent tubes achieve CRIs of anywhere from 50 to 98. Fluorescent lamps with low CRI have phosphors that emit too little red light. Skin appears less pink, and hence "unhealthy" compared with incandescent lighting. Colored objects appear muted. For example, a low CRI 6800 K halophosphate tube (an extreme example) will make reds appear dull red or even brown. Since the eye is relatively less efficient at detecting red light, an improvement in color rendering index, with increased energy in the red part of the spectrum, may reduce the overall luminous efficacy.[31]: 8
Lighting arrangements use fluorescent tubes in an assortment of tints of white. Mixing tube types within fittings can improve the color reproduction of lower quality tubes.
Phosphor composition
Some of the least pleasant light comes from tubes containing the older, halophosphate-type phosphors (chemical formula Ca5(PO4)3(F, Cl):Sb3+, Mn2+). This phosphor mainly emits yellow and blue light, and relatively little green and red. In the absence of a reference, this mixture appears white to the eye, but the light has an incomplete spectrum. The color rendering index (CRI) of such lamps is around 60.
Since the 1990s, higher-quality fluorescent lamps use triphosphor mixture, based on europium and terbium ions, which have emission bands more evenly distributed over the spectrum of visible light. Triphosphor tubes gives a more natural color reproduction to the human eye. The CRI of such lamps is typically 85.
| Typical fluorescent lamp with rare-earth phosphor |  |
A typical "cool white" fluorescent lamp utilizing two rare-earth-doped phosphors, Tb3+, Ce3+:LaPO4 for green and blue emission and Eu:Y2O3 for red. For an explanation of the origin of the individual peaks click on the image. Several of the spectral peaks are directly generated from the mercury arc. This is likely the most common type of fluorescent lamp in use today. |
| An older-style halophosphate-phosphor fluorescent lamp | .png) |
Halophosphate phosphors in these lamps usually consist of trivalent antimony- and divalent manganese-doped calcium halophosphate (Ca5(PO4)3(Cl, F):Sb3+, Mn2+). The color of the light output can be adjusted by altering the ratio of the blue-emitting antimony dopant and orange-emitting manganese dopant. The color rendering ability of these older-style lamps is quite poor. Halophosphate phosphors were invented by A. H. McKeag et al. in 1942. |
| "Natural sunshine" fluorescent light |  |
Peaks with stars are mercury lines. |
| Yellow fluorescent lights |  |
The spectrum is nearly identical to a normal fluorescent lamp except for a near total lack of light shorter than 500 nanometers. This effect can be achieved through either specialized phosphor use or more commonly by the use of a simple yellow light filter. These lamps are commonly used as lighting for photolithography work in cleanrooms and as "bug repellent" outdoor lighting (the efficacy of which is questionable). |
| Spectrum of a "blacklight" lamp |  |
There is typically only one phosphor present in a blacklight lamp, usually consisting of europium-doped strontium fluoroborate, which is contained in an envelope of Wood's glass. |
Applications
Fluorescent lamps come in many shapes and sizes.[49] The compact fluorescent lamp (CFL) is becoming more popular. Many compact fluorescent lamps integrate the auxiliary electronics into the base of the lamp, allowing them to fit into a regular light bulb socket.
In US residences, fluorescent lamps are mostly found in kitchens, basements, or garages, but schools and businesses find the cost savings of fluorescent lamps to be significant and rarely use incandescent lights. Electricity costs, tax incentives and building codes result in higher use in places such as California. Fluorescent use is declining as LED lighting, which is more energy efficient and doesn't contain mercury, is replacing fluorescents.[citation needed]
In other countries, residential use of fluorescent lighting varies depending on the price of energy, financial and environmental concerns of the local population, and acceptability of the light output. In East and Southeast Asia it is very rare to see incandescent bulbs in buildings anywhere.
Many countries are encouraging the phase-out of incandescent light bulbs and substitution of incandescent lamps with fluorescent lamps or LED and other types of energy-efficient lamps.
In addition to general lighting, special fluorescent lights are often used in stage lighting for film and video production. They are cooler than traditional halogen light sources, and use high-frequency ballasts to prevent video flickering and high color-rendition index lamps to approximate daylight color temperatures.
Comparison to incandescent lamps
Luminous efficacy
Fluorescent lamps convert more of the input power to visible light than incandescent lamps. A typical 100 watt tungsten filament incandescent lamp may convert only 5% of its power input to visible white light (400–700 nm wavelength), whereas typical fluorescent lamps convert about 22% of the power input to visible white light.[31]: 20
The efficacy of fluorescent tubes ranges from about 16 lumens per watt for a 4 watt tube with an ordinary ballast to over 100 lumens per watt[50] with a modern electronic ballast, commonly averaging 50 to 67 lm/W overall.[51] Ballast loss can be about 25% of the lamp power with magnetic ballasts, and around 10% with electronic ballasts.
Fluorescent lamp efficacy is dependent on lamp temperature at the coldest part of the lamp. In T8 lamps this is in the center of the tube. In T5 lamps this is at the end of the tube with the text stamped on it. The ideal temperature for a T8 lamp is 25 °C (77 °F) while the T5 lamp is ideally at 35 °C (95 °F).
Life
Typically a fluorescent lamp will last 10 to 20 times as long as an equivalent incandescent lamp when operated several hours at a time. Under standard test conditions fluorescent lamps last 6,000 to 80,000 hours (2 to 27 years at 8 hours per day).[52]
The higher initial cost of a fluorescent lamp compared with an incandescent lamp is usually compensated for by lower energy consumption over its life.[53][needs update]
Lower luminance
Compared with an incandescent lamp, a fluorescent tube is a more diffuse and physically larger light source. In suitably designed lamps, light can be more evenly distributed without point source of glare such as seen from an undiffused incandescent filament; the lamp is large compared to the typical distance between lamp and illuminated surfaces.
Lower heat
Fluorescent lamps give off about one-fifth the heat of equivalent incandescent lamps. This greatly reduces the size, cost, and energy consumption devoted to air conditioning for office buildings that would typically have many lights and few windows.
Disadvantages
Frequent switching
Frequent switching (more than every 3 hours) will shorten the life of lamps. [54] Each start cycle slightly erodes the electron-emitting surface of the cathodes; when all the emission material is gone, the lamp cannot start with the available ballast voltage. Fixtures for flashing lights (such as for advertising) use a ballast that maintains cathode temperature when the arc is off, preserving the life of the lamp.
The extra energy used to start a fluorescent lamp is equivalent to a few seconds of normal operation; it is more energy-efficient to switch off lamps when not required for several minutes.[55][56]
Mercury content
If a fluorescent lamp is broken, a very small amount of mercury can contaminate the surrounding environment. About 99% of the mercury is typically contained in the phosphor, especially on lamps that are near the end of their life.[57] Broken lamps may release mercury if not cleaned with correct methods.[58][failed verification]
Due to the mercury content, discarded fluorescent lamps must be treated as hazardous waste. For large users of fluorescent lamps, recycling services are available in some areas, and may be required by regulation.[59][60] In some areas, recycling is also available to consumers.[61]
Ultraviolet emission
Fluorescent lamps emit a small amount of ultraviolet (UV) light. A 1993 study in the US found that ultraviolet exposure from sitting under fluorescent lights for eight hours is equivalent to one minute of sun exposure.[62] Ultraviolet radiation from compact fluorescent lamps may exacerbate symptoms in photosensitive individuals.[63][64][65]
Museum artifacts may need protection from UV light to prevent degradation of pigments or textiles. [66]
Ballast
Fluorescent lamps require a ballast to stabilize the current through the lamp, and to provide the initial striking voltage required to start the arc discharge. Often one ballast is shared between two or more lamps. Electromagnetic ballasts can produce an audible humming or buzzing noise. In North America, magnetic ballasts are usually filled with a tar-like potting compound to reduce emitted noise. Hum is eliminated in lamps with a high-frequency electronic ballast. Energy lost in magnetic ballasts is around 10% of lamp input power according to GE literature from 1978.[31] Electronic ballasts reduce this loss.
Power quality and radio interference
Simple inductive fluorescent lamp ballasts have a power factor of less than unity. Inductive ballasts include power factor correction capacitors. Simple electronic ballasts may also have low power factor due to their rectifier input stage.
Fluorescent lamps are a non-linear load and generate harmonic currents in the electrical power supply. The arc within the lamp may generate radio frequency noise, which can be conducted through power wiring. Suppression of radio interference is possible. Very good suppression is possible, but adds to the cost of the fluorescent fixtures.
Fluorescent lamps near end of life can present a serious radio frequency interference hazard. Oscillations are generated from the negative differential resistance of the arc, and the current flow through the tube can form a tuned circuit whose frequency depends on path length. [67]
Operating temperature
Fluorescent lamps operate best around room temperature. At lower or higher temperatures, efficacy decreases. At below-freezing temperatures standard lamps may not start. Special lamps may be used for reliable service outdoors in cold weather.
Lamp shape
Fluorescent tubes are long, low-luminance sources compared with high intensity discharge lamps, incandescent and halogen lamps and high power LEDs. However, low luminous intensity of the emitting surface is useful because it reduces glare. Lamp fixture design must control light from a long tube instead of a compact globe. The compact fluorescent lamp (CFL) replaces regular incandescent bulbs in many light fixtures where space permits.
Flicker
Fluorescent lamps with magnetic ballasts flicker at a normally unnoticeable frequency of 100 or 120 Hz and this flickering can cause problems for some individuals with light sensitivity;[68] they are listed as problematic for some individuals with autism, epilepsy,[69] lupus,[70] chronic fatigue syndrome, Lyme disease,[71] and vertigo.[72]
A stroboscopic effect can be noticed, where something spinning at just the right speed may appear stationary if illuminated solely by a single fluorescent lamp. This effect is eliminated by paired lamps operating on a lead-lag ballast. Unlike a true strobe lamp, the light level drops in appreciable time and so substantial "blurring" of the moving part would be evident.
Fluorescent lamps may produce flicker at the power supply frequency (50 or 60 Hz), which is noticeable by more people. This happens if a damaged or failed cathode results in slight rectification and uneven light output in positive and negative going AC cycles. Power frequency flicker can be emitted from the ends of the tubes, if each tube electrode produces a slightly different light output pattern on each half-cycle. Flicker at power frequency is more noticeable in the peripheral vision than it is when viewed directly.
Near the end of life, fluorescent lamps can start flickering at a frequency lower than the power frequency. This is due to instability in the negative resistance of arc discharge,[73] which can be from a bad lamp or ballast or poor connection.
New fluorescent lamps may show a twisting spiral pattern of light in a part of the lamp. This effect is due to loose cathode material and usually disappears after a few hours of operation.[31]: 22
Electromagnetic ballasts may also cause problems for video recording as there can be a so-called beat effect between the video frame rate and the fluctuations in intensity of the fluorescent lamp.
Fluorescent lamps with electronic ballasts do not flicker, since above about 5 kHz, the excited electron state half-life is longer than a half cycle,[citation needed] and light production becomes continuous. Operating frequencies of electronic ballasts are selected to avoid interference with infrared remote controls. Poor quality or faulty electronic ballasts may have considerable 100/120 Hz modulation of the light.
Dimming
Fluorescent light fixtures cannot be connected to dimmer switches intended for incandescent lamps. Two effects are responsible for this: the waveform of the voltage emitted by a standard phase-control dimmer interacts badly with many ballasts, and it becomes difficult to sustain an arc in the fluorescent tube at low power levels. Dimming installations require a compatible dimming ballast. Some models of compact fluorescent lamps can be dimmed; in the United States, such lamps are identified as complying with UL standard 1993.[74]
Lamp sizes and designations
Systematic nomenclature identifies mass-market lamps as to general shape, power rating, length, color, and other electrical and illuminating characteristics.
In the United States and Canada, lamps are typically identified by a code such as FxxTy, where F is for fluorescent, the first number (xx) indicates either the power in watts or length in inches, the T indicates that the shape of the bulb is tubular, and the last number (y) is the diameter in eighths of an inch (sometimes in millimeters, rounded-up to the nearest millimeter). Typical diameters are T12 or T38 (1+1⁄2 inch or 38 mm) for residential lamps, T8 or T26 (1 inch or 25 mm) for commercial energy-saving lamps.
Overdriving
Overdriving a fluorescent lamp is a method of getting more light from each tube than is obtained under rated conditions. ODNO (Overdriven Normal Output) fluorescent tubes are generally used when there isn't enough room to put in more bulbs to increase the light. The method is effective, but generates some additional issues. This technique has become popular among aquatic gardeners as a cost-effective way to add more light to their aquariums. Overdriving is done by rewiring lamp fixtures to increase lamp current; however, lamp life is reduced.[75]
Other fluorescent lamps
Black light
Blacklights are a subset of fluorescent lamps that are used to provide UVA light (at about 360 nm wavelength). They are built in the same fashion as conventional fluorescent lamps but the glass tube is coated with a phosphor that converts the short-wave UV within the tube to long-wave UV rather than to visible light. They are used to provoke fluorescence (to provide dramatic effects using blacklight paint and to detect materials such as urine and certain dyes that would be invisible in visible light) as well as to attract insects to bug zappers.
So-called blacklite blue lamps are also made from more expensive deep purple glass known as Wood's glass rather than clear glass. The deep purple glass filters out most of the visible colors of light directly emitted by the mercury-vapor discharge, producing proportionally less visible light compared with UV light. This allows UV-induced fluorescence to be seen more easily (thereby allowing blacklight posters to seem much more dramatic). The blacklight lamps used in bug zappers do not require this refinement so it is usually omitted in the interest of cost; they are called simply blacklite (and not blacklite blue).
Tanning lamp
The lamps used in tanning beds contain a different phosphor blend (typically 3 to 5 or more phosphors) that emits both UVA and UVB, provoking a tanning response in most human skin. Typically, the output is rated as 3–10% UVB (5% most typical) with the remaining UV as UVA. These are mainly high output 100W lamps, although 160W very high output are somewhat common. One common phosphor used in these lamps is lead-activated barium disilicate, but a europium-activated strontium fluoroborate is also used. Early lamps used thallium as an activator, but emissions of thallium during manufacture were toxic.[76]
UVB medical lamps
The lamps used in phototherapy contain a phosphor that emits only UVB ultraviolet light.[citation needed] There are two types: broadband UVB that gives 290–320 nanometer with peak wavelength of 306 nm, and narrowband UVB that gives 311–313 nanometer. Because of the longer wavelength, the narrowband UVB bulbs do not cause erythema in the skin like the broadband.[dubious ] They requires a 10–20 times higher dose to the skin and they require more bulbs and longer exposure time. The narrowband is good for psoriasis, eczema (atopic dermatitis), vitiligo, lichen planus, and some other skin diseases.[citation needed] The broadband is better for increasing Vitamin D3 in the body.
Grow lamp
Grow lamps contain phosphor blends that encourage photosynthesis, growth, or flowering in plants, algae, photosynthetic bacteria, and other light-dependent organisms. These often emit light primarily in the red and blue color range, which is absorbed by chlorophyll and used for photosynthesis in plants.[77]
Infrared lamps
Lamps can be made with a lithium metaluminate phosphor activated with iron. This phosphor has peak emissions between 675 and 875 nanometers, with lesser emissions in the deep red part of the visible spectrum.[78]
Bilirubin lamps
Deep blue light generated from a europium-activated phosphor is used in the light therapy treatment of jaundice; light of this color penetrates skin and helps in the breakup of excess bilirubin.[78]
Germicidal lamp
Germicidal lamps contain no phosphor at all, making them mercury vapor gas discharge lamps rather than fluorescent. Their tubes are made of fused quartz transparent to the UVC light emitted by the mercury discharge. The 254 nm UVC emitted by these tubes will kill germs and the 184.45 nm far UV will ionize oxygen to ozone. Lamps labeled OF block the 184.45 nm far UV and do not produce significant ozone. In addition the UVC can cause eye and skin damage. They are sometimes used by geologists to identify certain species of minerals by the color of their fluorescence when fitted with filters that pass the short-wave UV and block visible light produced by the mercury discharge. They are also used in some EPROM erasers. Germicidal lamps have designations beginning with G, for example G30T8 for a 30-watt, 1-inch (2.5 cm) diameter, 36-inch (91 cm) long germicidal lamp (as opposed to an F30T8, which would be the fluorescent lamp of the same size and rating).
Electrodeless lamp
Electrodeless induction lamps are fluorescent lamps without internal electrodes. They have been commercially available since 1990. A current is induced into the gas column using electromagnetic induction. Because the electrodes are usually the life-limiting element of fluorescent lamps, such electrodeless lamps can have a very long service life, although they also have a higher purchase price.
Cold-cathode fluorescent lamp
Cold-cathode fluorescent lamps were used as backlighting for LCDs in computer monitors and televisions before the use of LED-backlit LCDs. They were also popular with computer case modders.
Science demonstrations
Fluorescent lamps can be illuminated by means other than a proper electrical connection. These other methods, however, result in very dim or very short-lived illumination, and so are seen mostly in science demonstrations. Static electricity or a Van de Graaff generator will cause a lamp to flash momentarily as it discharges a high-voltage capacitance. A Tesla coil will pass high-frequency current through the tube, and since it has a high voltage as well, the gases within the tube will ionize and emit light. This also works with plasma globes. Capacitive coupling with high-voltage power lines can light a lamp continuously at low intensity, depending on the intensity of the electric field.
See also
- Gas-filled tube
- LED tubes — made as drop-in replacement for fluorescents
- List of light sources
- Metal-halide lamp
References
All three of the 'FAST' (< .5 seconds) starter brands caused an audible 'BURRRRRRRP' noise in some light fittings as they started and this is an inherent problem caused by their use of the faster 'DC' heating. It is worse with higher wattage tubes and if there is any loose metal in the light fitting.
- Kane & Sell 2001, p. 122.
Sources
- Bright, Arthur Aaron Jr. (1949). The Electric-Lamp Industry: Technological Change and Economic Development from 1800 to 1947. Macmillan Co.
- Kane, Raymond; Sell, Heinz, eds. (2001). Revolution in lamps: a chronicle of 50 years of progress (2nd ed.). The Fairmont Press, Inc. ISBN 978-0-88173-378-5.
- Van Broekhoven, Jacob (2001). "Lamp Phosphors". In Kane, Raymond; Sell, Heinz (eds.). Revolution in lamps: a chronicle of 50 years of progress (2nd ed.). The Fairmont Press, Inc. pp. 93–126. ISBN 978-0-88173-378-5.
Further reading
- Emanuel Gluskin, “The fluorescent lamp circuit”, (Circuits & Systems Expositions)
- IEEE Transactions on Circuits and Systems, Part I: Fundamental Theory and Applications 46(5), 1999 (529–544).
External links
- Popular Science, January 1940 Fluorescent Lamps
- T5 Fluorescent Systems — Lighting Research Center Research about the improved T5 relative to the previous T8 standard
- NASA: The Fluorescent Lamp: A plasma you can use
- How Fluorescent Tubes are Manufactured on YouTube
- Museum of Electric Lamp Technology
- R. N. Thayer (October 25, 1991). "The Fluorescent Lamp: Early U. S. Development". The Report courtesy of General Electric Company. Archived from the original on 2007-03-24. Retrieved 2007-03-18.
- Wiebe E. Bijker,Of bicycles, bakelites, and bulbs: toward a theory of sociotechnical change MIT Press, 1995, Chapter 4, preview available at Google Books, on the social construction of fluorescent lighting
- Explanations and schematics of some fluorescent lamps
- Fluorescence
- Gas discharge lamps
- Glass applications
- Types of lamp
- Plasma physics
- American inventions
- Mercury (element)
https://en.wikipedia.org/wiki/Fluorescent_lamp#Cold-cathode_fluorescent_lamps
A glow switch starter or glowbottle starter is a type of preheat starter used with a fluorescent lamp. It is commonly filled with neon gas or argon gas and contains a bimetallic strip and a stationary electrode. The operating principle is simple, when current is applied, the gas inside ionizes and heats a bimetallic strip which in turn bends toward the stationary electrode thus shorting the starter between the electrodes of the fluorescent lamp. After about a second the starter's bimetallic strip cools and opens the circuit between the electrodes, and the process repeats until the lamp has lit. One disadvantage of glow switch starters is that when the lamp is at the end of its life it will continuously blink on and off until the glow switch starter wears out or an electrode on the fluorescent lamp burns out. Glow starters have a relatively short life, and light fittings enable the starter to be changed easily. Electronic starters, being interchangeable and using the same casing as a glow starter, last for many years.
The glow switch starter was invented by E. C. Dench in 1938.[1]
https://en.wikipedia.org/wiki/Glow_switch_starter
A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.
The invention of the bimetallic strip is generally credited to John Harrison, an eighteenth-century clockmaker who made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[1] Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
This effect is used in a range of mechanical and electrical devices.
https://en.wikipedia.org/wiki/Bimetallic_strip
Clocks
Mechanical clock mechanisms are sensitive to temperature changes as each part has tiny tolerance and it leads to errors in time keeping. A bimetallic strip is used to compensate this phenomenon in the mechanism of some timepieces. The most common method is to use a bimetallic construction for the circular rim of the balance wheel. What it does is move a weight in a radial way looking at the circular plane down by the balance wheel, varying then, the momentum of inertia of the balance wheel. As the spring controlling the balance becomes weaker with the increasing temperature, the balance becomes smaller in diameter to decrease the momentum of inertia and keep the period of oscillation (and hence timekeeping) constant.
Nowadays this system is not used anymore since the appearance of low temperature coefficient alloys like nivarox, parachrom and many others depending on each brand.
Thermostats
In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures are used. In these, one end of the bimetallic strip is mechanically fixed and attached to an electrical power source, while the other (moving) end carries an electrical contact. In adjustable thermostats another contact is positioned with a regulating knob or lever. The position so set controls the regulated temperature, called the set point.
Some thermostats use a mercury switch connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat.
Depending upon the application, a higher temperature may open a contact (as in a heater control) or it may close a contact (as in a refrigerator or air conditioner).
The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a relay or the supply of natural gas or fuel oil through an electrically operated valve. In some natural gas heaters the power may be provided with a thermocouple that is heated by a pilot light (a small, continuously burning, flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered spark generating device.
Thermometers
A direct indicating dial thermometer, common in household devices (such as a patio thermometer or a meat thermometer), uses a bimetallic strip wrapped into a coil in its most common design. The coil changes the linear movement of the metal expansion into a circular movement thanks to the helicoidal shape it draws. One end of the coil is fixed to the housing of the device as a fix point and the other drives an indicating needle inside a circular indicator. A bimetallic strip is also used in a recording thermometer. Breguet's thermometer consists of a tri-metallic helix in order to have a more accurate result.
Heat engine
Heat engines are not the most efficient ones, and with the use of bimetallic strips the efficiency of the heat engine is even lower as there is no chamber to contain the heat. Moreover, the bimetallic strips cannot produce strength in its moves, the reason why is that in order to achieve reasonables bendings (movements) both metallic strips have to be thin to make the difference between the expansion noticeable. So the uses for metallic strips in heat engines are mostly in simple toys that have been built to demonstrate how the principle can be used to drive a heat engine.[citation needed]
Electrical devices
Bimetal strips are used in miniature circuit breakers to protect circuits from excess current. A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches a spring-operated contact. This interrupts the circuit and can be reset when the bimetal strip has cooled down.
Bimetal strips are also used in time-delay relays, gas oven safety valves, thermal flashers for older turn signal lamps, and fluorescent lamp starters. In some devices, the current running directly through the bimetal strip is sufficient to heat it and operate contacts directly. It has also been used in mechanical PWM voltage regulators for automotive uses.[5]
See also
References
https://en.wikipedia.org/wiki/Bimetallic_strip
A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.
The invention of the bimetallic strip is generally credited to John Harrison, an eighteenth-century clockmaker who made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[1] Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
This effect is used in a range of mechanical and electrical devices.
Characteristics
The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled. The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals.
In some applications, the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.
The curvature of a bimetallic beam can be described by the following equation:
where and is the radius of curvature, and are the Young's modulus and height (thickness) of material one and and are the Young's modulus and height (thickness) of material two. is the misfit strain, calculated by:
where α1 is the coefficient of thermal expansion of material one and α2 is the coefficient of thermal expansion of material two. ΔT is the current temperature minus the reference temperature (the temperature where the beam has no flexure).[2][3]
| Derivation of the radius of curvature |
|
|---|
Insight may be gained if the result just given is multiplied on top and bottom by
where , and . Since for small , which is insensitive to because of the lack of first order terms, then we may approximate for close to unity (and insensitive to ), and for close to unity (and insensitive to ). Thus, unless or are very far from unity we can approximate .
History
The earliest surviving bimetallic strip was made by the eighteenth-century clockmaker John Harrison who is generally credited with its invention. He made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[4] It should not be confused with the bimetallic mechanism for correcting for thermal expansion in his gridiron pendulum. His earliest examples had two individual metal strips joined by rivets but he also invented the later technique of directly fusing molten brass onto a steel substrate. A strip of this type was fitted to his last timekeeper, H5. Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
https://en.wikipedia.org/wiki/Bimetallic_strip
A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs (tilt, displacement, radial, etc.) but they all share the common design strength of non-eroding switch contacts.
The most common is the mercury tilt switch. It is in one state (open or closed) when tilted one direction with respect to horizontal, and the other state when tilted the other direction. This is what older style thermostats used to turn a heater or air conditioner on or off.
The mercury displacement switch uses a 'plunger' that dips into a pool of mercury, raising the level in the container to contact at least one electrode. This design is used in relays in industrial applications that need to switch high current loads frequently. These relays use electromagnetic coils to pull steel sleeves inside hermetically sealed containers.
https://en.wikipedia.org/wiki/Mercury_switch
Thermostats
Mercury switches were once common in bimetal thermostats. The weight of the movable mercury drop provided some hysteresis by a degree of over-center action. The bimetal spring had to move further to overcome the weight of the mercury, tending to hold it in the open or closed position. The mercury also provided positive on-off switching, and could withstand millions of cycles without contact degradation.
Doorbells
Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interrupters.
Pressure switches
Some pressure switches use a Bourdon tube and a mercury switch. The small force generated by the tube reliably operates the switch.
Vending
Mercury switches are still used in electro-mechanical systems where physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines for tilt alarms that detect when someone tries to rock or tilt the machine to make it vend a product.
Bombs
A tilt switch can trigger a bomb.[6][7] Mercury tilt switches can be found in some bomb and landmine fuzes, typically in the form of anti-handling devices, for example, a variant of the VS-50 mine.
https://en.wikipedia.org/wiki/Mercury_switch
See also
- Mercury-arc valve, a rectifier device intended for high electrical voltages/currents
- Mercury battery, an electrochemical battery
- Mercury coulometer, an electro analytical chemistry device that determines the amount of matter transformed during a mercury reaction
- Mercury probe, an electrical probing device to sample for electrical characterization
- Mercury swivel commutator, an electrical circuit, current-reversing switch using the element mercury
- Mercury-wetted relay
- Mercury relay
https://en.wikipedia.org/wiki/Mercury_switch
https://en.wikipedia.org/wiki/Switch#Contact_bounce
https://en.wikipedia.org/wiki/Mercury_switch
https://en.wikipedia.org/wiki/Mercury_coulometer
https://en.wikipedia.org/wiki/Mercury_swivel_commutator
https://en.wikipedia.org/wiki/Mercury_relay
https://en.wikipedia.org/wiki/Mercury-arc_valve
https://en.wikipedia.org/wiki/Analogue_switch
https://en.wikipedia.org/wiki/Centrifugal_switch
https://en.wikipedia.org/wiki/Crossbar_switch
https://en.wikipedia.org/wiki/Crossover_switch
https://en.wikipedia.org/wiki/Cryotron
https://en.wikipedia.org/wiki/DIP_switch
https://en.wikipedia.org/wiki/Dry_contact
https://en.wikipedia.org/wiki/Float_switch
https://en.wikipedia.org/wiki/Infinite_switch
https://en.wikipedia.org/wiki/Kill_switch
https://en.wikipedia.org/wiki/Key_switch
https://en.wikipedia.org/wiki/Limit_switch
https://en.wikipedia.org/wiki/Latching_switch
https://en.wikipedia.org/wiki/Light_switch
https://en.wikipedia.org/wiki/Lightning_switch
https://en.wikipedia.org/wiki/Magnetic_proximity_fuze
https://en.wikipedia.org/wiki/Magnetic_starter
https://en.wikipedia.org/wiki/Magnetic_switch
https://en.wikipedia.org/wiki/Miniature_snap-action_switch
https://en.wikipedia.org/wiki/Optical_transistor
https://en.wikipedia.org/wiki/Photoswitch
https://en.wikipedia.org/wiki/Placebo_button
https://en.wikipedia.org/wiki/Pull_switch
https://en.wikipedia.org/wiki/Push_switch
https://en.wikipedia.org/wiki/Railroad_switch
https://en.wikipedia.org/wiki/Reed_switch
https://en.wikipedia.org/wiki/Rotary_switch
https://en.wikipedia.org/wiki/Sail_switch
https://en.wikipedia.org/wiki/Sense_switch
https://en.wikipedia.org/wiki/Sea_switch
https://en.wikipedia.org/wiki/Silicone_rubber_keypad
https://en.wikipedia.org/wiki/Spark_gap
https://en.wikipedia.org/wiki/Staircase_timer
https://en.wikipedia.org/wiki/Stepping_switch
https://en.wikipedia.org/wiki/Zero_speed_switch
https://en.wikipedia.org/wiki/Wireless_light_switch
https://en.wikipedia.org/wiki/Vacuum_switch
https://en.wikipedia.org/wiki/Transfer_switch
https://en.wikipedia.org/wiki/Time_switch
https://en.wikipedia.org/wiki/Category:Appropriate_technology
https://en.wikipedia.org/wiki/Village-level_operation_and_maintenance_(pumps)
https://en.wikipedia.org/wiki/Single-wire_earth_return
https://en.wikipedia.org/wiki/Single-wire_transmission_line
https://en.wikipedia.org/wiki/Static_electricity
https://en.wikipedia.org/wiki/Antistatic_device
https://en.wikipedia.org/wiki/Corona_discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage.
https://en.wikipedia.org/wiki/Corona_discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp.
In many high voltage applications, corona is an unwanted side effect. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high voltage equipment like cathode ray tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O3) and nitric oxide (NO), and in turn, nitrogen dioxide (NO2), and thus nitric acid (HNO3) if water vapor is present. These gases are corrosive and can degrade and embrittle nearby materials, and are also toxic to humans and the environment.
Corona discharges can often be suppressed by improved insulation, corona rings, and making high voltage electrodes in smooth rounded shapes.with no sharp edges However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.
Introduction
A corona discharge is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.
When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the electric field strength around that point will be much higher than elsewhere. Air near the electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.
Along with the similar brush discharge, the corona is often called a "single-electrode discharge", as opposed to a "two-electrode discharge" – an electric arc.[1][2][3] A corona forms only when the conductor is widely enough separated from conductors at the opposite potential that an arc cannot jump between them. If the geometry and gradient are such that the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric spark or electric arc, depending upon the source of the electric field. If the source continues to supply current, a spark will evolve into a continuous discharge called an arc.
Corona discharge forms only when the electric field (potential gradient) at the surface of the conductor exceeds a critical value, the dielectric strength or disruptive potential gradient of the fluid. In air at sea level pressure of 101 kPa, the critical value is roughly 30 kV/cm,[1] but this decreases with pressure, therefore, corona discharge is more of a problem at high altitudes.[4] Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. On sharp points in the air, corona can start at potentials of 2–6 kV.[2] In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large-diameter rounded shapes like balls or toruses. Corona rings are often added to insulators of high voltage transmission lines.
Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly curved electrode. If the curved electrode is positive with respect to the flat electrode, it has a positive corona; if it is negative, it has a negative corona. (See below for more details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.
An important reason for considering coronas is the production of ozone around conductors undergoing corona processes in air. A negative corona generates much more ozone than the corresponding positive corona.
Applications
Corona discharge has a number of commercial and industrial applications:
- Removal of unwanted electric charges from the surface of aircraft in flight and thus avoiding the detrimental effect of uncontrolled electrical discharge pulses on the performance of avionic systems
- Manufacture of ozone
- Sanitization of pool water
- In an electrostatic precipitator, removal of solid pollutants from a waste gas stream, or scrubbing particles from the air in air-conditioning systems
- Photocopying
- Air ionisers
- Production of photons for Kirlian photography to expose photographic film
- EHD thrusters, lifters, and other ionic wind devices
- Nitrogen laser
- Ionization of a gaseous sample for subsequent analysis in a mass spectrometer or an ion mobility spectrometer
- Static charge neutralization, as applied through antistatic devices like ionizing bars
- Refrigeration of electronic devices by forced convection[5]
Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying). They can also be used to remove particulate matter from air streams by first charging the air, and then passing the charged stream through a comb of alternating polarity, to deposit the charged particles onto oppositely charged plates.
The free radicals and ions generated in corona reactions can be used to scrub the air of certain noxious products, through chemical reactions, and can be used to produce ozone.
Problems
Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. Therefore, power transmission equipment is designed to minimize the formation of corona discharge.
Corona discharge is generally undesirable in:
- Electric power transmission, where it causes:
- Electrical components such as transformers, capacitors, electric motors, and generators:
- Corona can progressively damage the insulation inside these devices, leading to equipment failure
- Elastomer items such as O-rings can suffer ozone cracking
- Plastic film capacitors operating at mains voltage can suffer progressive loss of capacitance as corona discharges cause local vaporization of the metallization[8]
In many cases, coronas can be suppressed by corona rings, toroidal devices that serve to spread the electric field over a larger areas and decrease the field gradient below the corona threshold.
Mechanism
 |
Corona discharge occurs when the electric field is strong enough to create a chain reaction; electrons in the air collide with atoms hard enough to ionize them, creating more free electrons that ionize more atoms. The diagrams below illustrate at a microscopic scale the process which creates a corona in the air next to a pointed electrode carrying a high negative voltage with respect to ground. The process is:
- A neutral atom or molecule, in a region of the strong electric
field (such as the high potential gradient near the curved electrode),
is ionized by a natural environmental event (for example, being struck
by an ultraviolet photon or cosmic ray particle), to create a positive ion and a free electron.
- The electric field accelerates these oppositely charged particles in opposite directions, separating them, preventing their recombination, and imparting kinetic energy to each of them.
- The electron has a much higher charge/mass ratio and so is
accelerated to a higher velocity than the positive ion. It gains enough
energy from the field that when it strikes another atom it ionizes it,
knocking out another electron, and creating another positive ion. These
electrons are accelerated and collide with other atoms, creating
further electron/positive-ion pairs, and these electrons collide with
more atoms, in a chain reaction process called an electron avalanche.
Both positive and negative coronas rely on electron avalanches. In a
positive corona, all the electrons are attracted inward toward the
nearby positive electrode and the ions are repelled outwards. In a
negative corona, the ions are attracted inward and the electrons are
repelled outwards.
- The glow of the corona is caused by electrons recombining with
positive ions to form neutral atoms. When the electron falls back to
its original energy level, it releases a photon of light. The photons
serve to ionize other atoms, maintaining the creation of electron
avalanches.
- At a certain distance from the electrode, the electric field becomes low enough that it no longer imparts enough energy to the electrons to ionize atoms when they collide. This is the outer edge of the corona. Outside this, the ions move through the air without creating new ions. The outward moving ions are attracted to the opposite electrode and eventually reach it and combine with electrons from the electrode to become neutral atoms again, completing the circuit.
Thermodynamically, a corona is a very nonequilibrium process, creating a non-thermal plasma. The avalanche mechanism does not release enough energy to heat the gas in the corona region generally and ionize it, as occurs in an electric arc or spark. Only a small number of gas molecules take part in the electron avalanches and are ionized, having energies close to the ionization energy of 1–3 ev, the rest of the surrounding gas is close to ambient temperature.
The onset voltage of corona or corona inception voltage (CIV) can be found with Peek's law (1929), formulated from empirical observations. Later papers derived more accurate formulas.
Positive coronas
Properties
A positive corona is manifested as a uniform plasma across the length of a conductor. It can often be seen glowing blue/white, though many of the emissions are in the ultraviolet. The uniformity of the plasma is caused by the homogeneous source of secondary avalanche electrons described in the mechanism section, below. With the same geometry and voltages, it appears a little smaller than the corresponding negative corona, owing to the lack of a non-ionising plasma region between the inner and outer regions.
A positive corona has a much lower density of free electrons compared to a negative corona; perhaps a thousandth of the electron density, and a hundredth of the total number of electrons. However, the electrons in a positive corona are concentrated close to the surface of the curved conductor, in a region of the high potential gradient (and therefore the electrons have high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas. Therefore, if electrons are to be used in an application which requires high activation energy, positive coronas may support a greater reaction constant than corresponding negative coronas; though the total number of electrons may be lower, the number of very high energy electrons may be higher.
Coronas are efficient producers of ozone in the air. A positive corona generates much less ozone than the corresponding negative corona, as the reactions which produce ozone are relatively low-energy. Therefore, the greater number of electrons of a negative corona leads to increased production.
Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat electrode.
Mechanism
As with a negative corona, a positive corona is initiated by an exogenous ionization event in a region of a high potential gradient. The electrons resulting from the ionization are attracted toward the curved electrode, and the positive ions repelled from it. By undergoing inelastic collisions closer and closer to the curved electrode, further molecules are ionized in an electron avalanche.
In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the fluid itself, in the region outside the plasma or avalanche region. They are created by ionization caused by the photons emitted from that plasma in the various de-excitation processes occurring within the plasma after electron collisions, the thermal energy liberated in those collisions creating photons which are radiated into the gas. The electrons resulting from the ionization of a neutral gas molecule are then electrically attracted back toward the curved electrode, attracted into the plasma, and so begins the process of creating further avalanches inside the plasma.
Negative coronas
Properties
A negative corona is manifested in a non-uniform corona, varying according to the surface features and irregularities of the curved conductor. It often appears as tufts of the corona at sharp edges, the number of tufts altering with the strength of the field. The form of negative coronas is a result of its source of secondary avalanche electrons (see below). It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the ionizing region, and so the plasma continues some distance beyond it. The total number of electrons and electron density is much greater than in the corresponding positive corona. However, they are of predominantly lower energy, owing to being in a region of lower potential gradient. Therefore, whilst for many reactions, the increased electron density will increase the reaction rate, the lower energy of the electrons will mean that reactions which require higher electron energy may take place at a lower rate.
Mechanism
Negative coronas are more complex than positive coronas in construction. As with positive coronas, the establishing of a corona begins with an exogenous ionization event generating a primary electron, followed by an electron avalanche.
Electrons ionized from the neutral gas are not useful in sustaining the negative corona process by generating secondary electrons for further avalanches, as the general movement of electrons in a negative corona is outward from the curved electrode. For negative corona, instead, the dominant process generating secondary electrons is the photoelectric effect, from the surface of the electrode itself. The work function of the electrons (the energy required to liberate the electrons from the surface) is considerably lower than the ionization energy of air at standard temperatures and pressures, making it a more liberal source of secondary electrons under these conditions. Again, the source of energy for the electron-liberation is a high-energy photon from an atom within the plasma body relaxing after excitation from an earlier collision. The use of ionized neutral gas as a source of ionization is further diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode.
Under other conditions, the collision of the positive species with the curved electrode can also cause electron liberation.
The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode itself, the new secondary electrons travelling outward.
A further feature of the structure of negative coronas is that as the electrons drift outwards, they encounter neutral molecules and, with electronegative molecules (such as oxygen and water vapor), combine to produce negative ions. These negative ions are then attracted to the positive uncurved electrode, completing the 'circuit'.
Electrical wind
Ionized gases produced in a corona discharge are accelerated by the electric field, producing a movement of gas or electrical wind. The air movement associated with a discharge current of a few hundred microamperes can blow out a small candle flame within about 1 cm of a discharge point. A pinwheel, with radial metal spokes and pointed tips bent to point along the circumference of a circle, can be made to rotate if energized by a corona discharge; the rotation is due to the differential electric attraction between the metal spokes and the space charge shield region that surrounds the tips.[9]
See also
- Alternating current
- Atmospheric pressure chemical ionization
- Crookes tube
- Dielectric barrier discharge
- Kirlian photography
- List of plasma physics articles
- St. Elmo's fire
References
- Loeb, Leonard Benedict (1965). Electrical Coronas. University of California Press. pp. 406–409.
Further reading
- Chen, Junhong (August 2002). "Direct-Current Corona Enhanced Chemical Reactions". University of Minnesota.
- Peek, F.W. (1929). Dielectric Phenomena in High Voltage Engineering. McGraw-Hill. ISBN 0-9726596-6-8.
- Loeb, Leonard (1965). Electrical Coronas Their Basic Physical Mechanisms. University of California Press. ASIN B0006BM4LG.
- Cobine, James D. (1941). Gaseous Conductors; Theory and Engineering Applications. McGraw-Hill or Dover reprints. ASIN B000B9PK7S.
- Takacs, J. (1972). "Corona Stabilizer for Van De Graaff Accelerators". Nuclear Instruments and Methods. 103 (3): 587–600. Bibcode:1972NucIM.103..587T. doi:10.1016/0029-554X(72)90019-5. ISSN 0029-554X.
External links
https://en.wikipedia.org/wiki/Corona_discharge
https://en.wikipedia.org/wiki/Transmitter
https://en.wikipedia.org/wiki/Television_broadcasting
https://en.wikipedia.org/wiki/Local_insertion
https://en.wikipedia.org/wiki/Emergency_alert
https://en.wikipedia.org/wiki/Station_identification
https://en.wikipedia.org/wiki/Genlock
https://en.wiktionary.org/wiki/syntonize
https://en.wikipedia.org/wiki/Watermark
https://en.wikipedia.org/wiki/Signal_generator
https://en.wikipedia.org/wiki/Black_and_burst
https://en.wikipedia.org/wiki/Vision_mixer
Where composite video is in use, the phase of the chrominance subcarrier of each source being combined or switched should also be coincident. This is to avoid changes in colour hue and/or saturation during a transition between sources.
https://en.wikipedia.org/wiki/Genlock
https://en.wikipedia.org/wiki/Subcarrier
Scope
Generator locking can be used to synchronize as few as two isolated sources (e.g., a television camera and a videotape machine feeding a vision mixer (production switcher)), or in a wider facility where all the video sources are locked to a single synchronizing pulse generator (e.g., a fast-paced sporting event featuring multiple cameras and recording devices). Generator locking can also be used to ensure that multiple CRT monitors that appear in a movie are flicker-free. Generator locking is also used to synchronize two cameras for Stereoscopic 3D video recording.
In broadcast systems, an analog generator-lock signal usually consists of vertical and horizontal synchronizing pulses together with chrominance phase reference in the form of colorburst. No picture information is usually carried to avoid disturbing the timing signals, and the name reference, black and burst, color black, or black burst is usually given to such a signal. A composite colour video signal inherently carries the same reference signals and can be used as a generator-locking signal, albeit at the risk of being disturbed by out-of-specification picture signals.
Although some high-definition broadcast systems may use a standard-definition reference signal as a generator-locking reference signal, the use of tri-level synchronising pulses directly related to the frame and line rate is increasing within HD systems. A tri-level sync pulse is a signal that initially goes from 0 volts DC to a negative voltage, then a positive voltage, before returning to zero volts DC again. The voltage excursions are typically 300 mV either side of zero volts, and the duration each of the two excursions is the same as a particular number of digital picture samples.
https://en.wikipedia.org/wiki/Genlock
Connections
Most television studio and professional video cameras have dedicated generator-locking ports on the camera. If the camera is tethered with a triaxial cable or optical fibre cable, the analog generator-locking signal is used to lock the camera control unit, which in turn locks the camera head by means of information carried within a data channel transmitted along the cable. If the camera is an ENG-type camera, one without a triax/fibre connection or without a dockable head, the generator-locking signal is carried through a separate cable from the video.
Variants
Natlock is a picture-source synchronizing system using audio tone signals to describe the timing discrepancies between composite video signals, while Icelock uses digital information conveyed in the vertical blanking interval of a composite video signal.
See also
References
- Roe, John H. (March–April 1950). "The Genlock for Improved TV Programming" (PDF). RCA Broadcast News. Number 58: 11–12 – via americanradiohistory.com.
https://en.wikipedia.org/wiki/Genlock
Triaxial cable, often referred to as triax for short, is a type of electrical cable similar to coaxial cable, but with the addition of an extra layer of insulation and a second conducting sheath. It provides greater bandwidth and rejection of interference than coax, but is more expensive.[1][2]
https://en.wikipedia.org/wiki/Triaxial_cable
https://en.wikipedia.org/wiki/Triaxial_cable
https://en.wikipedia.org/wiki/Driven_guard
https://en.wikipedia.org/wiki/Buffer_amplifier#Voltage_buffer
https://en.wikipedia.org/wiki/Twinaxial_cabling
https://en.wikipedia.org/wiki/Coaxial_cable
https://en.wikipedia.org/wiki/Vertical_blanking_interval
https://en.wikipedia.org/wiki/Time_base_correction#frame_synchronizer
https://en.wikipedia.org/wiki/Time_base_correction
Time base correction (TBC) is a technique to reduce or eliminate errors caused by mechanical instability present in analog recordings on mechanical media.
Without time base correction, a signal from a videotape recorder (VTR) or videocassette recorder (VCR) cannot be mixed with other, more time-stable devices found in television studios and post-production facilities.
Most broadcast quality VCRs have simple time base correctors built in though external TBCs are often used. Some high-end domestic analog video recorders and camcorders also include a TBC circuit, which typically can be switched off if required.
Time base correction counteracts errors by buffering the video signal as it comes off the videotape at an unsteady rate, and releasing it at a steady rate. TBCs also allow a variable delay in the video stream. By adjusting the rate and delay using a waveform monitor and a vectorscope, the corrected signal can now match the timing of the other devices in the system. If all of the devices in a system are adjusted so their signals meet the video switcher at the same time and at the same rate, the signals can be mixed. A single master clock or "sync generator" provides the reference for all of the devices' clocks.
Video correction
As far back as 1956, professional reel-to-reel audio tape recorders relying on mechanical stability alone were stable enough that pitch distortion could be below audible level without time base correction. However, the higher sensitivity of video recordings meant that even the best mechanical solutions still resulted in detectable distortion of the video signals and difficulty locking to downstream devices.[1] A video signal consists of picture information but also sync and subcarrier signals which allow the image to be framed up square on the monitor, reproduce colors accurately[note 1] and, importantly, allow the combination and switching of two or more video signals.
https://en.wikipedia.org/wiki/Time_base_correction
A modern 5th and final type of TBC being achieved in the late 2010s is software defined, packaged inside the open source python based VHS-Decode[2] & CVBS-Decode[3] projects which evolved from the LD-Decode project[4] which uses FM RF captures of analouge media signals then de-modulates and corrects the signal in software.
https://en.wikipedia.org/wiki/Time_base_correction
https://en.wikipedia.org/wiki/Drop-out_compensator
https://en.wikipedia.org/wiki/Lossless_compression
https://en.wikipedia.org/wiki/Arithmetic_coding
https://en.wikipedia.org/wiki/Integrated_circuit
https://en.wikipedia.org/wiki/Indexed_color
https://en.wikipedia.org/wiki/19-inch_rack
 On consumer products a yellow RCA connector is typically used for composite video. | |||
| Type | Analog video connector | ||
|---|---|---|---|
| Production history | |||
| Designed | 1954[1]–1956[2] | ||
| General specifications | |||
| Length | Maximum of 50 m[citation needed] | ||
| External | Yes | ||
| Video signal | NTSC, PAL or SECAM video | ||
| Pins | 1 plus grounding shield | ||
| Connector | RCA connector | ||
| Electrical | |||
| Signal | 1 volt[3] | ||
| Pinout | |||
| Pin 1 | center | video | |
| Pin 2 | sheath | ground | |
Composite video is an analog video signal format that carries standard-definition video (typically at 525 lines or 625 lines) as a single channel. Video information is encoded on one channel, unlike the higher-quality S-Video (two channels) and the even higher-quality component video (three or more channels). In all of these video formats, audio is carried on a separate connection.
Composite video is also known by the initials CVBS for composite video baseband signal or color, video, blanking and sync,[4][5] or is simply referred to as SD video for the standard-definition television signal it conveys.
There are three dominant variants of composite video signals, corresponding to the analog color system used: NTSC, PAL, and SECAM. Usually composite video is carried by a yellow RCA connector, but other connections are used in professional settings, or on devices that are too small for an RCA connector, such as a digital camera.
https://en.wikipedia.org/wiki/Composite_video
https://en.wikipedia.org/wiki/525_lines
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems. The word raster comes from the Latin word rastrum (a rake), which is derived from radere (to scrape); see also rastrum, an instrument for drawing musical staff lines. The pattern left by the lines of a rake, when drawn straight, resembles the parallel lines of a raster: this line-by-line scanning is what creates a raster. It is a systematic process of covering the area progressively, one line at a time. Although often a great deal faster, it is similar in the most general sense to how one's gaze travels when one reads lines of text.
In most modern graphics cards the data to be drawn is stored internally in an area of semiconductor memory called the Framebuffer. This memory area holds the values for each pixel on the screen. These values are retrieved from the refresh buffer and painted onto the screen one row at a time.
https://en.wikipedia.org/wiki/Raster_scan
https://en.wikipedia.org/wiki/Bitmap
https://en.wikipedia.org/wiki/Image_tracing
https://en.wikipedia.org/wiki/Optical_character_recognition
https://en.wikipedia.org/wiki/Text_mining
https://en.wikipedia.org/wiki/Document_clustering
https://en.wikipedia.org/wiki/Information_retrieval
https://en.wikipedia.org/wiki/Search_engine
https://en.wikipedia.org/wiki/Singular_value_decomposition#Truncated_SVD
https://en.wikipedia.org/wiki/Latent_semantic_analysis#Latent_semantic_indexing
https://en.wikipedia.org/wiki/Dimensionality_reduction
https://en.wikipedia.org/wiki/Feature_selection
https://en.wikipedia.org/wiki/Curse_of_dimensionality
https://en.wikipedia.org/wiki/Sampling_(statistics)
https://en.wikipedia.org/wiki/Sampling_(statistics)
https://en.wikipedia.org/wiki/Minimax
https://en.wikipedia.org/wiki/Convenience_sampling
https://en.wikipedia.org/wiki/Self-selection_bias
https://en.wikipedia.org/wiki/Line-intercept_sampling
https://en.wikipedia.org/wiki/Sampling_error
https://en.wikipedia.org/wiki/Random_number_table
https://en.wikipedia.org/wiki/Pseudorandom_number_generator
https://en.wikipedia.org/wiki/Data_collection
https://en.wikipedia.org/wiki/Resampling_(statistics)
https://en.wikipedia.org/wiki/Matrix_norm
https://en.wikipedia.org/wiki/Analog_recording
https://en.wikipedia.org/wiki/Field_strength
https://en.wikipedia.org/wiki/Dipole_field_strength_in_free_space
https://en.wikipedia.org/wiki/Phonautograph
https://en.wikipedia.org/wiki/Phonograph
Scott coated a plate of glass with a thin layer of lampblack. He then took an acoustic trumpet, and at its tapered end affixed a thin membrane that served as the analog to the eardrum. At the center of that membrane, he attached a rigid boar's bristle approximately a centimeter long, placed so that it just grazed the lampblack. As the glass plate was slid horizontally in a well formed groove at a speed of one meter per second, a person would speak into the trumpet, causing the membrane to vibrate and the stylus to trace figures[13] that were scratched into the lampblack.[15] On March 25, 1857, Scott received the French patent[16] #17,897/31,470 for his device, which he called a phonautograph.[17] The earliest known surviving recorded sound of a human voice was conducted on April 9, 1860, when Scott recorded[15] someone singing the song "Au Clair de la Lune" ("By the Light of the Moon") on the device.[18] However, the device was not designed to play back sounds,[15][19] as Scott intended for people to read back the tracings,[20] which he called phonautograms.[14] This was not the first time someone had used a device to create direct tracings of the vibrations of sound-producing objects, as tuning forks had been used in this way by English physicist Thomas Young in 1807.[21] By late 1857, with support from the Société d'encouragement pour l'industrie nationale, Scott's phonautograph was recording sounds with sufficient precision to be adopted by the scientific community, paving the way for the nascent science of acoustics.[14]
The device's true significance in the history of recorded sound was not fully realized prior to March 2008, when it was discovered and resurrected in a Paris patent office by First Sounds, an informal collaborative of American audio historians, recording engineers, and sound archivists founded to make the earliest sound recordings available to the public. The phonautograms were then digitally converted by scientists at the Lawrence Berkeley National Laboratory in California, who were able to play back the recorded sounds, something Scott had never conceived of. Prior to this point, the earliest known record of a human voice was thought to be an 1877 phonograph recording by Thomas Edison.[15][22] The phonautograph would play a role in the development of the gramophone, whose inventor, Emile Berliner, worked with the phonautograph in the course of developing his own device.[23]
https://en.wikipedia.org/wiki/Phonograph
https://en.wikipedia.org/wiki/Telephone
https://en.wikipedia.org/wiki/Pantograph
https://en.wikipedia.org/wiki/Phonograph_cylinder
https://en.wikipedia.org/wiki/Graphophone
https://en.wikipedia.org/wiki/Arbitrary_waveform_generator
https://en.wikipedia.org/wiki/Radio_frequency
https://en.wikipedia.org/wiki/Function_generator
https://en.wikipedia.org/wiki/Digital_signal_processing
https://en.wikipedia.org/wiki/Digital-to-analog_converter
https://en.wikipedia.org/wiki/Triangle_wave
https://en.wikipedia.org/wiki/Continuous_wave
https://en.wikipedia.org/wiki/Modulation#Pulse_modulation_methods
https://en.wikipedia.org/wiki/Phase_modulation
https://en.wikipedia.org/wiki/Total_harmonic_distortion
https://en.wikipedia.org/wiki/Modulation#Digital_modulation_methods
https://en.wikipedia.org/wiki/GSM
https://en.wikipedia.org/wiki/Sound_recording_and_reproduction
https://en.wikipedia.org/wiki/Intermodulation
https://en.wikipedia.org/wiki/Metadata
https://en.wikipedia.org/wiki/Jitter
https://en.wikipedia.org/wiki/Bit_error_rate
https://en.wikipedia.org/wiki/Synthesizer
https://en.wikipedia.org/wiki/Sound_card
https://en.wikipedia.org/wiki/Video-signal_generator
https://en.wikipedia.org/wiki/AN/URM-25D_signal_generator
https://en.wikipedia.org/wiki/Digital_pattern_generator
https://en.wikipedia.org/wiki/Inductive_amplifier
https://en.wikipedia.org/wiki/Bus_analyzer
https://en.wikipedia.org/wiki/Sweep_generator
https://en.wikipedia.org/wiki/Category:Electronic_test_equipment
https://en.wikipedia.org/wiki/Network_analyzer_(electrical)
https://en.wikipedia.org/wiki/Spectrum_analyzer
https://en.wikipedia.org/wiki/Spectrum_analyzer
https://en.wikipedia.org/wiki/Signal_analyzer
https://en.wikipedia.org/wiki/Vectorscope
https://en.wikipedia.org/wiki/Sound_module
https://en.wikipedia.org/wiki/Sampler_(musical_instrument)
https://en.wikipedia.org/wiki/Demultiplex
https://en.wikipedia.org/wiki/Oscilloscope#X-Y_mode
https://en.wikipedia.org/wiki/Waveform_monitor
https://en.wikipedia.org/wiki/Fade_(audio_engineering)#Crossfading
On older vectorscopes that use cathode ray tubes (CRTs), the graticule was often a silk-screened overlay superimposed over the front surface of the screen. One notable exception was the Tektronix WFM601 series of instruments, which are combined waveform monitors and vectorscopes used to measure CCIR 601 television signals. The waveform-mode graticule of these instruments is implemented with a silkscreen, whereas the vectorscope graticule (consisting only of bar targets, as this family did not support composite video) was drawn on the CRT by the electron beam. Modern instruments have graticules drawn using computer graphics, and both graticule and trace are rendered on an external VGA monitor or an internal VGA-compatible LCD display.
https://en.wikipedia.org/wiki/Vectorscope
https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
https://en.wikipedia.org/wiki/Test_card
Audio
In audio applications, a vectorscope is used to measure the difference between channels of stereo audio signals. One stereo channel drives the horizontal deflection of the display, and the other drives the vertical deflection. A monaural signal, consisting of identical left and right signals, results in a straight line with a gradient of +1. Any stereo separation is visible as a deviation from this line, creating a Lissajous figure. If a straight line appears with a gradient of −1, this indicates that the left and right channels are 180° out of phase.https://en.wikipedia.org/wiki/Vectorscope
https://en.wikipedia.org/wiki/Liquid-crystal_display
https://en.wikipedia.org/wiki/Video_Graphics_Array
https://en.wikipedia.org/wiki/Cathode_ray
https://en.wikipedia.org/wiki/180-line_television_system
Laser-powered phosphor display (LPD) is a large-format display technology similar to the cathode ray tube (CRT). Prysm, Inc., a video wall designer and manufacturer in Silicon Valley, California, invented and patented[1] the LPD technology.[2][3] The key components of the LPD technology are its TD2 tiles, its image processor, and its backing frame that supports LPD tile arrays.[4] The company unveiled the LPD in January 2010.[4][5][6]
https://en.wikipedia.org/wiki/Laser-powered_phosphor_display
A surface-conduction electron-emitter display (SED) is a display technology for flat panel displays developed by a number of companies. SEDs use nanoscopic-scale electron emitters to energize colored phosphors and produce an image. In a general sense, a SED consists of a matrix of tiny cathode-ray tubes, each "tube" forming a single sub-pixel on the screen, grouped in threes to form red-green-blue (RGB) pixels. SEDs combine the advantages of CRTs, namely their high contrast ratios, wide viewing angles, and very fast response times, with the packaging advantages of LCD and other flat panel displays. They also use much less power than an LCD television of the same size.
After considerable time and effort in the early and mid-2000s, SED efforts started winding down in 2009 as LCD became the dominant technology. In August 2010, Canon announced they were shutting down their joint effort to develop SEDs commercially, signaling the end of development efforts.[1] SEDs are closely related to another developing display technology, the field emission display, or FED, differing primarily in the details of the electron emitters. Sony, the main backer of FED, has similarly backed off from their development efforts.[2]
https://en.wikipedia.org/wiki/Surface-conduction_electron-emitter_display
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen.[2] The images may represent electrical waveforms (oscilloscope), pictures (television set, computer monitor), radar targets, or other phenomena. A CRT on a television set is commonly called a picture tube. CRTs have also been used as memory devices, in which case the screen is not intended to be visible to an observer. The term cathode ray was used to describe electron beams when they were first discovered, before it was understood that what was emitted from the cathode was a beam of electrons.
In CRT television sets and computer monitors, the entire front area of the tube is scanned repeatedly and systematically in a fixed pattern called a raster. In color devices, an image is produced by controlling the intensity of each of three electron beams, one for each additive primary color (red, green, and blue) with a video signal as a reference.[3] In modern CRT monitors and televisions the beams are bent by magnetic deflection, using a deflection yoke. Electrostatic deflection is commonly used in oscilloscopes.[3]
A CRT is a glass envelope which is deep (i.e., long from front screen face to rear end), heavy, and fragile. The interior is evacuated to 0.01 pascals (1×10−7 atm)[4] to 0.1 micropascals (1×10−12 atm) or less,[5] to facilitate the free flight of electrons from the gun(s) to the tube's face without scattering due to collisions with air molecules. As such, handling a CRT carries the risk of violent implosion that can hurl glass at great velocity. The face is typically made of thick lead glass or special barium-strontium glass to be shatter-resistant and to block most X-ray emissions. CRTs make up most of the weight of CRT TVs and computer monitors.[6][7]
Since the mid-late 2000's, CRTs have been superseded by flat-panel display technologies such as LCD, plasma display, and OLED displays which are cheaper to manufacture and run, as well as significantly lighter and less bulky. Flat-panel displays can also be made in very large sizes whereas 40 in (100 cm) to 45 in (110 cm)[8] was about the largest size of a CRT.[9]
A CRT works by electrically heating a tungsten coil[10] which in turn heats a cathode in the rear of the CRT, causing it to emit electrons which are modulated and focused by electrodes. The electrons are steered by deflection coils or plates, and an anode accelerates them towards the phosphor-coated screen, which generates light when hit by the electrons.[11][12][13]
History
Discoveries
Cathode rays were discovered by Julius Plücker and Johann Wilhelm Hittorf.[14] Hittorf observed that some unknown rays were emitted from the cathode (negative electrode) which could cast shadows on the glowing wall of the tube, indicating the rays were traveling in straight lines. In 1890, Arthur Schuster demonstrated cathode rays could be deflected by electric fields, and William Crookes showed they could be deflected by magnetic fields. In 1897, J. J. Thomson succeeded in measuring the charge-mass-ratio of cathode rays, showing that they consisted of negatively charged particles smaller than atoms, the first "subatomic particles", which had already been named electrons by Irish physicist George Johnstone Stoney in 1891. The earliest version of the CRT was known as the "Braun tube", invented by the German physicist Ferdinand Braun in 1897.[15] It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen. Braun was the first to conceive the use of a CRT as a display device.[16]
In 1908, Alan Archibald Campbell-Swinton, fellow of the Royal Society (UK), published a letter in the scientific journal Nature, in which he described how "distant electric vision" could be achieved by using a cathode-ray tube (or "Braun" tube) as both a transmitting and receiving device.[17] He expanded on his vision in a speech given in London in 1911 and reported in The Times[18] and the Journal of the Röntgen Society.[19][20]
The first cathode-ray tube to use a hot cathode was developed by John Bertrand Johnson (who gave his name to the term Johnson noise) and Harry Weiner Weinhart of Western Electric, and became a commercial product in 1922.[21] The introduction of hot cathodes allowed for lower acceleration anode voltages and higher electron beam currents, since the anode now only accelerated the electrons emitted by the hot cathode, and no longer had to have a very high voltage to induce electron emission from the cold cathode.[22]
Development
In 1926, Kenjiro Takayanagi demonstrated a CRT television receiver with a mechanical video camera that received images with a 40-line resolution.[23] By 1927, he improved the resolution to 100 lines, which was unrivaled until 1931.[24] By 1928, he was the first to transmit human faces in half-tones on a CRT display.[25] In 1927, Philo Farnsworth created a television prototype.[26][27][28][29][30] The CRT was named in 1929 by inventor Vladimir K. Zworykin.[25]: 84 RCA was granted a trademark for the term (for its cathode-ray tube) in 1932; it voluntarily released the term to the public domain in 1950.[31]
In the 1930s, Allen B. DuMont made the first CRTs to last 1,000 hours of use, which was one of the factors that led to the widespread adoption of television.[32]
The first commercially made electronic television sets with cathode-ray tubes were manufactured by Telefunken in Germany in 1934.[33][34]
In 1947, the cathode-ray tube amusement device, the earliest known interactive electronic game as well as the first to incorporate a cathode-ray tube screen, was created.[35]
From 1949 to the early 1960s, there was a shift from circular CRTs to rectangular CRTs, although the first rectangular CRTs were made in 1938 by Telefunken.[36][22][37][38][39][40] While circular CRTs were the norm, European TV sets often blocked portions of the screen to make it appear somewhat rectangular while American sets often left the entire front of the CRT exposed or only blocked the upper and lower portions of the CRT.[41][42]
In 1954, RCA produced some of the first color CRTs, the 15GP22 CRTs used in the CT-100,[43] the first color TV set to be mass-produced.[44] The first rectangular color CRTs were also made in 1954.[45][46] However, the first rectangular color CRTs to be offered to the public were made in 1963. One of the challenges that had to be solved to produce the rectangular color CRT was convergence at the corners of the CRT.[39][38] In 1965, brighter rare earth phosphors began replacing dimmer and cadmium-containing red and green phosphors. Eventually blue phosphors were replaced as well.[47][48][49][50][51][52]
The size of CRTs increased over time, from 20 inches in 1938,[53] to 21 inches in 1955,[54][55] 35 inches by 1985,[56] and 43 inches by 1989.[57] However, experimental 31 inch CRTs were made as far back as 1938.[58]
In 1960, the Aiken tube was invented. It was a CRT in a flat-panel display format with a single electron gun.[59][60] Deflection was electrostatic and magnetic, but due to patent problems, it was never put into production. It was also envisioned as a head-up display in aircraft.[61] By the time patent issues were solved, RCA had already invested heavily in conventional CRTs.[62]
1968 marks the release of Sony Trinitron brand with the model KV-1310, which was based on Aperture Grille technology. It was acclaimed to have improved the output brightness. The Trinitron screen was identical with its upright cylindrical shape due to its unique triple cathode single gun construction.
In 1987, flat-screen CRTs were developed by Zenith for computer monitors, reducing reflections and helping increase image contrast and brightness.[63][64] Such CRTs were expensive, which limited their use to computer monitors.[65] Attempts were made to produce flat-screen CRTs using inexpensive and widely available float glass.[66]
In 1990, the first CRTs with HD resolution were released to the market by Sony.[67]
In the mid-1990s, some 160 million CRTs were made per year.[68]
In the mid-2000s, Canon and Sony presented the surface-conduction electron-emitter display and field-emission displays, respectively. They both were flat-panel displays that had one (SED) or several (FED) electron emitters per subpixel in place of electron guns. The electron emitters were placed on a sheet of glass and the electrons were accelerated to a nearby sheet of glass with phosphors using an anode voltage. The electrons were not focused, making each subpixel essentially a flood beam CRT. They were never put into mass production as LCD technology was significantly cheaper, eliminating the market for such displays.[69]
The last large-scale manufacturer of (in this case, recycled)[70] CRTs, Videocon, ceased in 2015.[71][72] CRT TVs stopped being made around the same time.[73]
In 2015, several CRT manufacturers were convicted in the US for price fixing. The same occurred in Canada in 2018.[74][75]
Worldwide sales of CRT computer monitors peaked in 2000, at 90 million units, while those of CRT TVs peaked in 2005 at 130 million units.[76]
Decline
Beginning in the late 90s to the early 2000s, CRTs began to be replaced with LCDs, starting first with computer monitors smaller than 15 inches in size,[77] largely because of their lower bulk.[78] Among the first manufacturers to stop CRT production was Hitachi in 2001,[79][80] followed by Sony in Japan in 2004,[81] Flat-panel displays dropped in price and started significantly displacing cathode-ray tubes in the 2000s. LCD monitor sales began exceeding those of CRTs in 2003–2004[82][83][84] and LCD TV sales started exceeding those of CRTs in some markets in 2005.[85]
Despite being a mainstay of display technology for decades, CRT-based computer monitors and televisions are now virtually a dead technology. Demand for CRT screens dropped in the late 2000s.[86] Despite efforts from Samsung and LG to make CRTs competitive with their LCD and plasma counterparts, offering slimmer and cheaper models to compete with similarly sized and more expensive LCDs,[87][88][89][90][91] CRTs eventually became obsolete and were relegated to developing markets once LCDs fell in price, with their lower bulk, weight and ability to be wall mounted coming as pluses.
Some industries still use CRTs because it is either too much effort, downtime, and/or cost to replace them, or there is no substitute available; a notable example is the airline industry. Planes such as the Boeing 747-400 and the Airbus A320 used CRT instruments in their glass cockpits instead of mechanical instruments.[92] Airlines such as Lufthansa still use CRT technology, which also uses floppy disks for navigation updates.[93] They are also used in some military equipment for similar reasons.
As of 2022, at least one company manufactures new CRTs for these markets.[94]
A popular consumer usage of CRTs is for retrogaming. Some games are impossible to play without CRT display hardware, and some games play better. Reasons for this include:
- CRTs refresh faster than LCDs, because they use interlaced lines.
- CRTs are able to correctly display certain oddball resolutions, such as the 256x224 resolution of the Nintendo Entertainment System (NES).[95]
- Light guns only work on CRTs because they depend on the progressive timing properties of CRTs.
Construction
Body
The body of a CRT is usually made up of three parts: A screen/faceplate/panel, a cone/funnel, and a neck.[96][97][98][99][100] The joined screen, funnel and neck are known as the bulb or envelope.[38]
The neck is made from a glass tube[101] while the funnel and screen are made by pouring and then pressing glass into a mold.[102][103][104][105][106] The glass, known as CRT glass[107][108] or TV glass,[109] needs special properties to shield against x-rays while providing adequate light transmission in the screen or being very electrically insulating in the funnel and neck. The formulation that gives the glass its properties is also known as the melt. The glass is of very high quality, being almost contaminant and defect free. Most of the costs associated with glass production come from the energy used to melt the raw materials into glass. Glass furnaces for CRT glass production have several taps to allow molds to be replaced without stopping the furnace, to allow production of CRTs of several sizes. Only the glass used on the screen needs to have precise optical properties. The optical properties of the glass used on the screen affects color reproduction and purity in Color CRTs. Transmittance, or how transparent the glass is, may be adjusted to be more transparent to certain colors (wavelengths) of light. Transmittance is measured at the center of the screen with a 546 nm wavelength light, and a 10.16mm thick screen. Transmittance goes down with increasing thickness. Standard transmittances for Color CRT screens are 86%, 73%, 57%, 46%, 42% and 30%. Lower transmittances are used to improve image contrast but they put more stress on the electron gun, requiring more power on the electron gun for a higher electron beam power to light the phosphors more brightly to compensate for the reduced transmittance.[65][110] The transmittance must be uniform across the screen to ensure color purity. The radius (curvature) of screens has increased (grown less curved) over time, from 30 to 68 inches, ultimately evolving into completely flat screens, reducing reflections. The thickness of both curved[111] and flat screens gradually increases from the center outwards, and with it, transmittance is gradually reduced. This means that flat-screen CRTs may not be completely flat on the inside.[111][112] The glass used in CRTs arrives from the glass factory to the CRT factory as either separate screens and funnels with fused necks, for Color CRTs, or as bulbs made up of a fused screen, funnel and neck. There were several glass formulations for different types of CRTs, that were classified using codes specific to each glass manufacturer. The compositions of the melts were also specific to each manufacturer.[113] Those optimized for high color purity and contrast were doped with Neodymium, while those for monochrome CRTs were tinted to differing levels, depending on the formulation used and had transmittances of 42% or 30%.[114] Purity is ensuring that the correct colors are activated (for example, ensuring that red is displayed uniformly across the screen) while convergence ensures that images are not distorted. Convergence may be modified using a cross hatch pattern.[115][116][117]
CRT glass used to be made by dedicated companies[118] such as AGC Inc.,[119][120][121] O-I Glass,[122] Samsung Corning Precision Materials,[123] Corning Inc.,[124][125] and Nippon Electric Glass;[126] others such as Videocon, Sony for the US market and Thomson made their own glass.[127][128][129][130][131]
The funnel and the neck are made of leaded potash-soda glass or lead silicate glass[7] formulation to shield against x-rays generated by high voltage electrons as they decelerate after striking a target, such as the phosphor screen or shadow mask of a color CRT. The velocity of the electrons depends on the anode voltage of the CRT; the higher the voltage, the higher the speed.[132] The amount of x-rays emitted by a CRT can also lowered by reducing the brightness of the image.[133][134][135][99] Leaded glass is used because it is inexpensive, while also shielding heavily against x-rays, although some funnels may also contain barium.[136][137][138][114] The screen is usually instead made out of a special lead-free silicate[7] glass formulation with barium and strontium to shield against x-rays. Another glass formulation uses 2-3% of lead on the screen.[99] Monochrome CRTs may have a tinted barium-lead glass formulation in both the screen and funnel, with a potash-soda lead glass in the neck; the potash-soda and barium-lead formulations have different thermal expansion coefficients. The glass used in the neck must be an excellent electrical insulator to contain the voltages used in the electron optics of the electron gun, such as focusing lenses. The lead in the glass causes it to brown (darken) with use due to x-rays, usually the CRT cathode wears out due to cathode poisoning before browning becomes apparent. The glass formulation determines the highest possible anode voltage and hence the maximum possible CRT screen size. For color, maximum voltages are often 24 to 32 kV, while for monochrome it is usually 21 or 24.5 kV,[139] limiting the size of monochrome CRTs to 21 inches, or approx. 1 kV per inch. The voltage needed depends on the size and type of CRT.[140] Since the formulations are different, they must be compatible with one another, having similar thermal expansion coefficients.[114] The screen may also have an anti-glare or anti-reflective coating,[141][110][142] or be ground to prevent reflections.[143] CRTs may also have an anti-static coating.[110][144][65]
The leaded glass in the funnels of CRTs may contain 21 to 25% of lead oxide (PbO),[145][146][113] The neck may contain 30 to 40% of lead oxide,[147][148] and the screen may contain 12% of barium oxide, and 12% of strontium oxide.[7] A typical CRT contains several kilograms of lead as lead oxide in the glass[100] depending on its size; 12 inch CRTs contain 0.5 kg of lead in total while 32 inch CRTs contain up to 3 kg.[7] Strontium oxide began being used in CRTs, its major application, in the 1970s.[149][150][151]
Some early CRTs used a metal funnel insulated with polyethylene instead of glass with conductive material.[54] Others had ceramic or blown pyrex instead of pressed glass funnels.[152][153][40][154][155] Early CRTs did not have a dedicated anode cap connection; the funnel was the anode connection, so it was live during operation.[156]
The funnel is coated on the inside and outside with a conductive coating,[157][158] making the funnel a capacitor, helping stabilize and filter the anode voltage of the CRT, and significantly reducing the amount of time needed to turn on a CRT. The stability provided by the coating solved problems inherent to early power supply designs, as they used vacuum tubes. Because the funnel is used as a capacitor, the glass used in the funnel must be an excellent electrical insulator (dielectric). The inner coating has a positive voltage (the anode voltage that can be several kV) while the outer coating is connected to ground. CRTs powered by more modern power supplies do not need to be connected to ground, due to the more robust design of modern power supplies. The value of the capacitor formed by the funnel is .005-.01uF, although at the voltage the anode is normally supplied with. The capacitor formed by the funnel can also suffer from dielectric absorption, similarly to other types of capacitors.[159][139][160][161][157][114] Because of this CRTs have to be discharged[162] before handling to prevent injury.
The depth of a CRT is related to its screen size.[163] Usual deflection angles were 90° for computer monitor CRTs and small CRTs and 110° which was the standard in larger TV CRTs, with 120 or 125° being used in slim CRTs made since 2001–2005 in an attempt to compete with LCD TVs. [164][110][90][98][165] Over time, deflection angles increased as they became practical, from 50° in 1938 to 110° in 1959,[22] and 125° in the 2000s. 140° deflection CRTs were researched but never commercialized, as convergence problems were never resolved.[166]
A monochrome CRT with 110° deflection
A monochrome CRT with 90° deflection
Size and weight
The size of the screen of a CRT is measured in two ways: the size of the screen or the face diagonal, and the viewable image size/area or viewable screen diagonal, which is the part of the screen with phosphor. The size of the screen is the viewable image size plus its black edges which are not coated with phosphor.[167][158][168] The viewable image may be perfectly square or rectangular while the edges of the CRT are black and have a curvature (such as in black stripe CRTs) or the edges may be black and truly flat (such as in Flatron CRTs),[111][131][169] or the edges of the image may follow the curvature of the edges of the CRT, which may be the case in CRTs without and with black edges and curved edges.[170][171][172] Black stripe CRTs were first made by Toshiba in 1972.[131]
Small CRTs below 3 inches were made for handheld televisions such as the MTV-1 and viewfinders in camcorders. In these, there may be no black edges, that are however truly flat.[173][160][174][175][176]
Most of the weight of a CRT comes from the thick glass screen, which comprises 65% of the total weight of a CRT. The funnel and neck glass comprise the remaining 30% and 5% respectively. The glass in the funnel is thinner than on the screen.[7][6] Chemically or thermally tempered glass may be used to reduce the weight of the CRT glass.[177][178][179][180]
Anode
The outer conductive coating is connected to ground while the inner conductive coating is connected using the anode button/cap through a series of capacitors and diodes (a Cockcroft–Walton generator) to the high voltage flyback transformer; the inner coating is the anode of the CRT,[181] which, together with an electrode in the electron gun, is also known as the final anode.[182][183] The inner coating is connected to the electrode using springs. The electrode forms part of a bipotential lens.[183][184] The capacitors and diodes serve as a voltage multiplier for the current delivered by the flyback.
For the inner funnel coating, monochrome CRTs use aluminum while color CRTs use aquadag;[114] Some CRTs may use iron oxide on the inside.[7] On the outside, most CRTs (but not all)[185] use aquadag.[186] Aquadag is an electrically conductive graphite-based paint. In color CRTs, the aquadag is sprayed onto the interior of the funnel[187][114] whereas historically aquadag was painted into the interior of monochrome CRTs.[22]
The anode is used to accelerate the electrons towards the screen and also collects the secondary electrons that are emitted by the phosphor particles in the vacuum of the CRT.[188][189][190][191][22]
The anode cap connection in modern CRTs must be able to handle up to 55–60 kV depending on the size and brightness of the CRT. Higher voltages allow for larger CRTs, higher image brightness, or a tradeoff between the two.[192][140] It consists of a metal clip that expands on the inside of an anode button that is embedded on the funnel glass of the CRT.[193][194] The connection is insulated by a silicone suction cup, possibly also using silicone grease to prevent corona discharge.[195][196]
The anode button must be specially shaped to establish a hermetic seal between the button and funnel. X-rays may leak through the anode button, although that may not be the case in newer CRTs starting from the late 1970s to early 1980s, thanks to a new button and clip design.[140] The button may consist of a set of 3 nested cups, with the outermost cup being made of a Nickel–Chromium–Iron alloy containing 40 to 49% of Nickel and 3 to 6% of Chromium to make the button easy to fuse to the funnel glass, with a first inner cup made of thick inexpensive iron to shield against x-rays, and with the second innermost cup also being made of iron or any other electrically conductive metal to connect to the clip. The cups must be heat resistant enough and have similar thermal expansion coefficients similar to that of the funnel glass to withstand being fused to the funnel glass. The inner side of the button is connected to the inner conductive coating of the CRT.[189] The anode button may be attached to the funnel while its being pressed into shape in a mold.[197][198][140] Alternatively, the x-ray shielding may instead be built into the clip.[199]
The flyback transformer is also known as an IHVT (Integrated High Voltage Transformer) if it includes a voltage multiplier. The flyback uses a ceramic or powdered iron core to enable efficient operation at high frequencies. The flyback contains one primary and many secondary windings that provide several different voltages. The main secondary winding supplies the voltage multiplier with voltage pulses to ultimately supply the CRT with the high anode voltage it uses, while the remaining windings supply the CRT's filament voltage, keying pulses, focus voltage and voltages derived from the scan raster. When the transformer is turned off, the flyback's magnetic field quickly collapses which induces high voltage in its windings. The speed at which the magnetic field collapses determines the voltage that is induced, so the voltage increases alongside its speed. A capacitor (Retrace Timing Capacitor) or series of capacitors (to provide redundancy) is used to slow the collapse of the magnetic field.[200][201]
The design of the high voltage power supply in a product using a CRT has an influence in the amount of x-rays emitted by the CRT. The amount of emitted x-rays increases with both higher voltages and currents. If the product such as a TV set uses an unregulated high voltage power supply, meaning that anode and focus voltage go down with increasing electron current when displaying a bright image, the amount of emitted x-rays is as its highest when the CRT is displaying a moderately bright images, since when displaying dark or bright images, the higher anode voltage counteracts the lower electron beam current and vice versa respectively. The high voltage regulator and rectifier vacuum tubes in some old CRT TV sets may also emit x-rays.[202]
Electron gun
The electron gun emits the electrons that ultimately hit the phosphors on the screen of the CRT. The electron gun contains a heater, which heats a cathode, which generates electrons that, using grids, are focused and ultimately accelerated into the screen of the CRT. The acceleration occurs in conjunction with the inner aluminum or aquadag coating of the CRT. The electron gun is positioned so that it aims at the center of the screen.[183] It is inside the neck of the CRT, and it is held together and mounted to the neck using glass beads or glass support rods, which are the glass strips on the electron gun.[22][183][203] The electron gun is made separately and then placed inside the neck through a process called "winding", or sealing.[66][204][205][206][207][208] The electron gun has a glass wafer that is fused to the neck of the CRT. The connections to the electron gun penetrate the glass wafer.[205][209] Once the electron gun is inside the neck, its metal parts (grids) are arced between each other using high voltage to smooth any rough edges in a process called spot knocking, to prevent the rough edges in the grids from generating secondary electrons.[210][211][212]
Construction and method of operation
It has a hot cathode that is heated by a tungsten filament heating element; the heater may draw 0.5 to 2 A of current depending on the CRT. The voltage applied to the heater can affect the life of the CRT.[213][214] Heating the cathode energizes the electrons in it, aiding electron emission,[215] while at the same time current is supplied to the cathode; typically anywhere from 140 mA at 1.5 V to 600 mA at 6.3 V.[216] The cathode creates an electron cloud (emits electrons) whose electrons are extracted, accelerated and focused into an electron beam.[22] Color CRTs have three cathodes: one for red, green and blue. The heater sits inside the cathode but does not touch it; the cathode has its own separate electrical connection. The cathode is coated onto a piece of nickel which provides the electrical connection and structural support; the heater sits inside this piece without touching it.[181][217][218][219]
There are several shortcircuits that can occur in a CRT electron gun. One is a heater-to-cathode short, that causes the cathode to permanently emit electrons which may cause an image with a bright red, green or blue tint with retrace lines, depending on the cathode (s) affected. Alternatively, the cathode may short to the control grid, possibly causing similar effects, or, the control grid and screen grid (G2)[220] can short causing a very dark image or no image at all. The cathode may be surrounded by a shield to prevent sputtering.[221][222]
The cathode is a layer of barium oxide which is coated on a piece of nickel for electrical and mechanical support.[223][139] The barium oxide must be activated by heating to enable it to release electrons. Activation is necessary because barium oxide is not stable in air, so it is applied to the cathode as barium carbonate, which cannot emit electrons. Activation heats the barium carbonate to decompose it into barium oxide and carbon dioxide while forming a thin layer of metallic barium on the cathode.[224][223] Activation occurs during evacuation of (at the same time a vacuum is formed in) the CRT. After activation the oxide can become damaged by several common gases such as water vapor, carbon dioxide, and oxygen.[225] Alternatively, barium strontium calcium carbonate may be used instead of barium carbonate, yielding barium, strontium and calcium oxides after activation.[226][22] During operation, the barium oxide is heated to 800-1000°C, at which point it starts shedding electrons.[227][139][215]
Since it is a hot cathode, it is prone to cathode poisoning, which is the formation of a positive ion layer that prevents the cathode from emitting electrons, reducing image brightness significantly or completely and causing focus and intensity to be affected by the frequency of the video signal preventing detailed images from being displayed by the CRT. The positive ions come from leftover air molecules inside the CRT or from the cathode itself[22] that react over time with the surface of the hot cathode.[228][222] Reducing metals such as manganese, zirconium, magnesium, aluminum or titanium may be added to the piece of nickel to lengthen the life of the cathode, as during activation, the reducing metals diffuse into the barium oxide, improving its lifespan, especially at high electron beam currents.[229] In color CRTs with red, green and blue cathodes, one or more cathodes may be affected independently of the others, causing total or partial loss of one or more colors.[222] CRTs can wear or burn out due to cathode poisoning. Cathode poisoning is accelerated by increased cathode current (overdriving).[230] In color CRTs, since there are three cathodes, one for red, green and blue, a single or more poisoned cathode may cause the partial or complete loss of one or more colors, tinting the image.[222] The layer may also act as a capacitor in series with the cathode, inducing thermal lag. The cathode may instead be made of scandium oxide or incorporate it as a dopant, to delay cathode poisoning, extending the life of the cathode by up to 15%.[231][139][232]
The amount of electrons generated by the cathodes is related to their surface area. A cathode with more surface area creates more electrons, in a larger electron cloud, which makes focusing the electron cloud into an electron beam more difficult.[230] Normally, only a part of the cathode emits electrons unless the CRT displays images with parts that are at full image brightness; only the parts at full brightness cause all of the cathode to emit electrons. The area of the cathode that emits electrons grows from the center outwards as brightness increases, so cathode wear may be uneven. When only the center of the cathode is worn, the CRT may light brightly those parts of images that have full image brightness but not show darker parts of images at all, in such a case the CRT displays a poor gamma characteristic.[222]
The second (screen) grid of the gun (G2) accelerates the electrons towards the screen using several hundred DC volts. A negative current[233] is applied to the first (control) grid (G1) to converge the electron beam. G1 in practice is a Wehnelt cylinder.[216][234] The brightness of the screen is not controlled by varying the anode voltage nor the electron beam current (they are never varied) despite them having an influence on image brightness, rather image brightness is controlled by varying the difference in voltage between the cathode and the G1 control grid. A third grid (G3) electrostatically focuses the electron beam before it is deflected and accelerated by the anode voltage onto the screen.[235] Electrostatic focusing of the electron beam may be accomplished using an Einzel lens energized at up to 600 volts.[236][224] Before electrostatic focusing, focusing the electron beam required a large, heavy and complex mechanical focusing system placed outside the electron gun.[156]
However, electrostatic focusing cannot be accomplished near the final anode of the CRT due to its high voltage in the dozens of Kilovolts, so a high voltage (≈600[237] to 8000 volt) electrode, together with an electrode at the final anode voltage of the CRT, may be used for focusing instead. Such an arrangement is called a bipotential lens, which also offers higher performance than an Einzel lens, or, focusing may be accomplished using a magnetic focusing coil together with a high anode voltage of dozens of kilovolts. However, magnetic focusing is expensive to implement, so it is rarely used in practice.[181][224][238][239] Some CRTs may use two grids and lenses to focus the electron beam.[231] The focus voltage is generated in the flyback using a subset of the flyback's high voltage winding in conjunction with a resistive voltage divider. The focus electrode is connected alongside the other connections that are in the neck of the CRT.[240]
There is a voltage called cutoff voltage which is the voltage that creates black on the screen since it causes the image on the screen created by the electron beam to disappear, the voltage is applied to G1. In a color CRT with three guns, the guns have different cutoff voltages. Many CRTs share grid G1 and G2 across all three guns, increasing image brightness and simplifying adjustment since on such CRTs there is a single cutoff voltage for all three guns (since G1 is shared across all guns).[183] but placing additional stress on the video amplifier used to feed video into the electron gun's cathodes, since the cutoff voltage becomes higher. Monochrome CRTs do not suffer from this problem. In monochrome CRTs video is fed to the gun by varying the voltage on the first control grid.[241][156]
During retracing of the electron beam, the preamplifier that feeds the video amplifier is disabled and the video amplifier is biased to a voltage higher than the cutoff voltage to prevent retrace lines from showing, or G1 can have a large negative voltage applied to it to prevent electrons from getting out of the cathode.[22] This is known as blanking. (see Vertical blanking interval and Horizontal blanking interval.) Incorrect biasing can lead to visible retrace lines on one or more colors, creating retrace lines that are tinted or white (for example, tinted red if the red color is affected, tinted magenta if the red and blue colors are affected, and white if all colors are affected).[242][243][244] Alternatively, the amplifier may be driven by a video processor that also introduces an OSD (On Screen Display) into the video stream that is fed into the amplifier, using a fast blanking signal.[245] TV sets and computer monitors that incorporate CRTs need a DC restoration circuit to provide a video signal to the CRT with a DC component, restoring the original brightness of different parts of the image.[246]
The electron beam may be affected by the earth's magnetic field, causing it to normally enter the focusing lens off-center; this can be corrected using astigmation controls. Astigmation controls are both magnetic and electronic (dynamic); magnetic does most of the work while electronic is used for fine adjustments.[247] One of the ends of the electron gun has a glass disk, the edges of which are fused with the edge of the neck of the CRT, possibly using frit;[248] the metal leads that connect the electron gun to the outside pass through the disk.[249]
Some electron guns have a quadrupole lens with dynamic focus to alter the shape and adjust the focus of the electron beam, varying the focus voltage depending on the position of the electron beam to maintain image sharpness across the entire screen, specially at the corners.[110][250][251][252][253] They may also have a bleeder resistor to derive voltages for the grids from the final anode voltage.[254][255][256]
After the CRTs were manufactured, they were aged to allow cathode emission to stabilize.[257][258]
The electron guns in color CRTs are driven by a video amplifier which takes a signal per color channel and amplifies it to 40-170v per channel, to be fed into the electron gun's cathodes;[244] each electron gun has its own channel (one per color) and all channels may be driven by the same amplifier, which internally has three separate channels.[259] The amplifier's capabilities limit the resolution, refresh rate and contrast ratio of the CRT, as the amplifier needs to provide high bandwidth and voltage variations at the same time; higher resolutions and refresh rates need higher bandwidths (speed at which voltage can be varied and thus switching between black and white) and higher contrast ratios need higher voltage variations or amplitude for lower black and higher white levels. 30Mhz of bandwidth can usually provide 720p or 1080i resolution, while 20Mhz usually provides around 600 (horizontal, from top to bottom) lines of resolution, for example.[260][244] The difference in voltage between the cathode and the control grid is what modulates the electron beam, modulating its current and thus the brightness of the image.[222] The phosphors used in color CRTs produce different amounts of light for a given amount of energy, so to produce white on a color CRT, all three guns must output differing amounts of energy. The gun that outputs the most energy is the red gun since the red phosphor emits the least amount of light.[244]
Gamma
CRTs have a pronounced triode characteristic, which results in significant gamma (a nonlinear relationship in an electron gun between applied video voltage and beam intensity).[261]
Deflection
There are two types of deflection: magnetic and electrostatic. Magnetic is usually used in TVs and monitors as it allows for higher deflection angles (and hence shallower CRTs) and deflection power (which allows for higher electron beam current and hence brighter images)[262] while avoiding the need for high voltages for deflection of up to 2000 volts,[165] while oscilloscopes often use electrostatic deflection since the raw waveforms captured by the oscilloscope can be applied directly (after amplification) to the vertical electrostatic deflection plates inside the CRT.[263]
Magnetic deflection
Those that use magnetic deflection may use a yoke that has two pairs of deflection coils; one pair for vertical, and another for horizontal deflection.[264] The yoke can be bonded (be integral) or removable. Those that were bonded used glue[265] or a plastic[266] to bond the yoke to the area between the neck and the funnel of the CRT while those with removable yokes are clamped.[267][116] The yoke generates heat whose removal is essential since the conductivity of glass goes up with increasing temperature, the glass needs to be insulating for the CRT to remain usable as a capacitor. The temperature of the glass below the yoke is thus checked during the design of a new yoke.[139] The yoke contains the deflection and convergence coils with a ferrite core to reduce loss of magnetic force[268][264] as well as the magnetized rings used to align or adjust the electron beams in color CRTs (The color purity and convergence rings, for example)[269] and monochrome CRTs.[270][271] The yoke may be connected using a connector, the order in which the deflection coils of the yoke are connected determines the orientation of the image displayed by the CRT.[162] The deflection coils may be held in place using polyurethane glue.[265]
The deflection coils are driven by sawtooth signals[272][273][244] that may be delivered through VGA as horizontal and vertical sync signals.[274] A CRT needs two deflection circuits: a horizontal and a vertical circuit, which are similar except that the horizontal circuit runs at a much higher frequency (a Horizontal scan rate) of 15 to 240 kHz depending on the refresh rate of the CRT and the number of horizontal lines to be drawn (the vertical resolution of the CRT). The higher frequency makes it more susceptible to interference, so an automatic frequency control (AFC) circuit may be used to lock the phase of the horizontal deflection signal to that of a sync signal, to prevent the image from becoming distorted diagonally. The vertical frequency varies according to the refresh rate of the CRT. So a CRT with a 60 Hz refresh rate has a vertical deflection circuit running at 60 Hz. The horizontal and vertical deflection signals may be generated using two circuits that work differently; the horizontal deflection signal may be generated using a voltage controlled oscillator (VCO) while the vertical signal may be generated using a triggered relaxation oscillator. In many TVs, the frequencies at which the deflection coils run is in part determined by the inductance value of the coils.[275][244] CRTs had differing deflection angles; the higher the deflection angle, the shallower the CRT[276] for a given screen size, but at the cost of more deflection power and lower optical performance.[139][277]
Higher deflection power means more current[278] is sent to the deflection coils to bend the electron beam at a higher angle,[110] which in turn may generate more heat or require electronics that can handle the increased power.[277] Heat is generated due to resistive and core losses.[279] The deflection power is measured in mA per inch.[244] The vertical deflection coils may require approximately 24 volts while the horizontal deflection coils require approx. 120 volts to operate.
The deflection coils are driven by deflection amplifiers.[280] The horizontal deflection coils may also be driven in part by the horizontal output stage of a TV set. The stage contains a capacitor that is in series with the horizontal deflection coils that performs several functions, among them are: shaping the sawtooth deflection signal to match the curvature of the CRT and centering the image by preventing a DC bias from developing on the coil. At the beginning of retrace, the magnetic field of the coil collapses, causing the electron beam to return to the center of the screen, while at the same time the coil returns energy into capacitors, the energy of which is then used to force the electron beam to go to the left of the screen. [200]
Due to the high frequency at which the horizontal deflection coils operate, the energy in the deflection coils must be recycled to reduce heat dissipation. Recycling is done by transferring the energy in the deflection coils' magnetic field to a set of capacitors.[200] The voltage on the horizontal deflection coils is negative when the electron beam is on the left side of the screen and positive when the electron beam is on the right side of the screen. The energy required for deflection is dependent on the energy of the electrons.[281] Higher energy (voltage and/or current) electron beams need more energy to be deflected,[132] and are used to achieve higher image brightness.[282][283][192]
Electrostatic deflection
Mostly used in oscilloscopes. Deflection is carried out by applying a voltage across two pairs of plates, one for horizontal, and the other for vertical deflection. The electron beam is steered by varying the voltage difference across plates in a pair; For example, applying a voltage to the upper plate of the vertical deflection pair, while keeping the voltage in the bottom plate at 0 volts, will cause the electron beam to be deflected towards the upper part of the screen; increasing the voltage in the upper plate while keeping the bottom plate at 0 will cause the electron beam to be deflected to a higher point in the screen (will cause the beam to be deflected at a higher deflection angle). The same applies with the horizontal deflection plates. Increasing the length and proximity between plates in a pair can also increase the deflection angle.[284]
Burn-in
Burn-in is when images are physically "burned" into the screen of the CRT; this occurs due to degradation of the phosphors due to prolonged electron bombardment of the phosphors, and happens when a fixed image or logo is left for too long on the screen, causing it to appear as a "ghost" image or, in severe cases, also when the CRT is off. To counter this, screensavers were used in computers to minimize burn-in.[285] Burn-in is not exclusive to CRTs, as it also happens to plasma displays and OLED displays.
Evacuation
CRTs are evacuated or exhausted (a vacuum is formed) inside an oven at approx. 375–475 °C, in a process called baking or bake-out.[286] The evacuation process also outgasses any materials inside the CRT, while decomposing others such as the polyvinyl alcohol used to apply the phosphors.[287] The heating and cooling are done gradually to avoid inducing stress, stiffening and possibly cracking the glass; the oven heats the gases inside the CRT, increasing the speed of the gas molecules which increases the chances of them getting drawn out by the vacuum pump. The temperature of the CRT is kept to below that of the oven, and the oven starts to cool just after the CRT reaches 400 °C, or, the CRT was kept at a temperature higher than 400 °C for up to 15–55 minutes. The CRT was heated during or after evacuation, and the heat may have been used simultaneously to melt the frit in the CRT, joining the screen and funnel.[288][289][290] The pump used is a turbomolecular pump or a diffusion pump.[291][292][293][294] Formerly mercury vacuum pumps were also used.[295][296] After baking, the CRT is disconnected ("sealed or tipped off") from the vacuum pump.[297][298][299] The getter is then fired using an RF (induction) coil. The getter is usually in the funnel or in the neck of the CRT.[300][301] The getter material which is often barium-based, catches any remaining gas particles as it evaporates due to heating induced by the RF coil (that may be combined with exothermic heating within the material); the vapor fills the CRT, trapping any gas molecules that it encounters and condenses on the inside of the CRT forming a layer that contains trapped gas molecules. Hydrogen may be present in the material to help distribute the barium vapor. The material is heated to temperatures above 1000 °C, causing it to evaporate.[302][303][225] Partial loss of vacuum in a CRT can result in a hazy image, blue glowing in the neck of the CRT, flashovers, loss of cathode emission or focusing problems.[156] The vacuum inside of a CRT causes atmospheric pressure to exert (in a 27-inch CRT) a pressure of 5,800 pounds (2,600 kg) in total.[304]
Rebuilding
CRTs used to be rebuilt; repaired or refurbished. The rebuilding process included the disassembly of the CRT, the disassembly and repair or replacement of the electron gun(s), the removal and redeposition of phosphors and aquadag, etc. Rebuilding was popular until the 1960s because CRTs were expensive and wore out quickly, making repair worth it.[300] The last CRT rebuilder in the US closed in 2010,[305] and the last in Europe, RACS, which was located in France, closed in 2013.[306]
Reactivation
Also known as rejuvenation, the goal is to temporarily restore the brightness of a worn CRT. This is often done by carefully increasing the voltage on the cathode heater and the current and voltage on the control grids of the electron gun manually[citation needed]. Some rejuvenators can also fix heater-to-cathode shorts by running a capacitive discharge through the short.[222]
Phosphors
Phosphors in CRTs emit secondary electrons due to them being inside the vacuum of the CRT. The secondary electrons are collected by the anode of the CRT.[191] Secondary electrons generated by phosphors need to be collected to prevent charges from developing in the screen, which would lead to reduced image brightness[22] since the charge would repel the electron beam.
The phosphors used in CRTs often contain rare earth metals,[307][308][285] replacing earlier dimmer phosphors. Early red and green phosphors contained Cadmium,[309] and some black and white CRT phosphors also contained beryllium in the form of Zinc beryllium silicate,[50] although white phosphors containing cadmium, zinc and magnesium with silver, copper or manganese as dopants were also used.[22] The rare earth phosphors used in CRTs are more efficient (produce more light) than earlier phosphors.[310] The phosphors adhere to the screen because of Van der Waals and electrostatic forces. Phosphors composed of smaller particles adhere more strongly to the screen. The phosphors together with the carbon used to prevent light bleeding (in color CRTs) can be easily removed by scratching.[136][311]
Several dozen types of phosphors were available for CRTs.[312] Phosphors were classified according to color, persistence, luminance rise and fall curves, color depending on anode voltage (for phosphors used in penetration CRTs), Intended use, chemical composition, safety, sensitivity to burn-in, and secondary emission properties.[313] Examples of rare earth phosphors are yittrium oxide for red[314] and yittrium silicide for blue,[citation needed] while examples of earlier phosphors are copper cadmium sulfide for red,
SMPTE-C phosphors have properties defined by the SMPTE-C standard, which defines a color space of the same name. The standard prioritizes accurate color reproduction, which was made difficult by the different phosphors and color spaces used in the NTSC and PAL color systems. PAL TV sets have subjectively better color reproduction due to the use of saturated green phosphors, which have relatively long decay times that are tolerated in PAL since there is more time in PAL for phosphors to decay, due to its lower framerate. SMPTE-C phosphors were used in professional video monitors.[315][316]
The phosphor coating on monochrome and color CRTs may have an aluminum coating on its rear side used to reflect light forward, provide protection against ions to prevent ion burn by negative ions on the phosphor, manage heat generated by electrons colliding against the phosphor,[317] prevent static build up that could repel electrons from the screen, form part of the anode and collect the secondary electrons generated by the phosphors in the screen after being hit by the electron beam, providing the electrons with a return path.[318][139][319][317][22] The electron beam passes through the aluminum coating before hitting the phosphors on the screen; the aluminum attenuates the electron beam voltage by about 1 kv.[320][22][313] A film or lacquer may be applied to the phosphors to reduce the surface roughness of the surface formed by the phosphors to allow the aluminum coating to have a uniform surface and prevent it from touching the glass of the screen.[321][322] This is known as filming.[172] The lacquer contains solvents that are later evaporated; the lacquer may be chemically roughened to cause an aluminum coating with holes to be created to allow the solvents to escape.[322]
Phosphor persistence
Various phosphors are available depending upon the needs of the measurement or display application. The brightness, color, and persistence of the illumination depends upon the type of phosphor used on the CRT screen. Phosphors are available with persistences ranging from less than one microsecond to several seconds.[323] For visual observation of brief transient events, a long persistence phosphor may be desirable. For events which are fast and repetitive, or high frequency, a short-persistence phosphor is generally preferable.[324] The phosphor persistence must be low enough to avoid smearing or ghosting artifacts at high refresh rates.[110]
Limitations and workarounds
Blooming
Variations in anode voltage can lead to variations in brightness in parts or all of the image, in addition to blooming, shrinkage or the image getting zoomed in or out. Lower voltages lead to blooming and zooming in, while higher voltages do the opposite.[325][326] Some blooming is unavoidable, which can be seen as bright areas of an image that expand, distorting or pushing aside surrounding darker areas of the same image. Blooming occurs because bright areas have a higher electron beam current from the electron gun, making the beam wider and harder to focus. Poor voltage regulation causes focus and anode voltage to go down with increasing electron beam current.[202]
Doming
Doming is a phenomenon found on some CRT televisions in which parts of the shadow mask become heated. In televisions that exhibit this behavior, it tends to occur in high-contrast scenes in which there is a largely dark scene with one or more localized bright spots. As the electron beam hits the shadow mask in these areas it heats unevenly. The shadow mask warps due to the heat differences, which causes the electron gun to hit the wrong colored phosphors and incorrect colors to be displayed in the affected area.[327] Thermal expansion causes the shadow mask to expand by around 100 microns.[328][329][330][331]
During normal operation, the shadow mask is heated to around 80–90 °C.[332] Bright areas of images heat the shadow mask more than dark areas, leading to uneven heating of the shadow mask and warping (blooming) due to thermal expansion caused by heating by increased electron beam current.[333][334] The shadow mask is usually made of steel but it can be made of Invar[115] (a low-thermal expansion Nickel-Iron alloy) as it withstands two to three times more current than conventional masks without noticeable warping,[110][335][64] while making higher resolution CRTs easier to achieve.[336] Coatings that dissipate heat may be applied on the shadow mask to limit blooming[337][338] in a process called blackening.[339][340]
Bimetal springs may be used in CRTs used in TVs to compensate for warping that occurs as the electron beam heats the shadow mask, causing thermal expansion.[63] The shadow mask is installed to the screen using metal pieces[341] or a rail or frame[342][343][344] that is fused to the funnel or the screen glass respectively,[251] holding the shadow mask in tension to minimize warping (if the mask is flat, used in flat-screen CRT computer monitors) and allowing for higher image brightness and contrast.
Aperture grille screens are brighter since they allow more electrons through, but they require support wires. They are also more resistant to warping.[110] Color CRTs need higher anode voltages than monochrome CRTs to achieve the same brightness since the shadow mask blocks most of the electron beam. Slot masks[51] and specially Aperture grilles do not block as many electrons resulting in a brighter image for a given anode voltage, but aperture grille CRTs are heavier.[115] Shadow masks block[345] 80–85%[333][332] of the electron beam while Aperture grilles allow more electrons to pass through.[346]
High voltage
Image brightness is related to the anode voltage and to the CRTs size, so higher voltages are needed for both larger screens[347] and higher image brightness. Image brightness is also controlled by the current of the electron beam.[230] Higher anode voltages and electron beam currents also mean higher amounts of x-rays and heat generation since the electrons have a higher speed and energy.[202] Leaded glass and special barium-strontium glass are used to block most x-ray emissions.
Size
Size is limited by anode voltage, as it would require a higher dielectric strength to prevent arcing (corona discharge) and the electrical losses and ozone generation it causes, without sacrificing image brightness. The weight of the CRT, which originates from the thick glass needed to safely sustain a vacuum, imposes a practical limit on the size of a CRT.[348] The 43-inch Sony PVM-4300 CRT monitor weighs 440 pounds (200 kg).[349] Smaller CRTs weigh significantly less, as an example, 32-inch CRTs weigh up to 163 pounds (74 kg) and 19-inch CRTs weigh up to 60 pounds (27 kg). For comparison, a 32-inch flat panel TV only weighs approx. 18 pounds (8.2 kg) and a 19-inch flat panel TV weighs 6.5 pounds (2.9 kg).[350]
Shadow masks become more difficult to make with increasing resolution and size.[336]
Limits imposed by deflection
At high deflection angles, resolutions and refresh rates (since higher resolutions and refresh rates require significantly higher frequencies to be applied to the horizontal deflection coils), the deflection yoke starts to produce large amounts of heat, due to the need to move the electron beam at a higher angle, which in turn requires exponentially larger amounts of power. As an example, to increase the deflection angle from 90 to 120°, power consumption of the yoke must also go up from 40 watts to 80 watts, and to increase it further from 120 to 150°, deflection power must again go up from 80 watts to 160 watts. This normally makes CRTs that go beyond certain deflection angles, resolutions and refresh rates impractical, since the coils would generate too much heat due to resistance caused by the skin effect, surface and eddy current losses, and/or possibly causing the glass underneath the coil to become conductive (as the electrical conductivity of glass decreases with increasing temperature). Some deflection yokes are designed to dissipate the heat that comes from their operation.[114][351][279][352][353][354] Higher deflection angles in color CRTs directly affect convergence at the corners of the screen which requires additional compensation circuitry to handle electron beam power and shape, leading to higher costs and power consumption.[355][356] Higher deflection angles allow a CRT of a given size to be slimmer, however they also impose more stress on the CRT envelope, specially on the panel, the seal between the panel and funnel and on the funnel. The funnel needs to be long enough to minimize stress, as a longer funnel can be better shaped to have lower stress.[98][357]
Comparison with other technologies
- LCD advantages over CRT: Lower bulk, power consumption and heat generation, higher refresh rates (up to 360 Hz),[358] higher contrast ratios
- CRT advantages over LCD: Better color reproduction, no motion blur, multisyncing available in many monitors, no input lag[359]
- OLED advantages over CRT: Lower bulk, similar color reproduction,[359] higher contrast ratios, similar refresh rates (over 60 Hz, up to 120 Hz)[360][361][362] except for computer monitors.[363]
On CRTs, refresh rate depends on resolution, both of which are ultimately limited by the maximum horizontal scanning frequency of the CRT. Motion blur also depends on the decay time of the phosphors. Phosphors that decay too slowly for a given refresh rate may cause smearing or motion blur on the image. In practice, CRTs are limited to a refresh rate of 160 Hz.[364] LCDs that can compete with OLED (Dual Layer, and mini-LED LCDs) are not available in high refresh rates, although quantum dot LCDs (QLEDs) are available in high refresh rates (up to 144 Hz)[365] and are competitive in color reproduction with OLEDs.[366]
CRT monitors can still outperform LCD and OLED monitors in input lag, as there is no signal processing between the CRT and the display connector of the monitor, since CRT monitors often use VGA which provides an analog signal that can be fed to a CRT directly. Video cards designed for use with CRTs may have a RAMDAC to generate the analog signals needed by the CRT.[367][11] Also, CRT monitors are often capable of displaying sharp images at several resolutions, an ability known as multisyncing.[368] Due to these reasons, CRTs are sometimes preferred by PC gamers in spite of their bulk, weight and heat generation.[369][359]
CRTs tend to be more durable than their flat panel counterparts,[11] though specialised LCDs that have similar durability also exist.
Types
CRTs were produced in two major categories, picture tubes and display tubes.[68] Picture tubes were used in TVs while display tubes were used in computer monitors. Display tubes had no overscan and were of higher resolution. Picture tube CRTs have overscan, meaning the actual edges of the image are not shown; this is deliberate to allow for adjustment variations between CRT TVs, preventing the ragged edges (due to blooming) of the image from being shown on screen. The shadow mask may have grooves that reflect away the electrons that do not hit the screen due to overscan.[370][110] Color picture tubes used in TVs were also known as CPTs.[371] CRTs are also sometimes called Braun tubes.[372][373]
Monochrome CRTs
If the CRT is a black and white (B&W or monochrome) CRT, there is a single electron gun in the neck and the funnel is coated on the inside with aluminum that has been applied by evaporation; the aluminum is evaporated in a vacuum and allowed to condense on the inside of the CRT.[172] Aluminum eliminates the need for ion traps, necessary to prevent ion burn on the phosphor, while also reflecting light generated by the phosphor towards the screen, managing heat and absorbing electrons providing a return path for them; previously funnels were coated on the inside with aquadag, used because it can be applied like paint;[161] the phosphors were left uncoated.[22] Aluminum started being applied to CRTs in the 1950s, coating the inside of the CRT including the phosphors, which also increased image brightness since the aluminum reflected light (that would otherwise be lost inside the CRT) towards the outside of the CRT.[22][374][375][376] In aluminized monochrome CRTs, Aquadag is used on the outside. There is a single aluminum coating covering the funnel and the screen.[172]
The screen, funnel and neck are fused together into a single envelope, possibly using lead enamel seals, a hole is made in the funnel onto which the anode cap is installed and the phosphor, aquadag and aluminum are applied afterwards.[66] Previously monochrome CRTs used ion traps that required magnets; the magnet was used to deflect the electrons away from the more difficult to deflect ions, letting the electrons through while letting the ions collide into a sheet of metal inside the electron gun.[377][156][317] Ion burn results in premature wear of the phosphor. Since ions are harder to deflect than electrons, ion burn leaves a black dot in the center of the screen.[156][317]
The interior aquadag or aluminum coating was the anode and served to accelerate the electrons towards the screen, collect them after hitting the screen while serving as a capacitor together with the outer aquadag coating. The screen has a single uniform phosphor coating and no shadow mask, technically having no resolution limit.[378][163][379]
Monochrome CRTs may use ring magnets to adjust the centering of the electron beam and magnets around the deflection yoke to adjust the geometry of the image.[271][380]
Older monochrome CRT[381] without aluminum, only aquadag
The electron gun of a monochrome CRT
Color CRTs
Color CRTs use three different phosphors which emit red, green, and blue light respectively. They are packed together in stripes (as in aperture grille designs) or clusters called "triads" (as in shadow mask CRTs).[382][383]
Color CRTs have three electron guns, one for each primary color, (red, green and blue) arranged either in a straight line (in-line) or in an equilateral triangular configuration (the guns are usually constructed as a single unit).[183][264][384][385][386] (The triangular configuration is often called "delta-gun", based on its relation to the shape of the Greek letter delta Δ.) The arrangement of the phosphors is the same as that of the electron guns.[183][387] A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor.[388]
A shadow mask tube uses a metal plate with tiny holes, typically in a delta configuration, placed so that the electron beam only illuminates the correct phosphors on the face of the tube;[382] blocking all other electrons.[99] Shadow masks that use slots instead of holes are known as slot masks.[11] The holes or slots are tapered[389][390] so that the electrons that strike the inside of any hole will be reflected back, if they are not absorbed (e.g. due to local charge accumulation), instead of bouncing through the hole to strike a random (wrong) spot on the screen. Another type of color CRT (Trinitron) uses an aperture grille of tensioned vertical wires to achieve the same result.[388] The shadow mask has a single hole for each triad.[183] The shadow mask is usually 1/2 inch behind the screen.[115]
Trinitron CRTs were different from other color CRTs in that they had a single electron gun with three cathodes, an aperture grille which lets more electrons through, increasing image brightness (since the aperture grille does not block as many electrons), and a vertically cylindrical screen, rather than a curved screen.[391]
The three electron guns are in the neck (except for Trinitrons) and the red, green and blue phosphors on the screen may be separated by a black grid or matrix (called black stripe by Toshiba).[65]
The funnel is coated with aquadag on both sides while the screen has a separate aluminum coating applied in a vacuum.[183][114] The aluminum coating protects the phosphor from ions, absorbs secondary electrons, providing them with a return path, preventing them from electrostatically charging the screen which would then repel electrons and reduce image brightness, reflects the light from the phosphors forwards and helps manage heat. It also serves as the anode of the CRT together with the inner aquadag coating. The inner coating is electrically connected to an electrode of the electron gun using springs, forming the final anode.[184][183] The outer aquadag coating is connected to ground, possibly using a series of springs or a harness that makes contact with the aquadag.[392][393]
Shadow mask
The shadow mask absorbs or reflects electrons that would otherwise strike the wrong phosphor dots,[379] causing color purity issues (discoloration of images); in other words, when set up correctly, the shadow mask helps ensure color purity.[183] When the electrons strike the shadow mask, they release their energy as heat and x-rays. If the electrons have too much energy due to an anode voltage that is too high for example, the shadow mask can warp due to the heat, which can also happen during the Lehr baking at approx. 435 °C of the frit seal between the faceplate and the funnel of the CRT.[345][394]
Shadow masks were replaced in TVs by slot masks in the 1970s, since slot masks let more electrons through, increasing image brightness. Shadow masks may be connected electrically to the anode of the CRT.[395][51][396][397] Trinitron used a single electron gun with three cathodes instead of three complete guns. CRT PC monitors usually use shadow masks, except for Sony's Trinitron, Mitsubishi's Diamondtron and NEC's Cromaclear; Trinitron and Diamondtron use aperture grilles while Cromaclear uses a slot mask. Some shadow mask CRTs have color phosphors that are smaller in diameter than the electron beams used to light them,[398] with the intention being to cover the entire phosphor, increasing image brightness.[399] Shadow masks may be pressed into a curved shape.[400][401][402]
Screen manufacture
Early color CRTs did not have a black matrix, which was introduced by Zenith in 1969, and Panasonic in 1970.[399][403][131] The black matrix eliminates light leaking from one phosphor to another since the black matrix isolates the phosphor dots from one another, so part of the electron beam touches the black matrix. This is also made necessary by warping of the shadow mask.[65][398] Light bleeding may still occur due to stray electrons striking the wrong phosphor dots. At high resolutions and refresh rates, phosphors only receive a very small amount of energy, limiting image brightness.[336]
Several methods were used to create the black matrix. One method coated the screen in photoresist such as dichromate-sensitized polyvinyl alcohol photoresist which was then dried and exposed; the unexposed areas were removed and the entire screen was coated in colloidal graphite to create a carbon film, and then hydrogen peroxide was used to remove the remaining photoresist alongside the carbon that was on top of it, creating holes that in turn created the black matrix. The photoresist had to be of the correct thickness to ensure sufficient adhesion to the screen, while the exposure step had to be controlled to avoid holes that were too small or large with ragged edges caused by light diffraction, ultimately limiting the maximum resolution of large color CRTs.[398] The holes were then filled with phosphor using the method described above. Another method used phosphors suspended in an aromatic diazonium salt that adhered to the screen when exposed to light; the phosphors were applied, then exposed to cause them to adhere to the screen, repeating the process once for each color. Then carbon was applied to the remaining areas of the screen while exposing the entire screen to light to create the black matrix, and a fixing process using an aqueous polymer solution was applied to the screen to make the phosphors and black matrix resistant to water.[403] Black chromium may be used instead of carbon in the black matrix.[398] Other methods were also used.[404][405][406][407]
The phosphors are applied using photolithography. The inner side of the screen is coated with phosphor particles suspended in PVA photoresist slurry,[408][409] which is then dried using infrared light,[410] exposed, and developed. The exposure is done using a "lighthouse" that uses an ultraviolet light source with a corrector lens to allow the CRT to achieve color purity. Removable shadow masks with spring-loaded clips are used as photomasks. The process is repeated with all colors. Usually the green phosphor is the first to be applied.[183][411][412][413] After phosphor application, the screen is baked to eliminate any organic chemicals (such as the PVA that was used to deposit the phosphor) that may remain on the screen.[403][414] Alternatively, the phosphors may be applied in a vacuum chamber by evaporating them and allowing them to condense on the screen, creating a very uniform coating.[231] Early color CRTs had their phosphors deposited using silkscreen printing.[43] Phosphors may have color filters over them (facing the viewer), contain pigment of the color emitted by the phosphor,[415][308] or be encapsulated in color filters to improve color purity and reproduction while reducing glare.[412][397] Poor exposure due to insufficient light leads to poor phosphor adhesion to the screen, which limits the maximum resolution of a CRT, as the smaller phosphor dots required for higher resolutions cannot receive as much light due to their smaller size.[416]
After the screen is coated with phosphor and aluminum and the shadow mask installed onto it the screen is bonded to the funnel using a glass frit that may contain 65 to 88% of lead oxide by weight. The lead oxide is necessary for the glass frit to have a low melting temperature. Boron oxide (III) may also present to stabilize the frit, with alumina powder as filler powder to control the thermal expansion of the frit.[417][145][7] The frit may be applied as a paste consisting of frit particles suspended in amyl acetate or in a polymer with an alkyl methacrylate monomer together with an organic solvent to dissolve the polymer and monomer.[418][419] The CRT is then baked in an oven in what is called a Lehr bake, to cure the frit, sealing the funnel and screen together. The frit contains a large quantity of lead, causing color CRTs to contain more lead than their monochrome counterparts. Monochrome CRTs on the other hand do not require frit; the funnel can be fused directly to the glass[99] by melting and joining the edges of the funnel and screen using gas flames. Frit is used in color CRTs to prevent deformation of the shadow mask and screen during the fusing process. The edges of the screen and funnel of the CRT are never melted.[183] A primer may be applied on the edges of the funnel and screen before the frit paste is applied to improve adhesion.[420] The Lehr bake consists of several successive steps that heat and then cool the CRT gradually until it reaches a temperature of 435 to 475 °C[418] (other sources may state different temperatures, such as 440 °C)[421] After the Lehr bake, the CRT is flushed with air or nitrogen to remove contaminants, the electron gun is inserted and sealed into the neck of the CRT, and a vacuum is formed on the CRT.[422][206]
Convergence and purity in color CRTs
Due to limitations in the dimensional precision with which CRTs can be manufactured economically, it has not been practically possible to build color CRTs in which three electron beams could be aligned to hit phosphors of respective color in acceptable coordination, solely on the basis of the geometric configuration of the electron gun axes and gun aperture positions, shadow mask apertures, etc. The shadow mask ensures that one beam will only hit spots of certain colors of phosphors, but minute variations in physical alignment of the internal parts among individual CRTs will cause variations in the exact alignment of the beams through the shadow mask, allowing some electrons from, for example, the red beam to hit, say, blue phosphors, unless some individual compensation is made for the variance among individual tubes.
Color convergence and color purity are two aspects of this single problem. Firstly, for correct color rendering it is necessary that regardless of where the beams are deflected on the screen, all three hit the same spot (and nominally pass through the same hole or slot) on the shadow mask.[clarification needed] This is called convergence.[423] More specifically, the convergence at the center of the screen (with no deflection field applied by the yoke) is called static convergence, and the convergence over the rest of the screen area (specially at the edges and corners) is called dynamic convergence.[116] The beams may converge at the center of the screen and yet stray from each other as they are deflected toward the edges; such a CRT would be said to have good static convergence but poor dynamic convergence. Secondly, each beam must only strike the phosphors of the color it is intended to strike and no others. This is called purity. Like convergence, there is static purity and dynamic purity, with the same meanings of "static" and "dynamic" as for convergence. Convergence and purity are distinct parameters; a CRT could have good purity but poor convergence, or vice versa. Poor convergence causes color "shadows" or "ghosts" along displayed edges and contours, as if the image on the screen were intaglio printed with poor registration. Poor purity causes objects on the screen to appear off-color while their edges remain sharp. Purity and convergence problems can occur at the same time, in the same or different areas of the screen or both over the whole screen, and either uniformly or to greater or lesser degrees over different parts of the screen.
The solution to the static convergence and purity problems is a set of color alignment ring magnets installed around the neck of the CRT.[424] These movable weak permanent magnets are usually mounted on the back end of the deflection yoke assembly and are set at the factory to compensate for any static purity and convergence errors that are intrinsic to the unadjusted tube. Typically there are two or three pairs of two magnets in the form of rings made of plastic impregnated with a magnetic material, with their magnetic fields parallel to the planes of the magnets, which are perpendicular to the electron gun axes. Often, one ring has two poles, another has 4, and the remaining ring has 6 poles.[425] Each pair of magnetic rings forms a single effective magnet whose field vector can be fully and freely adjusted (in both direction and magnitude). By rotating a pair of magnets relative to each other, their relative field alignment can be varied, adjusting the effective field strength of the pair. (As they rotate relative to each other, each magnet's field can be considered to have two opposing components at right angles, and these four components [two each for two magnets] form two pairs, one pair reinforcing each other and the other pair opposing and canceling each other. Rotating away from alignment, the magnets' mutually reinforcing field components decrease as they are traded for increasing opposed, mutually cancelling components.) By rotating a pair of magnets together, preserving the relative angle between them, the direction of their collective magnetic field can be varied. Overall, adjusting all of the convergence/purity magnets allows a finely tuned slight electron beam deflection or lateral offset to be applied, which compensates for minor static convergence and purity errors intrinsic to the uncalibrated tube. Once set, these magnets are usually glued in place, but normally they can be freed and readjusted in the field (e.g. by a TV repair shop) if necessary.
On some CRTs, additional fixed adjustable magnets are added for dynamic convergence or dynamic purity at specific points on the screen, typically near the corners or edges. Further adjustment of dynamic convergence and purity typically cannot be done passively, but requires active compensation circuits, one to correct convergence horizontally and another to correct it vertically. The deflection yoke contains convergence coils, a set of two per color, wound on the same core, to which the convergence signals are applied. That means 6 convergence coils in groups of 3, with 2 coils per group, with one coil for horizontal convergence correction and another for vertical convergence correction, with each group sharing a core. The groups are separated 120° from one another. Dynamic convergence is necessary because the front of the CRT and the shadow mask are not spherical, compensating for electron beam defocusing and astigmatism. The fact that the CRT screen is not spherical[426] leads to geometry problems which may be corrected using a circuit.[427] The signals used for convergence are parabolic waveforms derived from three signals coming from a vertical output circuit. The parabolic signal is fed into the convergence coils, while the other two are sawtooth signals that, when mixed with the parabolic signals, create the necessary signal for convergence. A resistor and diode are used to lock the convergence signal to the center of the screen to prevent it from being affected by the static convergence. The horizontal and vertical convergence circuits are similar. Each circuit has two resonators, one usually tuned to 15,625 Hz and the other to 31,250 Hz, which set the frequency of the signal sent to the convergence coils.[428] Dynamic convergence may be accomplished using electrostatic quadrupole fields in the electron gun.[429] Dynamic convergence means that the electron beam does not travel in a perfectly straight line between the deflection coils and the screen, since the convergence coils cause it to become curved to conform to the screen.
The convergence signal may instead be a sawtooth signal with a slight sine wave appearance, the sine wave part is created using a capacitor in series with each deflection coil. In this case, the convergence signal is used to drive the deflection coils. The sine wave part of the signal causes the electron beam to move more slowly near the edges of the screen. The capacitors used to create the convergence signal are known as the s-capacitors. This type of convergence is necessary due to the high deflection angles and flat screens of many CRT computer monitors. The value of the s-capacitors must be chosen based on the scan rate of the CRT, so multi-syncing monitors must have different sets of s-capacitors, one for each refresh rate.[110]
Dynamic convergence may instead be accomplished in some CRTs using only the ring magnets, magnets glued to the CRT, and by varying the position of the deflection yoke, whose position may be maintained using set screws, a clamp and rubber wedges.[116][430] 90° deflection angle CRTs may use "self-convergence" without dynamic convergence, which together with the in-line triad arrangement, eliminates the need for separate convergence coils and related circuitry, reducing costs. complexity and CRT depth by 10 millimeters. Self-convergence works by means of "nonuniform" magnetic fields. Dynamic convergence is necessary in 110° deflection angle CRTs, and quadrupole windings on the deflection yoke at a certain frequency may also be used for dynamic convergence.[431]
Dynamic color convergence and purity are one of the main reasons why until late in their history, CRTs were long-necked (deep) and had biaxially curved faces; these geometric design characteristics are necessary for intrinsic passive dynamic color convergence and purity. Only starting around the 1990s did sophisticated active dynamic convergence compensation circuits become available that made short-necked and flat-faced CRTs workable. These active compensation circuits use the deflection yoke to finely adjust beam deflection according to the beam target location. The same techniques (and major circuit components) also make possible the adjustment of display image rotation, skew, and other complex raster geometry parameters through electronics under user control.[110]
The guns are aligned with one another (converged) using convergence rings placed right outside the neck; there is one ring per gun. The rings have north and south poles. There are 4 sets of rings, one to adjust RGB convergence, a second to adjust Red and Blue convergence, a third to adjust vertical raster shift, and a fourth to adjust purity. The vertical raster shift adjusts the straightness of the scan line. CRTs may also employ dynamic convergence circuits, which ensure correct convergence at the edges of the CRT. Permalloy magnets may also be used to correct the convergence at the edges. Convergence is carried out with the help of a crosshatch (grid) pattern.[432][433] Other CRTs may instead use magnets that are pushed in and out instead of rings.[393] In early color CRTs, the holes in the shadow mask became progressively smaller as they extended outwards from the center of the screen, to aid in convergence.[399]
Magnetic shielding and degaussing
If the shadow mask or aperture grille becomes magnetized, its magnetic field alters the paths of the electron beams. This causes errors of "color purity" as the electrons no longer follow only their intended paths, and some will hit some phosphors of colors other than the one intended. For example, some electrons from the red beam may hit blue or green phosphors, imposing a magenta or yellow tint to parts of the image that are supposed to be pure red. (This effect is localized to a specific area of the screen if the magnetization is localized.) Therefore, it is important that the shadow mask or aperture grille not be magnetized. The earth's magnetic field may have an effect on the color purity of the CRT.[432] Because of this, some CRTs have external magnetic shields over their funnels. The magnetic shield may be made of soft iron or mild steel and contain a degaussing coil.[434] The magnetic shield and shadow mask may be permanently magnetized by the earth's magnetic field, adversely affecting color purity when the CRT is moved. This problem is solved with a built-in degaussing coil, found in many TVs and computer monitors. Degaussing may be automatic, occurring whenever the CRT is turned on.[435][183] The magnetic shield may also be internal, being on the inside of the funnel of the CRT.[436][437][110][438][439][440]
Color CRT displays in television sets and computer monitors often have a built-in degaussing (demagnetizing) coil mounted around the perimeter of the CRT face. Upon power-up of the CRT display, the degaussing circuit produces a brief, alternating current through the coil which fades to zero over a few seconds, producing a decaying alternating magnetic field from the coil. This degaussing field is strong enough to remove shadow mask magnetization in most cases, maintaining color purity.[441][442] In unusual cases of strong magnetization where the internal degaussing field is not sufficient, the shadow mask may be degaussed externally with a stronger portable degausser or demagnetizer. However, an excessively strong magnetic field, whether alternating or constant, may mechanically deform (bend) the shadow mask, causing a permanent color distortion on the display which looks very similar to a magnetization effect.
Resolution
Dot pitch defines the maximum resolution of the display, assuming delta-gun CRTs. In these, as the scanned resolution approaches the dot pitch resolution, moiré appears, as the detail being displayed is finer than what the shadow mask can render.[443] Aperture grille monitors do not suffer from vertical moiré, however, because their phosphor stripes have no vertical detail. In smaller CRTs, these strips maintain position by themselves, but larger aperture-grille CRTs require one or two crosswise (horizontal) support strips; one for smaller CRTs, and two for larger ones. The support wires block electrons, causing the wires to be visible.[444] In aperture grille CRTs, dot pitch is replaced by stripe pitch. Hitachi developed the Enhanced Dot Pitch (EDP) shadow mask, which uses oval holes instead of circular ones, with respective oval phosphor dots.[397] Moiré is reduced in shadow mask CRTs by arranging the holes in the shadow mask in a honeycomb-like pattern.[110]
Projection CRTs
Projection CRTs were used in CRT projectors and CRT rear-projection televisions, and are usually small (being 7 to 9 inches across);[260] have a phosphor that generates either red, green or blue light, thus making them monochrome CRTs;[445] and are similar in construction to other monochrome CRTs. Larger projection CRTs in general lasted longer, and were able to provide higher brightness levels and resolution, but were also more expensive.[446][447] Projection CRTs have an unusually high anode voltage for their size (such as 27 or 25 kV for a 5 or 7-inch projection CRT respectively),[448][449] and a specially made tungsten/barium cathode (instead of the pure barium oxide normally used) that consists of barium atoms embedded in 20% porous tungsten or barium and calcium aluminates or of barium, calcium and aluminum oxides coated on porous tungsten; the barium diffuses through the tungsten to emit electrons.[450] The special cathode can deliver 2mA of current instead of the 0.3mA of normal cathodes,[451][450][224][163] which makes them bright enough to be used as light sources for projection. The high anode voltage and the specially made cathode increase the voltage and current, respectively, of the electron beam, which increases the light emitted by the phosphors, and also the amount of heat generated during operation; this means that projector CRTs need cooling. The screen is usually cooled using a container (the screen forms part of the container) with glycol; the glycol may itself be dyed,[452] or colorless glycol may be used inside a container which may be colored (forming a lens known as a c-element). Colored lenses or glycol are used for improving color reproduction at the cost of brightness, and are only used on red and green CRTs.[453][454] Each CRT has its own glycol, which has access to an air bubble to allow the glycol to shrink and expand as it cools and warms. Projector CRTs may have adjustment rings just like color CRTs to adjust astigmatism,[455] which is flaring of the electron beam (stray light similar to shadows).[456] They have three adjustment rings; one with two poles, one with four poles, and another with 6 poles. When correctly adjusted, the projector can display perfectly round dots without flaring.[457] The screens used in projection CRTs were more transparent than usual, with 90% transmittance.[114] The first projection CRTs were made in 1933.[458]
Projector CRTs were available with electrostatic and electromagnetic focusing, the latter being more expensive. Electrostatic focusing used electronics to focus the electron beam, together with focusing magnets around the neck of the CRT for fine focusing adjustments. This type of focusing degraded over time. Electromagnetic focusing was introduced in the early 1990s and included an electromagnetic focusing coil in addition to the already existing focusing magnets. Electromagnetic focusing was much more stable over the lifetime of the CRT, retaining 95% of its sharpness by the end of life of the CRT.[459]
Beam-index tube
Beam-index tubes, also known as Uniray, Apple CRT or Indextron,[460] was an attempt in the 1950s by Philco to create a color CRT without a shadow mask, eliminating convergence and purity problems, and allowing for shallower CRTs with higher deflection angles.[461] It also required a lower voltage power supply for the final anode since it did not use a shadow mask, which normally blocks around 80% of the electrons generated by the electron gun. The lack of a shadow mask also made it immune to the earth's magnetic field while also making degaussing unnecessary and increasing image brightness.[462] It was constructed similarly to a monochrome CRT, with an aquadag outer coating, an aluminum inner coating, and a single electron gun but with a screen with an alternating pattern of red, green, blue and UV (index) phosphor stripes (similarly to a Trinitron) with a side mounted photomultiplier tube[463][462] or photodiode pointed towards the rear of the screen and mounted on the funnel of CRT, to track the electron beam to activate the phosphors separately from one another using the same electron beam. Only the index phosphor stripe was used for tracking, and it was the only phosphor that was not covered by an aluminum layer.[320] It was shelved because of the precision required to produce it.[464][465] It was revived by Sony in the 1980s as the Indextron but its adoption was limited, at least in part due to the development of LCD displays. Beam-index CRTs also suffered from poor contrast ratios of only around 50:1 since some light emission by the phosphors was required at all times by the photodiodes to track the electron beam. It allowed for single CRT color CRT projectors due to a lack of shadow mask; normally CRT projectors use three CRTs, one for each color,[466] since a lot of heat is generated due to the high anode voltage and beam current, making a shadow mask impractical and inefficient since it would warp under the heat produced (shadow masks absorb most of the electron beam, and, hence, most of the energy carried by the relativistic electrons); the three CRTs meant that an involved calibration and adjustment procedure[467] had to be carried out during installation of the projector, and moving the projector would require it to be recalibrated. A single CRT meant the need for calibration was eliminated, but brightness was decreased since the CRT screen had to be used for three colors instead of each color having its own CRT screen.[460] A stripe pattern also imposes a horizontal resolution limit; in contrast, three-screen CRT projectors have no theoretical resolution limit, due to them having single, uniform phosphor coatings.
Flat CRTs
Flat CRTs are those with a flat screen. Despite having a flat screen, they may not be completely flat, especially on the inside, instead having a greatly increased curvature. A notable exception is the LG Flatron (made by LG.Philips Displays, later LP Displays) which is truly flat on the outside and inside, but has a bonded glass pane on the screen with a tensioned rim band to provide implosion protection. Such completely flat CRTs were first introduced by Zenith in 1986, and used flat tensioned shadow masks, where the shadow mask is held under tension, providing increased resistance to blooming.[468][469][470][251][342][471] Flat CRTs have a number of challenges, like deflection. Vertical deflection boosters are required to increase the amount of current that is sent to the vertical deflection coils to compensate for the reduced curvature.[278] The CRTs used in the Sinclair TV80, and in many Sony Watchmans were flat in that they were not deep and their front screens were flat, but their electron guns were put to a side of the screen.[472][473] The TV80 used electrostatic deflection[474] while the Watchman used magnetic deflection with a phosphor screen that was curved inwards. Similar CRTs were used in video door bells.[475]
The side of a Sony Watchman monochrome CRT. One of the pairs of deflection coils is easily noticeable.
Radar CRTs
Radar CRTs such as the 7JP4 had a circular screen and scanned the beam from the center outwards. The deflection yoke rotated, causing the beam to rotate in a circular fashion.[476] The screen often had two colors, often a bright short persistence color that only appeared as the beam scanned the display and a long persistence phosphor afterglow. When the beam strikes the phosphor, the phosphor brightly illuminates, and when the beam leaves, the dimmer long persistence afterglow would remain lit where the beam struck the phosphor, alongside the radar targets that were "written" by the beam, until the beam re-struck the phosphor.[477][478]
Oscilloscope CRTs
In oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs. The beam is deflected horizontally by applying an electric field between a pair of plates to its left and right, and vertically by applying an electric field to plates above and below. Televisions use magnetic rather than electrostatic deflection because the deflection plates obstruct the beam when the deflection angle is as large as is required for tubes that are relatively short for their size. Some Oscilloscope CRTs incorporate post deflection anodes (PDAs) that are spiral-shaped to ensure even anode potential across the CRT and operate at up to 15,000 volts. In PDA CRTs the electron beam is deflected before it is accelerated, improving sensitivity and legibility, specially when analyzing voltage pulses with short duty cycles.[479][155][480]
Microchannel plate
When displaying fast one-shot events, the electron beam must deflect very quickly, with few electrons impinging on the screen, leading to a faint or invisible image on the display. Oscilloscope CRTs designed for very fast signals can give a brighter display by passing the electron beam through a micro-channel plate just before it reaches the screen. Through the phenomenon of secondary emission, this plate multiplies the number of electrons reaching the phosphor screen, giving a significant improvement in writing rate (brightness) and improved sensitivity and spot size as well.[481][482]
Graticules
Most oscilloscopes have a graticule as part of the visual display, to facilitate measurements. The graticule may be permanently marked inside the face of the CRT, or it may be a transparent external plate made of glass or acrylic plastic. An internal graticule eliminates parallax error, but cannot be changed to accommodate different types of measurements.[483] Oscilloscopes commonly provide a means for the graticule to be illuminated from the side, which improves its visibility.[484]
Image storage tubes
These are found in analog phosphor storage oscilloscopes. These are distinct from digital storage oscilloscopes which rely on solid state digital memory to store the image.
Where a single brief event is monitored by an oscilloscope, such an event will be displayed by a conventional tube only while it actually occurs. The use of a long persistence phosphor may allow the image to be observed after the event, but only for a few seconds at best. This limitation can be overcome by the use of a direct view storage cathode-ray tube (storage tube). A storage tube will continue to display the event after it has occurred until such time as it is erased. A storage tube is similar to a conventional tube except that it is equipped with a metal grid coated with a dielectric layer located immediately behind the phosphor screen. An externally applied voltage to the mesh initially ensures that the whole mesh is at a constant potential. This mesh is constantly exposed to a low velocity electron beam from a 'flood gun' which operates independently of the main gun. This flood gun is not deflected like the main gun but constantly 'illuminates' the whole of the storage mesh. The initial charge on the storage mesh is such as to repel the electrons from the flood gun which are prevented from striking the phosphor screen.
When the main electron gun writes an image to the screen, the energy in the main beam is sufficient to create a 'potential relief' on the storage mesh. The areas where this relief is created no longer repel the electrons from the flood gun which now pass through the mesh and illuminate the phosphor screen. Consequently, the image that was briefly traced out by the main gun continues to be displayed after it has occurred. The image can be 'erased' by resupplying the external voltage to the mesh restoring its constant potential. The time for which the image can be displayed was limited because, in practice, the flood gun slowly neutralises the charge on the storage mesh. One way of allowing the image to be retained for longer is temporarily to turn off the flood gun. It is then possible for the image to be retained for several days. The majority of storage tubes allow for a lower voltage to be applied to the storage mesh which slowly restores the initial charge state. By varying this voltage a variable persistence is obtained. Turning off the flood gun and the voltage supply to the storage mesh allows such a tube to operate as a conventional oscilloscope tube.[485]
Vector monitors
Vector monitors were used in early computer aided design systems[486] and are in some late-1970s to mid-1980s arcade games such as Asteroids.[487] They draw graphics point-to-point, rather than scanning a raster. Either monochrome or color CRTs can be used in vector displays, and the essential principles of CRT design and operation are the same for either type of display; the main difference is in the beam deflection patterns and circuits.
Data storage tubes
The Williams tube or Williams-Kilburn tube was a cathode-ray tube used to electronically store binary data. It was used in computers of the 1940s as a random-access digital storage device. In contrast to other CRTs in this article, the Williams tube was not a display device, and in fact could not be viewed since a metal plate covered its screen.
Cat's eye
In some vacuum tube radio sets, a "Magic Eye" or "Tuning Eye" tube was provided to assist in tuning the receiver. Tuning would be adjusted until the width of a radial shadow was minimized. This was used instead of a more expensive electromechanical meter, which later came to be used on higher-end tuners when transistor sets lacked the high voltage required to drive the device.[488] The same type of device was used with tape recorders as a recording level meter, and for various other applications including electrical test equipment.
Charactrons
Some displays for early computers (those that needed to display more text than was practical using vectors, or that required high speed for photographic output) used Charactron CRTs. These incorporate a perforated metal character mask (stencil), which shapes a wide electron beam to form a character on the screen. The system selects a character on the mask using one set of deflection circuits, but that causes the extruded beam to be aimed off-axis, so a second set of deflection plates has to re-aim the beam so it is headed toward the center of the screen. A third set of plates places the character wherever required. The beam is unblanked (turned on) briefly to draw the character at that position. Graphics could be drawn by selecting the position on the mask corresponding to the code for a space (in practice, they were simply not drawn), which had a small round hole in the center; this effectively disabled the character mask, and the system reverted to regular vector behavior. Charactrons had exceptionally long necks, because of the need for three deflection systems.[489][490]
Nimo
Nimo was the trademark of a family of small specialised CRTs manufactured by Industrial Electronic Engineers. These had 10 electron guns which produced electron beams in the form of digits in a manner similar to that of the charactron. The tubes were either simple single-digit displays or more complex 4- or 6- digit displays produced by means of a suitable magnetic deflection system. Having little of the complexities of a standard CRT, the tube required a relatively simple driving circuit, and as the image was projected on the glass face, it provided a much wider viewing angle than competitive types (e.g., nixie tubes).[491] However, their requirement for several voltages and their high voltage made them uncommon.
Flood-beam CRT
Flood-beam CRTs are small tubes that are arranged as pixels for large video walls like Jumbotrons. The first screen using this technology (called Diamond Vision by Mitsubishi Electric) was introduced by Mitsubishi Electric for the 1980 Major League Baseball All-Star Game. It differs from a normal CRT in that the electron gun within does not produce a focused controllable beam. Instead, electrons are sprayed in a wide cone across the entire front of the phosphor screen, basically making each unit act as a single light bulb.[492] Each one is coated with a red, green or blue phosphor, to make up the color sub-pixels. This technology has largely been replaced with light-emitting diode displays. Unfocused and undeflected CRTs were used as grid-controlled stroboscope lamps since 1958.[493] Electron-stimulated luminescence (ESL) lamps, which use the same operating principle, were released in 2011.[494]
Print-head CRT
CRTs with an unphosphored front glass but with fine wires embedded in it were used as electrostatic print heads in the 1960s. The wires would pass the electron beam current through the glass onto a sheet of paper where the desired content was therefore deposited as an electrical charge pattern. The paper was then passed near a pool of liquid ink with the opposite charge. The charged areas of the paper attract the ink and thus form the image.[495][496]
Zeus – thin CRT display
In the late 1990s and early 2000s Philips Research Laboratories experimented with a type of thin CRT known as the Zeus display, which contained CRT-like functionality in a flat-panel display.[497][498][499][500][501] The devices were demonstrated but never marketed.
Slimmer CRT
Some CRT manufacturers, both LG.Philips Displays (later LP Displays) and Samsung SDI, innovated CRT technology by creating a slimmer tube. Slimmer CRT had the trade names Superslim,[502] Ultraslim,[503] Vixlim (by Samsung)[504] and Cybertube and Cybertube+ (both by LG Philips displays).[505][506] A 21-inch (53 cm) flat CRT has a 447.2-millimetre (17.61 in) depth. The depth of Superslim was 352 millimetres (13.86 in)[507] and Ultraslim was 295.7 millimetres (11.64 in).[508]
Health concerns
Ionizing radiation
CRTs can emit a small amount of X-ray radiation; this is a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors, which produces bremsstrahlung (braking radiation) as the high-energy electrons are decelerated. The amount of radiation escaping the front of the monitor is widely considered to be not harmful. The Food and Drug Administration regulations in 21 CFR 1020.10 are used to strictly limit, for instance, television receivers to 0.5 milliroentgens per hour at a distance of 5 cm (2 in) from any external surface; since 2007, most CRTs have emissions that fall well below this limit.[509] Note that the roentgen is an outdated unit and does not account for dose absorption. The conversion rate is about .877 roentgen per rem.[510] Assuming that the viewer absorbed the entire dose (which is unlikely), and that they watched TV for 2 hours a day, a .5 milliroentgen hourly dose would increase the viewers yearly dose by 320 millirem. For comparison, the average background radiation in the United States is 310 millirem a year. Negative effects of chronic radiation are not generally noticeable until doses over 20,000 millirem.[511]
The density of the x-rays that would be generated by a CRT is low because the raster scan of a typical CRT distributes the energy of the electron beam across the entire screen. Voltages above 15,000 volts are enough to generate "soft" x-rays. However, since CRTs may stay on for several hours at a time, the amount of x-rays generated by the CRT may become significant, hence the importance of using materials to shield against x-rays, such as the thick leaded glass and barium-strontium glass used in CRTs.[135]
Concerns about x-rays emitted by CRTs began in 1967 when it was found that TV sets made by General Electric were emitting "X-radiation in excess of desirable levels". It was later found that TV sets from all manufacturers were also emitting radiation. This caused television industry representatives to be brought before a U.S. congressional committee, which later proposed a federal radiation regulation bill, which became the 1968 Radiation Control for Health and Safety Act. It was recommended to TV set owners to always be at a distance of at least 6 feet from the screen of the TV set, and to avoid "prolonged exposure" at the sides, rear or underneath a TV set. It was discovered that most of the radiation was directed downwards. Owners were also told to not modify their set's internals to avoid exposure to radiation. Headlines about "radioactive" TV sets continued until the end of the 1960s. There once was a proposal by two New York congressmen that would have forced TV set manufacturers to "go into homes to test all of the nation's 15 million color sets and to install radiation devices in them". The FDA eventually began regulating radiation emissions from all electronic products in the US.[512]
Toxicity
Older color and monochrome CRTs may have been manufactured with toxic substances, such as cadmium, in the phosphors.[50][513][514][515] The rear glass tube of modern CRTs may be made from leaded glass, which represent an environmental hazard if disposed of improperly.[516] Since 1970, glass in the front panel (the viewable portion of the CRT) used strontium oxide rather than lead, though the rear of the CRT was still produced from leaded glass. Monochrome CRTs typically do not contain enough leaded glass to fail EPA TCLP tests. While the TCLP process grinds the glass into fine particles in order to expose them to weak acids to test for leachate, intact CRT glass does not leach (The lead is vitrified, contained inside the glass itself, similar to leaded glass crystalware).
Flicker
At low refresh rates (60 Hz and below), the periodic scanning of the display may produce a flicker that some people perceive more easily than others, especially when viewed with peripheral vision. Flicker is commonly associated with CRT as most televisions run at 50 Hz (PAL) or 60 Hz (NTSC), although there are some 100 Hz PAL televisions that are flicker-free. Typically only low-end monitors run at such low frequencies, with most computer monitors supporting at least 75 Hz and high-end monitors capable of 100 Hz or more to eliminate any perception of flicker.[517] Though the 100 Hz PAL was often achieved using interleaved scanning, dividing the circuit and scan into two beams of 50 Hz. Non-computer CRTs or CRT for sonar or radar may have long persistence phosphor and are thus flicker free. If the persistence is too long on a video display, moving images will be blurred.
High-frequency audible noise
50 Hz/60 Hz CRTs used for television operate with horizontal scanning frequencies of 15,750 and 15,734.25 Hz (for NTSC systems) or 15,625 Hz (for PAL systems).[518] These frequencies are at the upper range of human hearing and are inaudible to many people; however, some people (especially children) will perceive a high-pitched tone near an operating CRT television.[519] The sound is due to magnetostriction in the magnetic core and periodic movement of windings of the flyback transformer[520] but the sound can also be created by movement of the deflection coils, yoke or ferrite beads.[521]
This problem does not occur on 100/120 Hz TVs and on non-CGA (Color Graphics Adapter) computer displays, because they use much higher horizontal scanning frequencies that produce sound which is inaudible to humans (22 kHz to over 100 kHz).
Implosion
High vacuum inside glass-walled cathode-ray tubes permits electron beams to fly freely—without colliding into molecules of air or other gas. If the glass is damaged, atmospheric pressure can collapse the vacuum tube into dangerous fragments which accelerate inward and then spray at high speed in all directions. Although modern cathode-ray tubes used in televisions and computer displays have epoxy-bonded face-plates or other measures to prevent shattering of the envelope, CRTs must be handled carefully to avoid personal injury.[522]
Implosion protection
Early CRTs had a glass plate over the screen that was bonded to it using glue,[139] creating a laminated glass screen: initially the glue was polyvinyl acetate (PVA),[523] while later versions such as the LG Flatron used a resin, perhaps a UV-curable resin.[524][342] The PVA degrades over time creating a "cataract", a ring of degraded glue around the edges of the CRT that does not allow light from the screen to pass through.[523] Later CRTs instead use a tensioned metal rim band mounted around the perimeter that also provides mounting points for the CRT to be mounted to a housing.[372] In a 19-inch CRT, the tensile stress in the rim band is 70 kg/cm².[525] Older CRTs were mounted to the TV set using a frame. The band is tensioned by heating it, then mounting it on the CRT; the band cools afterwards, shrinking in size and putting the glass under compression,[526][139][527] which strengthens the glass and reduces the necessary thickness (and hence weight) of the glass. This makes the band an integral component that should never be removed from an intact CRT that still has a vacuum; attempting to remove it may cause the CRT to implode.[317] The rim band prevents the CRT from imploding should the screen be broken. The rim band may be glued to the perimeter of the CRT using epoxy, preventing cracks from spreading beyond the screen and into the funnel.[528][527]
Electric shock
To accelerate the electrons from the cathode to the screen with enough energy[529] to achieve sufficient image brightness, a very high voltage (EHT or extra-high tension) is required,[530] from a few thousand volts for a small oscilloscope CRT to tens of thousands for a larger screen color TV. This is many times greater than household power supply voltage. Even after the power supply is turned off, some associated capacitors and the CRT itself may retain a charge for some time and therefore dissipate that charge suddenly through a ground such as an inattentive human grounding a capacitor discharge lead. An average monochrome CRT may use 1 to 1.5 kV of anode voltage per inch.[531][271]
Security concerns
Under some circumstances, the signal radiated from the electron guns, scanning circuitry, and associated wiring of a CRT can be captured remotely and used to reconstruct what is shown on the CRT using a process called Van Eck phreaking.[532] Special TEMPEST shielding can mitigate this effect. Such radiation of a potentially exploitable signal, however, occurs also with other display technologies[533] and with electronics in general.[citation needed]
Recycling
Due to the toxins contained in CRT monitors the United States Environmental Protection Agency created rules (in October 2001) stating that CRTs must be brought to special e-waste recycling facilities. In November 2002, the EPA began fining companies that disposed of CRTs through landfills or incineration. Regulatory agencies, local and statewide, monitor the disposal of CRTs and other computer equipment.[534]
As electronic waste, CRTs are considered one of the hardest types to recycle.[535] CRTs have relatively high concentration of lead and phosphors, both of which are necessary for the display. There are several companies in the United States that charge a small fee to collect CRTs, then subsidize their labor by selling the harvested copper, wire, and printed circuit boards. The United States Environmental Protection Agency (EPA) includes discarded CRT monitors in its category of "hazardous household waste"[536] but considers CRTs that have been set aside for testing to be commodities if they are not discarded, speculatively accumulated, or left unprotected from weather and other damage.[537]
Various states participate in the recycling of CRTs, each with their reporting requirements for collectors and recycling facilities. For example, in California the recycling of CRTs is governed by CALRecycle, the California Department of Resources Recycling and Recovery through their Payment System.[538] Recycling facilities that accept CRT devices from business and residential sector must obtain contact information such as address and phone number to ensure the CRTs come from a California source in order to participate in the CRT Recycling Payment System.
In Europe, disposal of CRT televisions and monitors is covered by the WEEE Directive.[539]
Multiple methods have been proposed for the recycling of CRT glass. The methods involve thermal, mechanical and chemical processes.[540][541][542][543] All proposed methods remove the lead oxide content from the glass. Some companies operated furnaces to separate the lead from the glass.[544] A coalition called the Recytube project was once formed by several European companies to devise a method to recycle CRTs.[6] The phosphors used in CRTs often contain rare earth metals.[545][546][547][307] A CRT contains about 7g of phosphor.[548]
The funnel can be separated from the screen of the CRT using laser cutting, diamond saws or wires or using a resistively heated nichrome wire.[549][550][551][552][553]
Leaded CRT glass was sold to be remelted into other CRTs,[76] or even broken down and used in road construction or used in tiles,[554][555] concrete, concrete and cement bricks,[556] fiberglass insulation or used as flux in metals smelting.[557][558]
A considerable portion of CRT glass is landfilled, where it can pollute the surrounding environment.[6] It is more common for CRT glass to be disposed of than being recycled.[559]
See also
Applying CRT in different display-purpose:
Historical aspects:
- Direct-view bistable storage tube
- Flat-panel display
- Geer tube
- History of display technology
- Image dissector
- LCD television, LED-backlit LCD, LED display
- Penetron
- Surface-conduction electron-emitter display
- Trinitron
Safety and precautions:
References
Evidence for the existence of "cathode-rays" was first found by Plücker and Hittorf ...
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24:TV viewers of the 1970s will see their programs on sets quite different from today's, if designs now being worked out are developed. At the Home Furnishings Market in Chicago, Illinois, on June 21, 1961, a thin TV screen is a feature of this design model. Another feature is an automatic timing device which would record TV programs during the viewers' absence to be played back later. The 32x22-inch color screen is four inches thick.
- Mehmet Ali Recai Önal (1 December 2018). "Recovering rare earths from old TVs and computer screens". Solvomet.
- "California CRT glass heads to disposal sites amid downstream challenges". 22 September 2016.
Selected patents
- U.S. Patent 1,691,324: Zworykin Television System
External links
- "CRTs". Virtual Valve Museum. Archived from the original on 10 October 2011. Retrieved 31 December 2006.
- Goldwasser, Samuel M. (28 February 2006). "TV and Monitor CRT (Picture Tube) Information". repairfaq.org. Archived from the original on 26 September 2006.
- "The Cathode Ray Tube site". crtsite.com.
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https://en.wikipedia.org/wiki/Cathode-ray_tube
A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Plasma televisions were the first large (over 32 inches diagonal) flat panel displays to be released to the public.
Until about 2007, plasma displays were commonly used in large televisions. By 2013, they had lost nearly all market share due to competition from low-cost LCDs and more expensive but high-contrast OLED flat-panel displays. Manufacturing of plasma displays for the United States retail market ended in 2014,[1][2] and manufacturing for the Chinese market ended in 2016.[3][4] Plasma displays are obsolete, having been superseded in most if not all aspects by OLED displays.[5]
https://en.wikipedia.org/wiki/Plasma_display
An LED-backlit LCD is a liquid-crystal display that uses LEDs for backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting.[1] LED-backlit displays use the same TFT LCD (thin-film-transistor liquid-crystal display) technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.
While not an LED display, a television using such a combination of an LED backlight with an LCD panel is advertised as an LED TV by some manufacturers and suppliers.[1][2]
https://en.wikipedia.org/wiki/LED-backlit_LCD
A thin-film-transistor liquid-crystal display (TFT LCD) is a variant of a liquid-crystal display that uses thin-film-transistor technology[1] to improve image qualities such as addressability and contrast. A TFT LCD is an active matrix LCD, in contrast to passive matrix LCDs or simple, direct-driven (i.e. with segments directly connected to electronics outside the LCD) LCDs with a few segments.
TFT LCDs are used in appliances including television sets, computer monitors, mobile phones, handheld devices, video game systems, personal digital assistants, navigation systems, projectors,[2] and dashboards in automobiles.
https://en.wikipedia.org/wiki/TFT_LCD
Lamps
Cold-cathode lamps include cold-cathode fluorescent lamps (CCFLs) and neon lamps. Neon lamps primarily rely on excitation of gas molecules to emit light; CCFLs use a discharge in mercury vapor to develop ultraviolet light, which in turn causes a fluorescent coating on the inside of the lamp to emit visible light.
Cold-cathode fluorescent lamps were used for backlighting of LCDs, for example computer monitors and television screens.
In the lighting industry, “cold cathode” historically refers to luminous tubing larger than 20 mm in diameter and operating on a current of 120 to 240 milliamperes. This larger-diameter tubing is often used for interior alcove and general lighting.[3][4] The term "neon lamp" refers to tubing that is smaller than 15 mm in diameter[citation needed] and typically operates at approximately 40 milliamperes. These lamps are commonly used for neon signs.
https://en.wikipedia.org/wiki/Cold_cathode#Lamps
Details
The cathode is the negative electrode. Any gas-discharge lamp has a positive (anode) and a negative electrode. Both electrodes alternate between acting as an anode and a cathode when these devices run with alternating current.
A cold cathode is distinguished from a hot cathode that is heated to induce thermionic emission of electrons. Discharge tubes with hot cathodes have an envelope filled with low-pressure gas and containing two electrodes. Hot cathode devices include common vacuum tubes, fluorescent lamps, high-pressure discharge lamps and vacuum fluorescent displays.
https://en.wikipedia.org/wiki/Cold_cathode#Lamps
A vacuum fluorescent display (VFD) is a display device once commonly used on consumer electronics equipment such as video cassette recorders, car radios, and microwave ovens.
A VFD operates on the principle of cathodoluminescence, roughly similar to a cathode ray tube, but operating at much lower voltages. Each tube in a VFD has a phosphor-coated carbon anode that is bombarded by electrons emitted from the cathode filament.[1][2] In fact, each tube in a VFD is a triode vacuum tube because it also has a mesh control grid.[3]
Unlike liquid crystal displays, a VFD emits very bright light with high contrast and can support display elements of various colors. Standard illumination figures for VFDs are around 640 cd/m2 with high-brightness VFDs operating at 4,000 cd/m2, and experimental units as high as 35,000 cd/m2 depending on the drive voltage and its timing.[3] The choice of color (which determines the nature of the phosphor) and display brightness significantly affect the lifetime of the tubes, which can range from as low as 1,500 hours for a vivid red VFD to 30,000 hours for the more common green ones.[3] Cadmium was commonly used in the phosphors of VFDs in the past, but the current RoHS-compliant VFDs have eliminated this metal from their construction, using instead phosphors consisting of a matrix of alkaline earth and very small amounts of group III metals, doped with very small amounts of rare earth metals.[4]
VFDs can display seven-segment numerals, multi-segment alpha-numeric characters or can be made in a dot-matrix to display different alphanumeric characters and symbols. In practice, there is little limit to the shape of the image that can be displayed: it depends solely on the shape of phosphor on the anode(s).
The first VFD was the single indication DM160 by Philips in 1959.[5] The first multi-segment VFD was a 1967 Japanese single-digit, seven-segment device. The displays became common on calculators and other consumer electronics devices.[6] In the late 1980s hundreds of millions of units were made yearly.[7]
Design
The device consists of a hot cathode (filaments), grids and anodes (phosphor) encased in a glass envelope under a high vacuum condition. The cathode is made up of fine tungsten wires, coated by alkaline earth metal oxides (barium,[2] strontium and calcium oxides[8][9]), which emit electrons when heated to 650 °C[2] by an electric current. These electrons are controlled and diffused by the grids (made using Photochemical machining), which are made up of thin (50 micron thick) stainless steel.[2] If electrons impinge on the phosphor-coated anode plates, they fluoresce, emitting light. Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much lower temperatures, and are therefore essentially invisible.[10] The anode consists of a glass plate with electrically conductive traces (each trace is connected to a single indicator segment), which is coated with an insulator, which is then partially etched to create holes which are then filled with a conductor like graphite, which in turn is coated with phosphor. This transfers energy from the trace to the segment. The shape of the phosphor will determine the shape of the VFD's segments. The most widely used phosphor is Zinc-doped copper-activated Zinc oxide,[2] which generates light at a peak wavelength of 505 nm.
The cathode wire to which the oxides are applied is made of tungsten or ruthenium-tungsten alloy. The oxides in the cathodes are not stable in air, so they are applied to the cathode as carbonates, the cathodes are assembled into the VFD, and the cathodes are heated by passing a current through them while inside the vacuum of the VFD to convert the carbonates into oxides.[2][9]
The principle of operation is identical to that of a vacuum tube triode. Electrons can only reach (and "illuminate") a given plate element if both the grid and the plate are at a positive potential with respect to the cathode.[11] This allows the displays to be organized as multiplexed displays where the multiple grids and plates form a matrix, minimizing the number of signal pins required. In the example of the VCR display shown to the right, the grids are arranged so that only one digit is illuminated at a time. All of the similar plates in all of the digits (for example, all of the lower-left plates in all of the digits) are connected in parallel. One by one, the microprocessor driving the display enables a digit by placing a positive voltage on that digit's grid and then placing a positive voltage on the appropriate plates. Electrons flow through that digit's grid and strike those plates that are at a positive potential. The microprocessor cycles through illuminating the digits in this way at a rate high enough to create the illusion of all digits glowing at once via persistence of vision.
The extra indicators (in our example, "VCR", "Hi-Fi", "STEREO", "SAP", etc.) are arranged as if they were segments of an additional digit or two or extra segments of existing digits and are scanned using the same multiplexed strategy as the real digits. Some of these extra indicators may use a phosphor that emits a different color of light, for example, orange.
The light emitted by most VFDs contains many colors and can often be filtered to enhance the color saturation providing a deep green or deep blue, depending on the whims of the product's designers. Phosphors used in VFDs are different from those in cathode-ray displays since they must emit acceptable brightness with only around 50 volts of electron energy, compared to several thousand volts in a CRT.[12] The insulating layer in a VFD is normally black, however it can be removed or made transparent to allow the display to be transparent. AMVFD displays that incorporate a driver IC are available for applications that require high image brightness and an increased number of pixels. Phosphors of different colors can be stacked on top of each other for achieving gradations and various color combinations. Hybrid VFDs include both fixed display segments and a graphic VFD in the same unit. VFDs may have display segments, grids and related circuitry on their front and rear plass panels, using a central cathode for both panels, allowing for increased segment density. The segments can also be placed exclusively on the front instead of on the back, improving viewing angles and brightness.[13][14][15][16][17][18][19][20][21]
The Russian IV-15 VFD tube is very similar to the DM160. The DM160, DM70/DM71 and Russian IV-15 can (like a VFD panel) be used as triodes. The DM160 is thus the smallest VFD and smallest triode valve. The IV-15 is slightly different shape (see photo of DM160 and IV-15 for comparison).
https://en.wikipedia.org/wiki/Vacuum_fluorescent_display
Cold-cathode fluorescent lamps
Most fluorescent lamps use electrodes that emit electrons into the tube by heat, known as hot cathodes. However, cold cathode tubes have cathodes that emit electrons only due to the large voltage between the electrodes. The cathodes will be warmed by current flowing through them, but are not hot enough for significant thermionic emission. Because cold cathode lamps have no thermionic emission coating to wear out, they can have much longer lives than hot cathode tubes. This makes them desirable for long-life applications (such as backlights in liquid crystal displays). Sputtering of the electrode may still occur, but electrodes can be shaped (e.g. into an internal cylinder) to capture most of the sputtered material so it is not lost from the electrode.
Cold cathode lamps are generally less efficient than thermionic emission lamps because the cathode fall voltage is much higher. Power dissipated due to cathode fall voltage does not contribute to light output. However, this is less significant with longer tubes. The increased power dissipation at tube ends also usually means cold cathode tubes have to be run at a lower loading than their thermionic emission equivalents. Given the higher tube voltage required anyway, these tubes can easily be made long, and even run as series strings. They are better suited for bending into special shapes for lettering and signage, and can also be instantly switched on or off.
Starting
The gas used in the fluorescent tube must be ionized before the arc can "strike" . For small lamps, it does not take much voltage to strike the arc and starting the lamp presents no problem, but larger tubes require a substantial voltage (in the range of a thousand volts). Many different starting circuits have been used. The choice of circuit is based on cost, AC voltage, tube length, instant versus non-instant starting, temperature ranges and parts availability.
Preheating
Preheating, also called switchstart, uses a combination filament–cathode at each end of the lamp in conjunction with a mechanical or automatic (bi-metallic) switch (see circuit diagram to the right) that initially connect the filaments in series with the ballast to preheat them; after a short preheating time the starting switch opens. If timed correctly relative to the phase of the supply AC, this causes the ballast to induce a voltage over the tube high enough to initiate the starting arc.[35] These systems are standard equipment in 200–240 V countries (and in the United States lamps up to about 30 watts).
Before the 1960s, four-pin thermal starters and manual switches were used.[citation needed] A glow switch starter automatically preheats the lamp cathodes. It consists of a normally open bi-metallic switch in a small sealed gas-discharge lamp containing inert gas (neon or argon). The glow switch will cyclically warm the filaments and initiate a pulse voltage to strike the arc; the process repeats until the lamp is lit. Once the tube strikes, the impinging main discharge keeps the cathodes hot, permitting continued electron emission. The starter switch does not close again because the voltage across the lit tube is insufficient to start a glow discharge in the starter.[35]
With glow switch starters a failing tube will cycle repeatedly. Some starter systems used a thermal over-current trip to detect repeated starting attempts and disable the circuit until manually reset.
A power factor correction (PFC) capacitor draws leading current from the mains to compensate for the lagging current drawn by the lamp circuit.[35]
Electronic starters use a different method to preheat the cathodes.[36] They may be plug-in interchangeable with glow starters. They use a semiconductor switch and "soft start" the lamp by preheating the cathodes before applying a starting pulse which strikes the lamp first time without flickering; this dislodges a minimal amount of material from the cathodes during starting, giving longer lamp life.[35] This is claimed to prolong lamp life by a factor of typically 3 to 4 times for a lamp frequently switched on as in domestic use,[37] and to reduce the blackening of the ends of the lamp typical of fluorescent tubes. While the circuit is complex, the complexity is built into an integrated circuit chip. Electronic starters may be optimized for fast starting (typical start time of 0.3 seconds),[37][38] or for most reliable starting even at low temperatures and with low supply voltages, with a startup time of 2–4 seconds.[39] The faster-start units may produce audible noise during start-up.[40]
Electronic starters only attempt to start a lamp for a short time when power is initially applied, and do not repeatedly attempt to restrike a lamp that is dead and unable to sustain an arc; some automatically stop trying to start a failed lamp.[36] This eliminates the re-striking of a lamp and the continuous flashing of a failing lamp with a glow starter. Electronic starters are not subject to wear and do not need replacing periodically, although they may fail like any other electronic circuit. Manufacturers typically quote lives of 20 years, or as long as the light fitting.[38][39]
Instant start
Instant start fluorescent tubes were invented in 1944. Instant start simply uses a high enough voltage to break down the gas column and thereby start arc conduction. Once the high-voltage spark "strikes" the arc, the current is boosted until a glow discharge forms. As the lamp warms and pressure increases, the current continues to rise and both resistance and voltage falls, until mains or line-voltage takes over and the discharge becomes an arc. These tubes have no filaments and can be identified by a single pin at each end of the tube (for common lamps; compact cold-cathode lamps may also have a single pin, but operate from a transformer rather than a ballast). The lamp holders have a "disconnect" socket at the low-voltage end which disconnects the ballast when the tube is removed, to prevent electric shock. Instant-start lamps are slightly more energy efficient than rapid start, because they do not constantly send a heating current to the cathodes during operation, but the cold cathodes starting increases sputter, and they take much longer to transition from a glow discharge to an arc during warm up, thus the lifespan is typically about half of those seen in comparable rapid-start lamps.[41]
Rapid start
Because the formation of an arc requires the thermionic emission of large quantities of electrons from the cathode, rapid start ballast designs provide windings within the ballast that continuously warm the cathode filaments. Usually operating at a lower arc voltage than the instant start design; no inductive voltage spike is produced for starting, so the lamps must be mounted near a grounded (earthed) reflector to allow the glow discharge to propagate through the tube and initiate the arc discharge via capacitive coupling. In some lamps a grounded "starting aid" strip is attached to the outside of the lamp glass. This ballast type is incompatible with the European energy saver T8 fluorescent lamps because these lamps requires a higher starting voltage than that of the open circuit voltage of rapid start ballasts.
Quick-start
Quick-start ballasts use a small auto-transformer to heat the filaments when power is first applied. When an arc strikes, the filament heating power is reduced and the tube will start within half a second. The auto-transformer is either combined with the ballast or may be a separate unit. Tubes need to be mounted near an earthed metal reflector in order for them to strike. Quick-start ballasts are more common in commercial installations because of lower maintenance costs. A quick-start ballast eliminates the need for a starter switch, a common source of lamp failures. Nonetheless, Quick-start ballasts are also used in domestic (residential) installations because of the desirable feature that a Quick-start ballast light turns on nearly immediately after power is applied (when a switch is turned on). Quick-start ballasts are used only on 240 V circuits and are designed for use with the older, less efficient T12 tubes.
Semi-resonant start
The semi-resonant start circuit was invented by Thorn Lighting for use with T12 fluorescent tubes. This method uses a double wound transformer and a capacitor. With no arc current, the transformer and capacitor resonate at line frequency and generate about twice the supply voltage across the tube, and a small electrode heating current.[42] This tube voltage is too low to strike the arc with cold electrodes, but as the electrodes heat up to thermionic emission temperature, the tube striking voltage falls below that of the ringing voltage, and the arc strikes. As the electrodes heat, the lamp slowly, over three to five seconds, reaches full brightness. As the arc current increases and tube voltage drops, the circuit provides current limiting.
Semi-resonant start circuits are mainly restricted to use in commercial installations because of the higher initial cost of circuit components. However, there are no starter switches to be replaced and cathode damage is reduced during starting making lamps last longer, reducing maintenance costs. Because of the high open circuit tube voltage, this starting method is particularly good for starting tubes in cold locations. Additionally, the circuit power factor is almost 1.0, and no additional power factor correction is needed in the lighting installation. As the design requires that twice the supply voltage must be lower than the cold-cathode striking voltage (or the tubes would erroneously instant-start), this design cannot be used with 240 volt AC power unless the tubes are at least 1.2 m (3 ft 11 in) length. Semi-resonant start fixtures are generally incompatible with energy saving T8 retrofit tubes, because such tubes have a higher starting voltage than T12 lamps and may not start reliably, especially in low temperatures. Recent proposals in some countries to phase out T12 tubes will reduce the application of this starting method.
Electronic ballasts
Electronic ballasts employ transistors to change the supply frequency into high-frequency AC while regulating the current flow in the lamp. These ballasts take advantage of the higher efficacy of lamps, which rises by almost 10% at 10 kHz, compared to efficacy at normal power frequency. When the AC period is shorter than the relaxation time to de-ionize mercury atoms in the discharge column, the discharge stays closer to optimum operating condition.[43] Electronic ballasts convert supply frequency AC power to variable frequency AC. The conversion can reduce lamp brightness modulation at twice the power supply frequency.
Low cost ballasts contain only a simple oscillator and series resonant LC circuit. This principle is called the current resonant inverter circuit. After a short time the voltage across the lamp reaches about 1 kV and the lamp instant-starts in cold cathode mode. The cathode filaments are still used for protection of the ballast from overheating if the lamp does not ignite. A few manufacturers use positive temperature coefficient (PTC) thermistors to disable instant starting and give some time to preheat the filaments.
More complex electronic ballasts use programmed start. The output frequency is started above the resonance frequency of the output circuit of the ballast; and after the filaments are heated, the frequency is rapidly decreased. If the frequency approaches the resonant frequency of the ballast, the output voltage will increase so much that the lamp will ignite. If the lamp does not ignite, an electronic circuit stops the operation of the ballast.
Many electronic ballasts are controlled by a microcontroller, and these are sometimes called digital ballasts. Digital ballasts can apply quite complex logic to lamp starting and operation. This enables functions such as testing for broken electrodes and missing tubes before attempting to start, detection of tube replacement, and detection of tube type, such that a single ballast can be used with several different tubes. Features such as dimming can be included in the embedded microcontroller software, and can be found in various manufacturers' products.
Since introduction in the 1990s, high-frequency ballasts have been used in general lighting fixtures with either rapid start or pre-heat lamps. These ballasts convert the incoming power to an output frequency in excess of 20 kHz. This increases lamp efficiency.[44] These ballasts operate with voltages that can be almost 600 volts, requiring some consideration in housing design, and can cause a minor limitation in the length of the wire leads from the ballast to the lamp ends.
End of life
The life expectancy of a fluorescent lamp is primarily limited by the life of the cathode electrodes. To sustain an adequate current level, the electrodes are coated with an emission mixture of metal oxides. Every time the lamp is started, and during operation, some small amount of the cathode coating is sputtered off the electrodes by the impact of electrons and heavy ions within the tube. The sputtered material collects on the walls of the tube, darkening it. The starting method and frequency affect cathode sputtering. A filament may also break, disabling the lamp.
Low-mercury designs of lamps may fail when mercury is absorbed by the glass tube, phosphor, and internal components, and is no longer available to vaporize in the fill gas. Loss of mercury initially causes an extended warm-up time to full light output, and finally causes the lamp to glow a dim pink when the argon gas takes over as the primary discharge.[45]
Subjecting the tube to asymmetric current flow, effectively operates it under a DC bias, and causes asymmetric distribution of mercury ions along the tube. The localized depletion of mercury vapor pressure manifests itself as pink luminescence of the base gas in the vicinity of one of the electrodes, and the operating lifetime of the lamp may be dramatically shortened. This can be an issue with some poorly designed inverters.[46]
The phosphors lining the lamp degrade with time as well, until a lamp no longer produces an acceptable fraction of its initial light output.
Failure of the integral electronic ballast of a compact fluorescent bulb will also end its usable life.
Phosphors and the spectrum of emitted light
The spectrum of light emitted from a fluorescent lamp is the combination of light directly emitted by the mercury vapor, and light emitted by the phosphorescent coating. The spectral lines from the mercury emission and the phosphorescence effect give a combined spectral distribution of light that is different from those produced by incandescent sources. The relative intensity of light emitted in each narrow band of wavelengths over the visible spectrum is in different proportions compared to that of an incandescent source. Colored objects are perceived differently under light sources with differing spectral distributions. For example, some people find the color rendition produced by some fluorescent lamps to be harsh and displeasing. A healthy person can sometimes appear to have an unhealthy skin tone under fluorescent lighting. The extent to which this phenomenon occurs is related to the light's spectral composition, and may be gauged by its color rendering index (CRI).
Color temperature
Correlated color temperature (CCT) is a measure of the "shade" of whiteness of a light source compared with a blackbody. Typical incandescent lighting is 2700 K, which is yellowish-white.[47] Halogen lighting is 3000 K.[48] Fluorescent lamps are manufactured to a chosen CCT by altering the mixture of phosphors inside the tube. Warm-white fluorescents have CCT of 2700 K and are popular for residential lighting. Neutral-white fluorescents have a CCT of 3000 K or 3500 K. Cool-white fluorescents have a CCT of 4100 K and are popular for office lighting. Daylight fluorescents have a CCT of 6500 K, which is bluish-white.
Color rendering index
Color rendering index (CRI) is a measure of how well colors can be perceived using light from a source, relative to light from a reference source such as daylight or a blackbody of the same color temperature. By definition, an incandescent lamp has a CRI of 100. Real-life fluorescent tubes achieve CRIs of anywhere from 50 to 98. Fluorescent lamps with low CRI have phosphors that emit too little red light. Skin appears less pink, and hence "unhealthy" compared with incandescent lighting. Colored objects appear muted. For example, a low CRI 6800 K halophosphate tube (an extreme example) will make reds appear dull red or even brown. Since the eye is relatively less efficient at detecting red light, an improvement in color rendering index, with increased energy in the red part of the spectrum, may reduce the overall luminous efficacy.[31]: 8
Lighting arrangements use fluorescent tubes in an assortment of tints of white. Mixing tube types within fittings can improve the color reproduction of lower quality tubes.
Phosphor composition
Some of the least pleasant light comes from tubes containing the older, halophosphate-type phosphors (chemical formula Ca5(PO4)3(F, Cl):Sb3+, Mn2+). This phosphor mainly emits yellow and blue light, and relatively little green and red. In the absence of a reference, this mixture appears white to the eye, but the light has an incomplete spectrum. The color rendering index (CRI) of such lamps is around 60.
Since the 1990s, higher-quality fluorescent lamps use triphosphor mixture, based on europium and terbium ions, which have emission bands more evenly distributed over the spectrum of visible light. Triphosphor tubes gives a more natural color reproduction to the human eye. The CRI of such lamps is typically 85.
| Typical fluorescent lamp with rare-earth phosphor | |
A typical "cool white" fluorescent lamp utilizing two rare-earth-doped phosphors, Tb3+, Ce3+:LaPO4 for green and blue emission and Eu:Y2O3 for red. For an explanation of the origin of the individual peaks click on the image. Several of the spectral peaks are directly generated from the mercury arc. This is likely the most common type of fluorescent lamp in use today. |
| An older-style halophosphate-phosphor fluorescent lamp | |
Halophosphate phosphors in these lamps usually consist of trivalent antimony- and divalent manganese-doped calcium halophosphate (Ca5(PO4)3(Cl, F):Sb3+, Mn2+). The color of the light output can be adjusted by altering the ratio of the blue-emitting antimony dopant and orange-emitting manganese dopant. The color rendering ability of these older-style lamps is quite poor. Halophosphate phosphors were invented by A. H. McKeag et al. in 1942. |
| "Natural sunshine" fluorescent light | |
Peaks with stars are mercury lines. |
| Yellow fluorescent lights | |
The spectrum is nearly identical to a normal fluorescent lamp except for a near total lack of light shorter than 500 nanometers. This effect can be achieved through either specialized phosphor use or more commonly by the use of a simple yellow light filter. These lamps are commonly used as lighting for photolithography work in cleanrooms and as "bug repellent" outdoor lighting (the efficacy of which is questionable). |
| Spectrum of a "blacklight" lamp | |
There is typically only one phosphor present in a blacklight lamp, usually consisting of europium-doped strontium fluoroborate, which is contained in an envelope of Wood's glass. |
Applications
Fluorescent lamps come in many shapes and sizes.[49] The compact fluorescent lamp (CFL) is becoming more popular. Many compact fluorescent lamps integrate the auxiliary electronics into the base of the lamp, allowing them to fit into a regular light bulb socket.
In US residences, fluorescent lamps are mostly found in kitchens, basements, or garages, but schools and businesses find the cost savings of fluorescent lamps to be significant and rarely use incandescent lights. Electricity costs, tax incentives and building codes result in higher use in places such as California. Fluorescent use is declining as LED lighting, which is more energy efficient and doesn't contain mercury, is replacing fluorescents.[citation needed]
In other countries, residential use of fluorescent lighting varies depending on the price of energy, financial and environmental concerns of the local population, and acceptability of the light output. In East and Southeast Asia it is very rare to see incandescent bulbs in buildings anywhere.
Many countries are encouraging the phase-out of incandescent light bulbs and substitution of incandescent lamps with fluorescent lamps or LED and other types of energy-efficient lamps.
In addition to general lighting, special fluorescent lights are often used in stage lighting for film and video production. They are cooler than traditional halogen light sources, and use high-frequency ballasts to prevent video flickering and high color-rendition index lamps to approximate daylight color temperatures.
Comparison to incandescent lamps
Luminous efficacy
Fluorescent lamps convert more of the input power to visible light than incandescent lamps. A typical 100 watt tungsten filament incandescent lamp may convert only 5% of its power input to visible white light (400–700 nm wavelength), whereas typical fluorescent lamps convert about 22% of the power input to visible white light.[31]: 20
The efficacy of fluorescent tubes ranges from about 16 lumens per watt for a 4 watt tube with an ordinary ballast to over 100 lumens per watt[50] with a modern electronic ballast, commonly averaging 50 to 67 lm/W overall.[51] Ballast loss can be about 25% of the lamp power with magnetic ballasts, and around 10% with electronic ballasts.
Fluorescent lamp efficacy is dependent on lamp temperature at the coldest part of the lamp. In T8 lamps this is in the center of the tube. In T5 lamps this is at the end of the tube with the text stamped on it. The ideal temperature for a T8 lamp is 25 °C (77 °F) while the T5 lamp is ideally at 35 °C (95 °F).
Life
Typically a fluorescent lamp will last 10 to 20 times as long as an equivalent incandescent lamp when operated several hours at a time. Under standard test conditions fluorescent lamps last 6,000 to 80,000 hours (2 to 27 years at 8 hours per day).[52]
The higher initial cost of a fluorescent lamp compared with an incandescent lamp is usually compensated for by lower energy consumption over its life.[53][needs update]
Lower luminance
Compared with an incandescent lamp, a fluorescent tube is a more diffuse and physically larger light source. In suitably designed lamps, light can be more evenly distributed without point source of glare such as seen from an undiffused incandescent filament; the lamp is large compared to the typical distance between lamp and illuminated surfaces.
Lower heat
Fluorescent lamps give off about one-fifth the heat of equivalent incandescent lamps. This greatly reduces the size, cost, and energy consumption devoted to air conditioning for office buildings that would typically have many lights and few windows.
Disadvantages
Frequent switching
Frequent switching (more than every 3 hours) will shorten the life of lamps. [54] Each start cycle slightly erodes the electron-emitting surface of the cathodes; when all the emission material is gone, the lamp cannot start with the available ballast voltage. Fixtures for flashing lights (such as for advertising) use a ballast that maintains cathode temperature when the arc is off, preserving the life of the lamp.
The extra energy used to start a fluorescent lamp is equivalent to a few seconds of normal operation; it is more energy-efficient to switch off lamps when not required for several minutes.[55][56]
Mercury content
If a fluorescent lamp is broken, a very small amount of mercury can contaminate the surrounding environment. About 99% of the mercury is typically contained in the phosphor, especially on lamps that are near the end of their life.[57] Broken lamps may release mercury if not cleaned with correct methods.[58][failed verification]
Due to the mercury content, discarded fluorescent lamps must be treated as hazardous waste. For large users of fluorescent lamps, recycling services are available in some areas, and may be required by regulation.[59][60] In some areas, recycling is also available to consumers.[61]
Ultraviolet emission
Fluorescent lamps emit a small amount of ultraviolet (UV) light. A 1993 study in the US found that ultraviolet exposure from sitting under fluorescent lights for eight hours is equivalent to one minute of sun exposure.[62] Ultraviolet radiation from compact fluorescent lamps may exacerbate symptoms in photosensitive individuals.[63][64][65]
Museum artifacts may need protection from UV light to prevent degradation of pigments or textiles. [66]
Ballast
Fluorescent lamps require a ballast to stabilize the current through the lamp, and to provide the initial striking voltage required to start the arc discharge. Often one ballast is shared between two or more lamps. Electromagnetic ballasts can produce an audible humming or buzzing noise. In North America, magnetic ballasts are usually filled with a tar-like potting compound to reduce emitted noise. Hum is eliminated in lamps with a high-frequency electronic ballast. Energy lost in magnetic ballasts is around 10% of lamp input power according to GE literature from 1978.[31] Electronic ballasts reduce this loss.
Power quality and radio interference
Simple inductive fluorescent lamp ballasts have a power factor of less than unity. Inductive ballasts include power factor correction capacitors. Simple electronic ballasts may also have low power factor due to their rectifier input stage.
Fluorescent lamps are a non-linear load and generate harmonic currents in the electrical power supply. The arc within the lamp may generate radio frequency noise, which can be conducted through power wiring. Suppression of radio interference is possible. Very good suppression is possible, but adds to the cost of the fluorescent fixtures.
Fluorescent lamps near end of life can present a serious radio frequency interference hazard. Oscillations are generated from the negative differential resistance of the arc, and the current flow through the tube can form a tuned circuit whose frequency depends on path length. [67]
Operating temperature
Fluorescent lamps operate best around room temperature. At lower or higher temperatures, efficacy decreases. At below-freezing temperatures standard lamps may not start. Special lamps may be used for reliable service outdoors in cold weather.
Lamp shape
Fluorescent tubes are long, low-luminance sources compared with high intensity discharge lamps, incandescent and halogen lamps and high power LEDs. However, low luminous intensity of the emitting surface is useful because it reduces glare. Lamp fixture design must control light from a long tube instead of a compact globe. The compact fluorescent lamp (CFL) replaces regular incandescent bulbs in many light fixtures where space permits.
Flicker
Fluorescent lamps with magnetic ballasts flicker at a normally unnoticeable frequency of 100 or 120 Hz and this flickering can cause problems for some individuals with light sensitivity;[68] they are listed as problematic for some individuals with autism, epilepsy,[69] lupus,[70] chronic fatigue syndrome, Lyme disease,[71] and vertigo.[72]
A stroboscopic effect can be noticed, where something spinning at just the right speed may appear stationary if illuminated solely by a single fluorescent lamp. This effect is eliminated by paired lamps operating on a lead-lag ballast. Unlike a true strobe lamp, the light level drops in appreciable time and so substantial "blurring" of the moving part would be evident.
Fluorescent lamps may produce flicker at the power supply frequency (50 or 60 Hz), which is noticeable by more people. This happens if a damaged or failed cathode results in slight rectification and uneven light output in positive and negative going AC cycles. Power frequency flicker can be emitted from the ends of the tubes, if each tube electrode produces a slightly different light output pattern on each half-cycle. Flicker at power frequency is more noticeable in the peripheral vision than it is when viewed directly.
Near the end of life, fluorescent lamps can start flickering at a frequency lower than the power frequency. This is due to instability in the negative resistance of arc discharge,[73] which can be from a bad lamp or ballast or poor connection.
New fluorescent lamps may show a twisting spiral pattern of light in a part of the lamp. This effect is due to loose cathode material and usually disappears after a few hours of operation.[31]: 22
Electromagnetic ballasts may also cause problems for video recording as there can be a so-called beat effect between the video frame rate and the fluctuations in intensity of the fluorescent lamp.
Fluorescent lamps with electronic ballasts do not flicker, since above about 5 kHz, the excited electron state half-life is longer than a half cycle,[citation needed] and light production becomes continuous. Operating frequencies of electronic ballasts are selected to avoid interference with infrared remote controls. Poor quality or faulty electronic ballasts may have considerable 100/120 Hz modulation of the light.
Dimming
Fluorescent light fixtures cannot be connected to dimmer switches intended for incandescent lamps. Two effects are responsible for this: the waveform of the voltage emitted by a standard phase-control dimmer interacts badly with many ballasts, and it becomes difficult to sustain an arc in the fluorescent tube at low power levels. Dimming installations require a compatible dimming ballast. Some models of compact fluorescent lamps can be dimmed; in the United States, such lamps are identified as complying with UL standard 1993.[74]
Lamp sizes and designations
Systematic nomenclature identifies mass-market lamps as to general shape, power rating, length, color, and other electrical and illuminating characteristics.
In the United States and Canada, lamps are typically identified by a code such as FxxTy, where F is for fluorescent, the first number (xx) indicates either the power in watts or length in inches, the T indicates that the shape of the bulb is tubular, and the last number (y) is the diameter in eighths of an inch (sometimes in millimeters, rounded-up to the nearest millimeter). Typical diameters are T12 or T38 (1+1⁄2 inch or 38 mm) for residential lamps, T8 or T26 (1 inch or 25 mm) for commercial energy-saving lamps.
Overdriving
Overdriving a fluorescent lamp is a method of getting more light from each tube than is obtained under rated conditions. ODNO (Overdriven Normal Output) fluorescent tubes are generally used when there isn't enough room to put in more bulbs to increase the light. The method is effective, but generates some additional issues. This technique has become popular among aquatic gardeners as a cost-effective way to add more light to their aquariums. Overdriving is done by rewiring lamp fixtures to increase lamp current; however, lamp life is reduced.[75]
Other fluorescent lamps
Black light
Blacklights are a subset of fluorescent lamps that are used to provide UVA light (at about 360 nm wavelength). They are built in the same fashion as conventional fluorescent lamps but the glass tube is coated with a phosphor that converts the short-wave UV within the tube to long-wave UV rather than to visible light. They are used to provoke fluorescence (to provide dramatic effects using blacklight paint and to detect materials such as urine and certain dyes that would be invisible in visible light) as well as to attract insects to bug zappers.
So-called blacklite blue lamps are also made from more expensive deep purple glass known as Wood's glass rather than clear glass. The deep purple glass filters out most of the visible colors of light directly emitted by the mercury-vapor discharge, producing proportionally less visible light compared with UV light. This allows UV-induced fluorescence to be seen more easily (thereby allowing blacklight posters to seem much more dramatic). The blacklight lamps used in bug zappers do not require this refinement so it is usually omitted in the interest of cost; they are called simply blacklite (and not blacklite blue).
Tanning lamp
The lamps used in tanning beds contain a different phosphor blend (typically 3 to 5 or more phosphors) that emits both UVA and UVB, provoking a tanning response in most human skin. Typically, the output is rated as 3–10% UVB (5% most typical) with the remaining UV as UVA. These are mainly high output 100W lamps, although 160W very high output are somewhat common. One common phosphor used in these lamps is lead-activated barium disilicate, but a europium-activated strontium fluoroborate is also used. Early lamps used thallium as an activator, but emissions of thallium during manufacture were toxic.[76]
UVB medical lamps
The lamps used in phototherapy contain a phosphor that emits only UVB ultraviolet light.[citation needed] There are two types: broadband UVB that gives 290–320 nanometer with peak wavelength of 306 nm, and narrowband UVB that gives 311–313 nanometer. Because of the longer wavelength, the narrowband UVB bulbs do not cause erythema in the skin like the broadband.[dubious ] They requires a 10–20 times higher dose to the skin and they require more bulbs and longer exposure time. The narrowband is good for psoriasis, eczema (atopic dermatitis), vitiligo, lichen planus, and some other skin diseases.[citation needed] The broadband is better for increasing Vitamin D3 in the body.
Grow lamp
Grow lamps contain phosphor blends that encourage photosynthesis, growth, or flowering in plants, algae, photosynthetic bacteria, and other light-dependent organisms. These often emit light primarily in the red and blue color range, which is absorbed by chlorophyll and used for photosynthesis in plants.[77]
Infrared lamps
Lamps can be made with a lithium metaluminate phosphor activated with iron. This phosphor has peak emissions between 675 and 875 nanometers, with lesser emissions in the deep red part of the visible spectrum.[78]
Bilirubin lamps
Deep blue light generated from a europium-activated phosphor is used in the light therapy treatment of jaundice; light of this color penetrates skin and helps in the breakup of excess bilirubin.[78]
Germicidal lamp
Germicidal lamps contain no phosphor at all, making them mercury vapor gas discharge lamps rather than fluorescent. Their tubes are made of fused quartz transparent to the UVC light emitted by the mercury discharge. The 254 nm UVC emitted by these tubes will kill germs and the 184.45 nm far UV will ionize oxygen to ozone. Lamps labeled OF block the 184.45 nm far UV and do not produce significant ozone. In addition the UVC can cause eye and skin damage. They are sometimes used by geologists to identify certain species of minerals by the color of their fluorescence when fitted with filters that pass the short-wave UV and block visible light produced by the mercury discharge. They are also used in some EPROM erasers. Germicidal lamps have designations beginning with G, for example G30T8 for a 30-watt, 1-inch (2.5 cm) diameter, 36-inch (91 cm) long germicidal lamp (as opposed to an F30T8, which would be the fluorescent lamp of the same size and rating).
Electrodeless lamp
Electrodeless induction lamps are fluorescent lamps without internal electrodes. They have been commercially available since 1990. A current is induced into the gas column using electromagnetic induction. Because the electrodes are usually the life-limiting element of fluorescent lamps, such electrodeless lamps can have a very long service life, although they also have a higher purchase price.
Cold-cathode fluorescent lamp
Cold-cathode fluorescent lamps were used as backlighting for LCDs in computer monitors and televisions before the use of LED-backlit LCDs. They were also popular with computer case modders.
Science demonstrations
Fluorescent lamps can be illuminated by means other than a proper electrical connection. These other methods, however, result in very dim or very short-lived illumination, and so are seen mostly in science demonstrations. Static electricity or a Van de Graaff generator will cause a lamp to flash momentarily as it discharges a high-voltage capacitance. A Tesla coil will pass high-frequency current through the tube, and since it has a high voltage as well, the gases within the tube will ionize and emit light. This also works with plasma globes. Capacitive coupling with high-voltage power lines can light a lamp continuously at low intensity, depending on the intensity of the electric field.
See also
- Gas-filled tube
- LED tubes — made as drop-in replacement for fluorescents
- List of light sources
- Metal-halide lamp
References
All three of the 'FAST' (< .5 seconds) starter brands caused an audible 'BURRRRRRRP' noise in some light fittings as they started and this is an inherent problem caused by their use of the faster 'DC' heating. It is worse with higher wattage tubes and if there is any loose metal in the light fitting.
- Kane & Sell 2001, p. 122.
Sources
- Bright, Arthur Aaron Jr. (1949). The Electric-Lamp Industry: Technological Change and Economic Development from 1800 to 1947. Macmillan Co.
- Kane, Raymond; Sell, Heinz, eds. (2001). Revolution in lamps: a chronicle of 50 years of progress (2nd ed.). The Fairmont Press, Inc. ISBN 978-0-88173-378-5.
- Van Broekhoven, Jacob (2001). "Lamp Phosphors". In Kane, Raymond; Sell, Heinz (eds.). Revolution in lamps: a chronicle of 50 years of progress (2nd ed.). The Fairmont Press, Inc. pp. 93–126. ISBN 978-0-88173-378-5.
Further reading
- Emanuel Gluskin, “The fluorescent lamp circuit”, (Circuits & Systems Expositions)
- IEEE Transactions on Circuits and Systems, Part I: Fundamental Theory and Applications 46(5), 1999 (529–544).
External links
- Popular Science, January 1940 Fluorescent Lamps
- T5 Fluorescent Systems — Lighting Research Center Research about the improved T5 relative to the previous T8 standard
- NASA: The Fluorescent Lamp: A plasma you can use
- How Fluorescent Tubes are Manufactured on YouTube
- Museum of Electric Lamp Technology
- R. N. Thayer (October 25, 1991). "The Fluorescent Lamp: Early U. S. Development". The Report courtesy of General Electric Company. Archived from the original on 2007-03-24. Retrieved 2007-03-18.
- Wiebe E. Bijker,Of bicycles, bakelites, and bulbs: toward a theory of sociotechnical change MIT Press, 1995, Chapter 4, preview available at Google Books, on the social construction of fluorescent lighting
- Explanations and schematics of some fluorescent lamps
- Fluorescence
- Gas discharge lamps
- Glass applications
- Types of lamp
- Plasma physics
- American inventions
- Mercury (element)
https://en.wikipedia.org/wiki/Fluorescent_lamp#Cold-cathode_fluorescent_lamps
A glow switch starter or glowbottle starter is a type of preheat starter used with a fluorescent lamp. It is commonly filled with neon gas or argon gas and contains a bimetallic strip and a stationary electrode. The operating principle is simple, when current is applied, the gas inside ionizes and heats a bimetallic strip which in turn bends toward the stationary electrode thus shorting the starter between the electrodes of the fluorescent lamp. After about a second the starter's bimetallic strip cools and opens the circuit between the electrodes, and the process repeats until the lamp has lit. One disadvantage of glow switch starters is that when the lamp is at the end of its life it will continuously blink on and off until the glow switch starter wears out or an electrode on the fluorescent lamp burns out. Glow starters have a relatively short life, and light fittings enable the starter to be changed easily. Electronic starters, being interchangeable and using the same casing as a glow starter, last for many years.
The glow switch starter was invented by E. C. Dench in 1938.[1]
https://en.wikipedia.org/wiki/Glow_switch_starter
A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.
The invention of the bimetallic strip is generally credited to John Harrison, an eighteenth-century clockmaker who made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[1] Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
This effect is used in a range of mechanical and electrical devices.
https://en.wikipedia.org/wiki/Bimetallic_strip
Clocks
Mechanical clock mechanisms are sensitive to temperature changes as each part has tiny tolerance and it leads to errors in time keeping. A bimetallic strip is used to compensate this phenomenon in the mechanism of some timepieces. The most common method is to use a bimetallic construction for the circular rim of the balance wheel. What it does is move a weight in a radial way looking at the circular plane down by the balance wheel, varying then, the momentum of inertia of the balance wheel. As the spring controlling the balance becomes weaker with the increasing temperature, the balance becomes smaller in diameter to decrease the momentum of inertia and keep the period of oscillation (and hence timekeeping) constant.
Nowadays this system is not used anymore since the appearance of low temperature coefficient alloys like nivarox, parachrom and many others depending on each brand.
Thermostats
In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures are used. In these, one end of the bimetallic strip is mechanically fixed and attached to an electrical power source, while the other (moving) end carries an electrical contact. In adjustable thermostats another contact is positioned with a regulating knob or lever. The position so set controls the regulated temperature, called the set point.
Some thermostats use a mercury switch connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat.
Depending upon the application, a higher temperature may open a contact (as in a heater control) or it may close a contact (as in a refrigerator or air conditioner).
The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a relay or the supply of natural gas or fuel oil through an electrically operated valve. In some natural gas heaters the power may be provided with a thermocouple that is heated by a pilot light (a small, continuously burning, flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered spark generating device.
Thermometers
A direct indicating dial thermometer, common in household devices (such as a patio thermometer or a meat thermometer), uses a bimetallic strip wrapped into a coil in its most common design. The coil changes the linear movement of the metal expansion into a circular movement thanks to the helicoidal shape it draws. One end of the coil is fixed to the housing of the device as a fix point and the other drives an indicating needle inside a circular indicator. A bimetallic strip is also used in a recording thermometer. Breguet's thermometer consists of a tri-metallic helix in order to have a more accurate result.
Heat engine
Heat engines are not the most efficient ones, and with the use of bimetallic strips the efficiency of the heat engine is even lower as there is no chamber to contain the heat. Moreover, the bimetallic strips cannot produce strength in its moves, the reason why is that in order to achieve reasonables bendings (movements) both metallic strips have to be thin to make the difference between the expansion noticeable. So the uses for metallic strips in heat engines are mostly in simple toys that have been built to demonstrate how the principle can be used to drive a heat engine.[citation needed]
Electrical devices
Bimetal strips are used in miniature circuit breakers to protect circuits from excess current. A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches a spring-operated contact. This interrupts the circuit and can be reset when the bimetal strip has cooled down.
Bimetal strips are also used in time-delay relays, gas oven safety valves, thermal flashers for older turn signal lamps, and fluorescent lamp starters. In some devices, the current running directly through the bimetal strip is sufficient to heat it and operate contacts directly. It has also been used in mechanical PWM voltage regulators for automotive uses.[5]
See also
References
https://en.wikipedia.org/wiki/Bimetallic_strip
A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.
The invention of the bimetallic strip is generally credited to John Harrison, an eighteenth-century clockmaker who made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[1] Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
This effect is used in a range of mechanical and electrical devices.
Characteristics
The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled. The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals.
In some applications, the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.
The curvature of a bimetallic beam can be described by the following equation:
where and is the radius of curvature, and are the Young's modulus and height (thickness) of material one and and are the Young's modulus and height (thickness) of material two. is the misfit strain, calculated by:
where α1 is the coefficient of thermal expansion of material one and α2 is the coefficient of thermal expansion of material two. ΔT is the current temperature minus the reference temperature (the temperature where the beam has no flexure).[2][3]
| Derivation of the radius of curvature |
|
|---|
Insight may be gained if the result just given is multiplied on top and bottom by
where , and . Since for small , which is insensitive to because of the lack of first order terms, then we may approximate for close to unity (and insensitive to ), and for close to unity (and insensitive to ). Thus, unless or are very far from unity we can approximate .
History
The earliest surviving bimetallic strip was made by the eighteenth-century clockmaker John Harrison who is generally credited with its invention. He made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.[4] It should not be confused with the bimetallic mechanism for correcting for thermal expansion in his gridiron pendulum. His earliest examples had two individual metal strips joined by rivets but he also invented the later technique of directly fusing molten brass onto a steel substrate. A strip of this type was fitted to his last timekeeper, H5. Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.
https://en.wikipedia.org/wiki/Bimetallic_strip
A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs (tilt, displacement, radial, etc.) but they all share the common design strength of non-eroding switch contacts.
The most common is the mercury tilt switch. It is in one state (open or closed) when tilted one direction with respect to horizontal, and the other state when tilted the other direction. This is what older style thermostats used to turn a heater or air conditioner on or off.
The mercury displacement switch uses a 'plunger' that dips into a pool of mercury, raising the level in the container to contact at least one electrode. This design is used in relays in industrial applications that need to switch high current loads frequently. These relays use electromagnetic coils to pull steel sleeves inside hermetically sealed containers.
https://en.wikipedia.org/wiki/Mercury_switch
Thermostats
Mercury switches were once common in bimetal thermostats. The weight of the movable mercury drop provided some hysteresis by a degree of over-center action. The bimetal spring had to move further to overcome the weight of the mercury, tending to hold it in the open or closed position. The mercury also provided positive on-off switching, and could withstand millions of cycles without contact degradation.
Doorbells
Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interrupters.
Pressure switches
Some pressure switches use a Bourdon tube and a mercury switch. The small force generated by the tube reliably operates the switch.
Vending
Mercury switches are still used in electro-mechanical systems where physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines for tilt alarms that detect when someone tries to rock or tilt the machine to make it vend a product.
Bombs
A tilt switch can trigger a bomb.[6][7] Mercury tilt switches can be found in some bomb and landmine fuzes, typically in the form of anti-handling devices, for example, a variant of the VS-50 mine.
https://en.wikipedia.org/wiki/Mercury_switch
See also
- Mercury-arc valve, a rectifier device intended for high electrical voltages/currents
- Mercury battery, an electrochemical battery
- Mercury coulometer, an electro analytical chemistry device that determines the amount of matter transformed during a mercury reaction
- Mercury probe, an electrical probing device to sample for electrical characterization
- Mercury swivel commutator, an electrical circuit, current-reversing switch using the element mercury
- Mercury-wetted relay
- Mercury relay
https://en.wikipedia.org/wiki/Mercury_switch
https://en.wikipedia.org/wiki/Switch#Contact_bounce
https://en.wikipedia.org/wiki/Mercury_switch
https://en.wikipedia.org/wiki/Mercury_coulometer
https://en.wikipedia.org/wiki/Mercury_swivel_commutator
https://en.wikipedia.org/wiki/Mercury_relay
https://en.wikipedia.org/wiki/Mercury-arc_valve
https://en.wikipedia.org/wiki/Analogue_switch
https://en.wikipedia.org/wiki/Centrifugal_switch
https://en.wikipedia.org/wiki/Crossbar_switch
https://en.wikipedia.org/wiki/Crossover_switch
https://en.wikipedia.org/wiki/Cryotron
https://en.wikipedia.org/wiki/DIP_switch
https://en.wikipedia.org/wiki/Dry_contact
https://en.wikipedia.org/wiki/Float_switch
https://en.wikipedia.org/wiki/Infinite_switch
https://en.wikipedia.org/wiki/Kill_switch
https://en.wikipedia.org/wiki/Key_switch
https://en.wikipedia.org/wiki/Limit_switch
https://en.wikipedia.org/wiki/Latching_switch
https://en.wikipedia.org/wiki/Light_switch
https://en.wikipedia.org/wiki/Lightning_switch
https://en.wikipedia.org/wiki/Magnetic_proximity_fuze
https://en.wikipedia.org/wiki/Magnetic_starter
https://en.wikipedia.org/wiki/Magnetic_switch
https://en.wikipedia.org/wiki/Miniature_snap-action_switch
https://en.wikipedia.org/wiki/Optical_transistor
https://en.wikipedia.org/wiki/Photoswitch
https://en.wikipedia.org/wiki/Placebo_button
https://en.wikipedia.org/wiki/Pull_switch
https://en.wikipedia.org/wiki/Push_switch
https://en.wikipedia.org/wiki/Railroad_switch
https://en.wikipedia.org/wiki/Reed_switch
https://en.wikipedia.org/wiki/Rotary_switch
https://en.wikipedia.org/wiki/Sail_switch
https://en.wikipedia.org/wiki/Sense_switch
https://en.wikipedia.org/wiki/Sea_switch
https://en.wikipedia.org/wiki/Silicone_rubber_keypad
https://en.wikipedia.org/wiki/Spark_gap
https://en.wikipedia.org/wiki/Staircase_timer
https://en.wikipedia.org/wiki/Stepping_switch
https://en.wikipedia.org/wiki/Zero_speed_switch
https://en.wikipedia.org/wiki/Wireless_light_switch
https://en.wikipedia.org/wiki/Vacuum_switch
https://en.wikipedia.org/wiki/Transfer_switch
https://en.wikipedia.org/wiki/Time_switch
https://en.wikipedia.org/wiki/Category:Appropriate_technology
https://en.wikipedia.org/wiki/Village-level_operation_and_maintenance_(pumps)
https://en.wikipedia.org/wiki/Single-wire_earth_return
https://en.wikipedia.org/wiki/Single-wire_transmission_line
https://en.wikipedia.org/wiki/Static_electricity
https://en.wikipedia.org/wiki/Antistatic_device
https://en.wikipedia.org/wiki/Corona_discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage.
https://en.wikipedia.org/wiki/Corona_discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp.
In many high voltage applications, corona is an unwanted side effect. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high voltage equipment like cathode ray tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O3) and nitric oxide (NO), and in turn, nitrogen dioxide (NO2), and thus nitric acid (HNO3) if water vapor is present. These gases are corrosive and can degrade and embrittle nearby materials, and are also toxic to humans and the environment.
Corona discharges can often be suppressed by improved insulation, corona rings, and making high voltage electrodes in smooth rounded shapes.with no sharp edges However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.
Introduction
A corona discharge is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.
When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the electric field strength around that point will be much higher than elsewhere. Air near the electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.
Along with the similar brush discharge, the corona is often called a "single-electrode discharge", as opposed to a "two-electrode discharge" – an electric arc.[1][2][3] A corona forms only when the conductor is widely enough separated from conductors at the opposite potential that an arc cannot jump between them. If the geometry and gradient are such that the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric spark or electric arc, depending upon the source of the electric field. If the source continues to supply current, a spark will evolve into a continuous discharge called an arc.
Corona discharge forms only when the electric field (potential gradient) at the surface of the conductor exceeds a critical value, the dielectric strength or disruptive potential gradient of the fluid. In air at sea level pressure of 101 kPa, the critical value is roughly 30 kV/cm,[1] but this decreases with pressure, therefore, corona discharge is more of a problem at high altitudes.[4] Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. On sharp points in the air, corona can start at potentials of 2–6 kV.[2] In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large-diameter rounded shapes like balls or toruses. Corona rings are often added to insulators of high voltage transmission lines.
Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly curved electrode. If the curved electrode is positive with respect to the flat electrode, it has a positive corona; if it is negative, it has a negative corona. (See below for more details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.
An important reason for considering coronas is the production of ozone around conductors undergoing corona processes in air. A negative corona generates much more ozone than the corresponding positive corona.
Applications
Corona discharge has a number of commercial and industrial applications:
- Removal of unwanted electric charges from the surface of aircraft in flight and thus avoiding the detrimental effect of uncontrolled electrical discharge pulses on the performance of avionic systems
- Manufacture of ozone
- Sanitization of pool water
- In an electrostatic precipitator, removal of solid pollutants from a waste gas stream, or scrubbing particles from the air in air-conditioning systems
- Photocopying
- Air ionisers
- Production of photons for Kirlian photography to expose photographic film
- EHD thrusters, lifters, and other ionic wind devices
- Nitrogen laser
- Ionization of a gaseous sample for subsequent analysis in a mass spectrometer or an ion mobility spectrometer
- Static charge neutralization, as applied through antistatic devices like ionizing bars
- Refrigeration of electronic devices by forced convection[5]
Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying). They can also be used to remove particulate matter from air streams by first charging the air, and then passing the charged stream through a comb of alternating polarity, to deposit the charged particles onto oppositely charged plates.
The free radicals and ions generated in corona reactions can be used to scrub the air of certain noxious products, through chemical reactions, and can be used to produce ozone.
Problems
Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. Therefore, power transmission equipment is designed to minimize the formation of corona discharge.
Corona discharge is generally undesirable in:
- Electric power transmission, where it causes:
- Electrical components such as transformers, capacitors, electric motors, and generators:
- Corona can progressively damage the insulation inside these devices, leading to equipment failure
- Elastomer items such as O-rings can suffer ozone cracking
- Plastic film capacitors operating at mains voltage can suffer progressive loss of capacitance as corona discharges cause local vaporization of the metallization[8]
In many cases, coronas can be suppressed by corona rings, toroidal devices that serve to spread the electric field over a larger areas and decrease the field gradient below the corona threshold.
Mechanism
|
Corona discharge occurs when the electric field is strong enough to create a chain reaction; electrons in the air collide with atoms hard enough to ionize them, creating more free electrons that ionize more atoms. The diagrams below illustrate at a microscopic scale the process which creates a corona in the air next to a pointed electrode carrying a high negative voltage with respect to ground. The process is:
- A neutral atom or molecule, in a region of the strong electric
field (such as the high potential gradient near the curved electrode),
is ionized by a natural environmental event (for example, being struck
by an ultraviolet photon or cosmic ray particle), to create a positive ion and a free electron.
- The electric field accelerates these oppositely charged particles in opposite directions, separating them, preventing their recombination, and imparting kinetic energy to each of them.
- The electron has a much higher charge/mass ratio and so is
accelerated to a higher velocity than the positive ion. It gains enough
energy from the field that when it strikes another atom it ionizes it,
knocking out another electron, and creating another positive ion. These
electrons are accelerated and collide with other atoms, creating
further electron/positive-ion pairs, and these electrons collide with
more atoms, in a chain reaction process called an electron avalanche.
Both positive and negative coronas rely on electron avalanches. In a
positive corona, all the electrons are attracted inward toward the
nearby positive electrode and the ions are repelled outwards. In a
negative corona, the ions are attracted inward and the electrons are
repelled outwards.
- The glow of the corona is caused by electrons recombining with
positive ions to form neutral atoms. When the electron falls back to
its original energy level, it releases a photon of light. The photons
serve to ionize other atoms, maintaining the creation of electron
avalanches.
- At a certain distance from the electrode, the electric field becomes low enough that it no longer imparts enough energy to the electrons to ionize atoms when they collide. This is the outer edge of the corona. Outside this, the ions move through the air without creating new ions. The outward moving ions are attracted to the opposite electrode and eventually reach it and combine with electrons from the electrode to become neutral atoms again, completing the circuit.
Thermodynamically, a corona is a very nonequilibrium process, creating a non-thermal plasma. The avalanche mechanism does not release enough energy to heat the gas in the corona region generally and ionize it, as occurs in an electric arc or spark. Only a small number of gas molecules take part in the electron avalanches and are ionized, having energies close to the ionization energy of 1–3 ev, the rest of the surrounding gas is close to ambient temperature.
The onset voltage of corona or corona inception voltage (CIV) can be found with Peek's law (1929), formulated from empirical observations. Later papers derived more accurate formulas.
Positive coronas
Properties
A positive corona is manifested as a uniform plasma across the length of a conductor. It can often be seen glowing blue/white, though many of the emissions are in the ultraviolet. The uniformity of the plasma is caused by the homogeneous source of secondary avalanche electrons described in the mechanism section, below. With the same geometry and voltages, it appears a little smaller than the corresponding negative corona, owing to the lack of a non-ionising plasma region between the inner and outer regions.
A positive corona has a much lower density of free electrons compared to a negative corona; perhaps a thousandth of the electron density, and a hundredth of the total number of electrons. However, the electrons in a positive corona are concentrated close to the surface of the curved conductor, in a region of the high potential gradient (and therefore the electrons have high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas. Therefore, if electrons are to be used in an application which requires high activation energy, positive coronas may support a greater reaction constant than corresponding negative coronas; though the total number of electrons may be lower, the number of very high energy electrons may be higher.
Coronas are efficient producers of ozone in the air. A positive corona generates much less ozone than the corresponding negative corona, as the reactions which produce ozone are relatively low-energy. Therefore, the greater number of electrons of a negative corona leads to increased production.
Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat electrode.
Mechanism
As with a negative corona, a positive corona is initiated by an exogenous ionization event in a region of a high potential gradient. The electrons resulting from the ionization are attracted toward the curved electrode, and the positive ions repelled from it. By undergoing inelastic collisions closer and closer to the curved electrode, further molecules are ionized in an electron avalanche.
In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the fluid itself, in the region outside the plasma or avalanche region. They are created by ionization caused by the photons emitted from that plasma in the various de-excitation processes occurring within the plasma after electron collisions, the thermal energy liberated in those collisions creating photons which are radiated into the gas. The electrons resulting from the ionization of a neutral gas molecule are then electrically attracted back toward the curved electrode, attracted into the plasma, and so begins the process of creating further avalanches inside the plasma.
Negative coronas
Properties
A negative corona is manifested in a non-uniform corona, varying according to the surface features and irregularities of the curved conductor. It often appears as tufts of the corona at sharp edges, the number of tufts altering with the strength of the field. The form of negative coronas is a result of its source of secondary avalanche electrons (see below). It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the ionizing region, and so the plasma continues some distance beyond it. The total number of electrons and electron density is much greater than in the corresponding positive corona. However, they are of predominantly lower energy, owing to being in a region of lower potential gradient. Therefore, whilst for many reactions, the increased electron density will increase the reaction rate, the lower energy of the electrons will mean that reactions which require higher electron energy may take place at a lower rate.
Mechanism
Negative coronas are more complex than positive coronas in construction. As with positive coronas, the establishing of a corona begins with an exogenous ionization event generating a primary electron, followed by an electron avalanche.
Electrons ionized from the neutral gas are not useful in sustaining the negative corona process by generating secondary electrons for further avalanches, as the general movement of electrons in a negative corona is outward from the curved electrode. For negative corona, instead, the dominant process generating secondary electrons is the photoelectric effect, from the surface of the electrode itself. The work function of the electrons (the energy required to liberate the electrons from the surface) is considerably lower than the ionization energy of air at standard temperatures and pressures, making it a more liberal source of secondary electrons under these conditions. Again, the source of energy for the electron-liberation is a high-energy photon from an atom within the plasma body relaxing after excitation from an earlier collision. The use of ionized neutral gas as a source of ionization is further diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode.
Under other conditions, the collision of the positive species with the curved electrode can also cause electron liberation.
The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode itself, the new secondary electrons travelling outward.
A further feature of the structure of negative coronas is that as the electrons drift outwards, they encounter neutral molecules and, with electronegative molecules (such as oxygen and water vapor), combine to produce negative ions. These negative ions are then attracted to the positive uncurved electrode, completing the 'circuit'.
Electrical wind
Ionized gases produced in a corona discharge are accelerated by the electric field, producing a movement of gas or electrical wind. The air movement associated with a discharge current of a few hundred microamperes can blow out a small candle flame within about 1 cm of a discharge point. A pinwheel, with radial metal spokes and pointed tips bent to point along the circumference of a circle, can be made to rotate if energized by a corona discharge; the rotation is due to the differential electric attraction between the metal spokes and the space charge shield region that surrounds the tips.[9]
See also
- Alternating current
- Atmospheric pressure chemical ionization
- Crookes tube
- Dielectric barrier discharge
- Kirlian photography
- List of plasma physics articles
- St. Elmo's fire
References
- Loeb, Leonard Benedict (1965). Electrical Coronas. University of California Press. pp. 406–409.
Further reading
- Chen, Junhong (August 2002). "Direct-Current Corona Enhanced Chemical Reactions". University of Minnesota.
- Peek, F.W. (1929). Dielectric Phenomena in High Voltage Engineering. McGraw-Hill. ISBN 0-9726596-6-8.
- Loeb, Leonard (1965). Electrical Coronas Their Basic Physical Mechanisms. University of California Press. ASIN B0006BM4LG.
- Cobine, James D. (1941). Gaseous Conductors; Theory and Engineering Applications. McGraw-Hill or Dover reprints. ASIN B000B9PK7S.
- Takacs, J. (1972). "Corona Stabilizer for Van De Graaff Accelerators". Nuclear Instruments and Methods. 103 (3): 587–600. Bibcode:1972NucIM.103..587T. doi:10.1016/0029-554X(72)90019-5. ISSN 0029-554X.
External links
https://en.wikipedia.org/wiki/Corona_discharge
https://en.wikipedia.org/wiki/Transmitter
https://en.wikipedia.org/wiki/Television_broadcasting
https://en.wikipedia.org/wiki/Local_insertion
https://en.wikipedia.org/wiki/Emergency_alert
https://en.wikipedia.org/wiki/Station_identification
https://en.wikipedia.org/wiki/Genlock
https://en.wiktionary.org/wiki/syntonize
https://en.wikipedia.org/wiki/Watermark
https://en.wikipedia.org/wiki/Signal_generator
https://en.wikipedia.org/wiki/Black_and_burst
https://en.wikipedia.org/wiki/Vision_mixer
Where composite video is in use, the phase of the chrominance subcarrier of each source being combined or switched should also be coincident. This is to avoid changes in colour hue and/or saturation during a transition between sources.
https://en.wikipedia.org/wiki/Genlock
https://en.wikipedia.org/wiki/Subcarrier
Scope
Generator locking can be used to synchronize as few as two isolated sources (e.g., a television camera and a videotape machine feeding a vision mixer (production switcher)), or in a wider facility where all the video sources are locked to a single synchronizing pulse generator (e.g., a fast-paced sporting event featuring multiple cameras and recording devices). Generator locking can also be used to ensure that multiple CRT monitors that appear in a movie are flicker-free. Generator locking is also used to synchronize two cameras for Stereoscopic 3D video recording.
In broadcast systems, an analog generator-lock signal usually consists of vertical and horizontal synchronizing pulses together with chrominance phase reference in the form of colorburst. No picture information is usually carried to avoid disturbing the timing signals, and the name reference, black and burst, color black, or black burst is usually given to such a signal. A composite colour video signal inherently carries the same reference signals and can be used as a generator-locking signal, albeit at the risk of being disturbed by out-of-specification picture signals.
Although some high-definition broadcast systems may use a standard-definition reference signal as a generator-locking reference signal, the use of tri-level synchronising pulses directly related to the frame and line rate is increasing within HD systems. A tri-level sync pulse is a signal that initially goes from 0 volts DC to a negative voltage, then a positive voltage, before returning to zero volts DC again. The voltage excursions are typically 300 mV either side of zero volts, and the duration each of the two excursions is the same as a particular number of digital picture samples.
https://en.wikipedia.org/wiki/Genlock
Connections
Most television studio and professional video cameras have dedicated generator-locking ports on the camera. If the camera is tethered with a triaxial cable or optical fibre cable, the analog generator-locking signal is used to lock the camera control unit, which in turn locks the camera head by means of information carried within a data channel transmitted along the cable. If the camera is an ENG-type camera, one without a triax/fibre connection or without a dockable head, the generator-locking signal is carried through a separate cable from the video.
Variants
Natlock is a picture-source synchronizing system using audio tone signals to describe the timing discrepancies between composite video signals, while Icelock uses digital information conveyed in the vertical blanking interval of a composite video signal.
See also
References
- Roe, John H. (March–April 1950). "The Genlock for Improved TV Programming" (PDF). RCA Broadcast News. Number 58: 11–12 – via americanradiohistory.com.
https://en.wikipedia.org/wiki/Genlock
Triaxial cable, often referred to as triax for short, is a type of electrical cable similar to coaxial cable, but with the addition of an extra layer of insulation and a second conducting sheath. It provides greater bandwidth and rejection of interference than coax, but is more expensive.[1][2]
https://en.wikipedia.org/wiki/Triaxial_cable
https://en.wikipedia.org/wiki/Triaxial_cable
https://en.wikipedia.org/wiki/Driven_guard
https://en.wikipedia.org/wiki/Buffer_amplifier#Voltage_buffer
https://en.wikipedia.org/wiki/Twinaxial_cabling
https://en.wikipedia.org/wiki/Coaxial_cable
https://en.wikipedia.org/wiki/Vertical_blanking_interval
https://en.wikipedia.org/wiki/Time_base_correction#frame_synchronizer
https://en.wikipedia.org/wiki/Time_base_correction
Time base correction (TBC) is a technique to reduce or eliminate errors caused by mechanical instability present in analog recordings on mechanical media.
Without time base correction, a signal from a videotape recorder (VTR) or videocassette recorder (VCR) cannot be mixed with other, more time-stable devices found in television studios and post-production facilities.
Most broadcast quality VCRs have simple time base correctors built in though external TBCs are often used. Some high-end domestic analog video recorders and camcorders also include a TBC circuit, which typically can be switched off if required.
Time base correction counteracts errors by buffering the video signal as it comes off the videotape at an unsteady rate, and releasing it at a steady rate. TBCs also allow a variable delay in the video stream. By adjusting the rate and delay using a waveform monitor and a vectorscope, the corrected signal can now match the timing of the other devices in the system. If all of the devices in a system are adjusted so their signals meet the video switcher at the same time and at the same rate, the signals can be mixed. A single master clock or "sync generator" provides the reference for all of the devices' clocks.
Video correction
As far back as 1956, professional reel-to-reel audio tape recorders relying on mechanical stability alone were stable enough that pitch distortion could be below audible level without time base correction. However, the higher sensitivity of video recordings meant that even the best mechanical solutions still resulted in detectable distortion of the video signals and difficulty locking to downstream devices.[1] A video signal consists of picture information but also sync and subcarrier signals which allow the image to be framed up square on the monitor, reproduce colors accurately[note 1] and, importantly, allow the combination and switching of two or more video signals.
https://en.wikipedia.org/wiki/Time_base_correction
A modern 5th and final type of TBC being achieved in the late 2010s is software defined, packaged inside the open source python based VHS-Decode[2] & CVBS-Decode[3] projects which evolved from the LD-Decode project[4] which uses FM RF captures of analouge media signals then de-modulates and corrects the signal in software.
https://en.wikipedia.org/wiki/Time_base_correction
https://en.wikipedia.org/wiki/Drop-out_compensator
https://en.wikipedia.org/wiki/Lossless_compression
https://en.wikipedia.org/wiki/Arithmetic_coding
https://en.wikipedia.org/wiki/Integrated_circuit
https://en.wikipedia.org/wiki/Indexed_color
https://en.wikipedia.org/wiki/19-inch_rack
|
On consumer products a yellow RCA connector is typically used for composite video. | |||
| Type | Analog video connector | ||
|---|---|---|---|
| Production history | |||
| Designed | 1954[1]–1956[2] | ||
| General specifications | |||
| Length | Maximum of 50 m[citation needed] | ||
| External | Yes | ||
| Video signal | NTSC, PAL or SECAM video | ||
| Pins | 1 plus grounding shield | ||
| Connector | RCA connector | ||
| Electrical | |||
| Signal | 1 volt[3] | ||
| Pinout | |||
| Pin 1 | center | video | |
| Pin 2 | sheath | ground | |
Composite video is an analog video signal format that carries standard-definition video (typically at 525 lines or 625 lines) as a single channel. Video information is encoded on one channel, unlike the higher-quality S-Video (two channels) and the even higher-quality component video (three or more channels). In all of these video formats, audio is carried on a separate connection.
Composite video is also known by the initials CVBS for composite video baseband signal or color, video, blanking and sync,[4][5] or is simply referred to as SD video for the standard-definition television signal it conveys.
There are three dominant variants of composite video signals, corresponding to the analog color system used: NTSC, PAL, and SECAM. Usually composite video is carried by a yellow RCA connector, but other connections are used in professional settings, or on devices that are too small for an RCA connector, such as a digital camera.
https://en.wikipedia.org/wiki/Composite_video
https://en.wikipedia.org/wiki/525_lines
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems. The word raster comes from the Latin word rastrum (a rake), which is derived from radere (to scrape); see also rastrum, an instrument for drawing musical staff lines. The pattern left by the lines of a rake, when drawn straight, resembles the parallel lines of a raster: this line-by-line scanning is what creates a raster. It is a systematic process of covering the area progressively, one line at a time. Although often a great deal faster, it is similar in the most general sense to how one's gaze travels when one reads lines of text.
In most modern graphics cards the data to be drawn is stored internally in an area of semiconductor memory called the Framebuffer. This memory area holds the values for each pixel on the screen. These values are retrieved from the refresh buffer and painted onto the screen one row at a time.
https://en.wikipedia.org/wiki/Raster_scan
https://en.wikipedia.org/wiki/Bitmap
https://en.wikipedia.org/wiki/Image_tracing
https://en.wikipedia.org/wiki/Optical_character_recognition
https://en.wikipedia.org/wiki/Text_mining
https://en.wikipedia.org/wiki/Document_clustering
https://en.wikipedia.org/wiki/Information_retrieval
https://en.wikipedia.org/wiki/Search_engine
https://en.wikipedia.org/wiki/Singular_value_decomposition#Truncated_SVD
https://en.wikipedia.org/wiki/Latent_semantic_analysis#Latent_semantic_indexing
https://en.wikipedia.org/wiki/Dimensionality_reduction
https://en.wikipedia.org/wiki/Feature_selection
https://en.wikipedia.org/wiki/Curse_of_dimensionality
https://en.wikipedia.org/wiki/Sampling_(statistics)
https://en.wikipedia.org/wiki/Sampling_(statistics)
https://en.wikipedia.org/wiki/Minimax
https://en.wikipedia.org/wiki/Convenience_sampling
https://en.wikipedia.org/wiki/Self-selection_bias
https://en.wikipedia.org/wiki/Line-intercept_sampling
https://en.wikipedia.org/wiki/Sampling_error
https://en.wikipedia.org/wiki/Random_number_table
https://en.wikipedia.org/wiki/Pseudorandom_number_generator
https://en.wikipedia.org/wiki/Data_collection
https://en.wikipedia.org/wiki/Resampling_(statistics)
https://en.wikipedia.org/wiki/Matrix_norm
https://en.wikipedia.org/wiki/Analog_recording
https://en.wikipedia.org/wiki/Field_strength
https://en.wikipedia.org/wiki/Dipole_field_strength_in_free_space
https://en.wikipedia.org/wiki/Phonautograph
https://en.wikipedia.org/wiki/Phonograph
Scott coated a plate of glass with a thin layer of lampblack. He then took an acoustic trumpet, and at its tapered end affixed a thin membrane that served as the analog to the eardrum. At the center of that membrane, he attached a rigid boar's bristle approximately a centimeter long, placed so that it just grazed the lampblack. As the glass plate was slid horizontally in a well formed groove at a speed of one meter per second, a person would speak into the trumpet, causing the membrane to vibrate and the stylus to trace figures[13] that were scratched into the lampblack.[15] On March 25, 1857, Scott received the French patent[16] #17,897/31,470 for his device, which he called a phonautograph.[17] The earliest known surviving recorded sound of a human voice was conducted on April 9, 1860, when Scott recorded[15] someone singing the song "Au Clair de la Lune" ("By the Light of the Moon") on the device.[18] However, the device was not designed to play back sounds,[15][19] as Scott intended for people to read back the tracings,[20] which he called phonautograms.[14] This was not the first time someone had used a device to create direct tracings of the vibrations of sound-producing objects, as tuning forks had been used in this way by English physicist Thomas Young in 1807.[21] By late 1857, with support from the Société d'encouragement pour l'industrie nationale, Scott's phonautograph was recording sounds with sufficient precision to be adopted by the scientific community, paving the way for the nascent science of acoustics.[14]
The device's true significance in the history of recorded sound was not fully realized prior to March 2008, when it was discovered and resurrected in a Paris patent office by First Sounds, an informal collaborative of American audio historians, recording engineers, and sound archivists founded to make the earliest sound recordings available to the public. The phonautograms were then digitally converted by scientists at the Lawrence Berkeley National Laboratory in California, who were able to play back the recorded sounds, something Scott had never conceived of. Prior to this point, the earliest known record of a human voice was thought to be an 1877 phonograph recording by Thomas Edison.[15][22] The phonautograph would play a role in the development of the gramophone, whose inventor, Emile Berliner, worked with the phonautograph in the course of developing his own device.[23]
https://en.wikipedia.org/wiki/Phonograph
https://en.wikipedia.org/wiki/Telephone
https://en.wikipedia.org/wiki/Pantograph
https://en.wikipedia.org/wiki/Phonograph_cylinder
https://en.wikipedia.org/wiki/Graphophone
https://en.wikipedia.org/wiki/Arbitrary_waveform_generator
https://en.wikipedia.org/wiki/Radio_frequency
https://en.wikipedia.org/wiki/Function_generator
https://en.wikipedia.org/wiki/Digital_signal_processing
https://en.wikipedia.org/wiki/Digital-to-analog_converter
https://en.wikipedia.org/wiki/Triangle_wave
https://en.wikipedia.org/wiki/Continuous_wave
https://en.wikipedia.org/wiki/Modulation#Pulse_modulation_methods
https://en.wikipedia.org/wiki/Phase_modulation
https://en.wikipedia.org/wiki/Total_harmonic_distortion
https://en.wikipedia.org/wiki/Modulation#Digital_modulation_methods
https://en.wikipedia.org/wiki/GSM
https://en.wikipedia.org/wiki/Sound_recording_and_reproduction
https://en.wikipedia.org/wiki/Intermodulation
https://en.wikipedia.org/wiki/Metadata
https://en.wikipedia.org/wiki/Jitter
https://en.wikipedia.org/wiki/Bit_error_rate
https://en.wikipedia.org/wiki/Synthesizer
https://en.wikipedia.org/wiki/Sound_card
https://en.wikipedia.org/wiki/Video-signal_generator
https://en.wikipedia.org/wiki/AN/URM-25D_signal_generator
https://en.wikipedia.org/wiki/Digital_pattern_generator
https://en.wikipedia.org/wiki/Inductive_amplifier
https://en.wikipedia.org/wiki/Bus_analyzer
https://en.wikipedia.org/wiki/Sweep_generator
https://en.wikipedia.org/wiki/Category:Electronic_test_equipment
https://en.wikipedia.org/wiki/Network_analyzer_(electrical)
https://en.wikipedia.org/wiki/Spectrum_analyzer
https://en.wikipedia.org/wiki/Spectrum_analyzer
https://en.wikipedia.org/wiki/Signal_analyzer
https://en.wikipedia.org/wiki/Vectorscope
https://en.wikipedia.org/wiki/Sound_module
https://en.wikipedia.org/wiki/Sampler_(musical_instrument)
https://en.wikipedia.org/wiki/Demultiplex
https://en.wikipedia.org/wiki/Oscilloscope#X-Y_mode
https://en.wikipedia.org/wiki/Waveform_monitor
https://en.wikipedia.org/wiki/Fade_(audio_engineering)#Crossfading
On older vectorscopes that use cathode ray tubes (CRTs), the graticule was often a silk-screened overlay superimposed over the front surface of the screen. One notable exception was the Tektronix WFM601 series of instruments, which are combined waveform monitors and vectorscopes used to measure CCIR 601 television signals. The waveform-mode graticule of these instruments is implemented with a silkscreen, whereas the vectorscope graticule (consisting only of bar targets, as this family did not support composite video) was drawn on the CRT by the electron beam. Modern instruments have graticules drawn using computer graphics, and both graticule and trace are rendered on an external VGA monitor or an internal VGA-compatible LCD display.
https://en.wikipedia.org/wiki/Vectorscope
https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
https://en.wikipedia.org/wiki/Test_card
Audio
In audio applications, a vectorscope is used to measure the difference between channels of stereo audio signals. One stereo channel drives the horizontal deflection of the display, and the other drives the vertical deflection. A monaural signal, consisting of identical left and right signals, results in a straight line with a gradient of +1. Any stereo separation is visible as a deviation from this line, creating a Lissajous figure. If a straight line appears with a gradient of −1, this indicates that the left and right channels are 180° out of phase.https://en.wikipedia.org/wiki/Vectorscope
https://en.wikipedia.org/wiki/Liquid-crystal_display
https://en.wikipedia.org/wiki/Video_Graphics_Array
https://en.wikipedia.org/wiki/Cathode_ray
https://en.wikipedia.org/wiki/180-line_television_system
https://en.wikipedia.org/wiki/Teletext
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https://en.wikipedia.org/wiki/180-line_television_system
https://en.wikipedia.org/wiki/Closed_captioning
https://en.wikipedia.org/wiki/Programme_Delivery_Control
https://en.wikipedia.org/wiki/Ghost-canceling_reference
https://en.wikipedia.org/wiki/Extended_Data_Services
https://en.wikipedia.org/wiki/Chirp
https://en.wikipedia.org/wiki/Multipath_propagation
https://en.wikipedia.org/wiki/VIT_signals
https://en.wikipedia.org/wiki/CCIR_System_B
https://en.wikipedia.org/wiki/Two-wave_with_diffuse_power_fading
https://en.wikipedia.org/wiki/Reflection_(physics)
https://en.wikipedia.org/wiki/Skywave
https://en.wikipedia.org/wiki/Atmospheric_duct
https://en.wikipedia.org/wiki/Refraction
https://en.wikipedia.org/wiki/Refraction_(metallurgy)
https://en.wikipedia.org/wiki/Signaling_(telecommunications)
https://en.wikipedia.org/wiki/Antenna_(radio)
https://en.wikipedia.org/wiki/Wave_interference
https://en.wikipedia.org/wiki/Rician_fading
https://en.wikipedia.org/wiki/Radio#Radio_communication
https://en.wikipedia.org/wiki/Microwave_transmission
https://en.wikipedia.org/wiki/Television_receive-only
https://en.wikipedia.org/wiki/Television_transmitter
https://en.wikipedia.org/wiki/Phase_(waves)
https://en.wikipedia.org/wiki/Transposer
https://en.wikipedia.org/wiki/Video-signal_generator
https://en.wikipedia.org/wiki/ATSC_standards
https://en.wikipedia.org/wiki/Rec._601
https://en.wikipedia.org/wiki/Vertical_interval_timecode
https://en.wikipedia.org/wiki/Video_Encoded_Invisible_Light
https://en.wikipedia.org/wiki/Steganography
https://en.wikipedia.org/wiki/Interactive_television
https://en.wikipedia.org/wiki/Vertical_blanking_interval
https://en.wikipedia.org/wiki/Interactive_television_(narrative_technique)
https://en.wikipedia.org/wiki/Vertical_blanking_interval
https://en.wikipedia.org/wiki/Nominal_analogue_blanking
https://en.wikipedia.org/wiki/Nominal_analogue_blanking
https://en.wikipedia.org/wiki/Overscan
https://en.wikipedia.org/wiki/1:1_pixel_mapping
https://en.wikipedia.org/wiki/Analog_television#Structure_of_a_video_signal
https://en.wikipedia.org/wiki/Fall_time
https://en.wikipedia.org/wiki/Analog_television
The earliest systems of analog television were mechanical television systems that used spinning disks with patterns of holes punched into the disc to scan an image. A similar disk reconstructed the image at the receiver. Synchronization of the receiver disc rotation was handled through sync pulses broadcast with the image information. Camera systems used similar spinning discs and required intensely bright illumination of the subject for the light detector to work. The reproduced images from these mechanical systems were dim, very low resolution and flickered severely.
Analog television did not really begin as an industry until the development of the cathode-ray tube (CRT), which uses a focused electron beam to trace lines across a phosphor coated surface. The electron beam could be swept across the screen much faster than any mechanical disc system, allowing for more closely spaced scan lines and much higher image resolution. Also, far less maintenance was required of an all-electronic system compared to a mechanical spinning disc system. All-electronic systems became popular with households after World War II.
Standards
Broadcasters of analog television encode their signal using different systems. The official systems of transmission were defined by the ITU in 1961 as: A, B, C, D, E, F, G, H, I, K, K1, L, M and N.[2] These systems determine the number of scan lines, frame rate, channel width, video bandwidth, video-audio separation, and so on. A color encoding scheme (NTSC, PAL, or SECAM) could be added to the base monochrome signal.[3] Using RF modulation the signal is then modulated onto a very high frequency (VHF) or ultra high frequency (UHF) carrier wave. Each frame of a television image is composed of scan lines drawn on the screen. The lines are of varying brightness; the whole set of lines is drawn quickly enough that the human eye perceives it as one image. The process repeats and next sequential frame is displayed, allowing the depiction of motion. The analog television signal contains timing and synchronization information so that the receiver can reconstruct a two-dimensional moving image from a one-dimensional time-varying signal.
The first commercial television systems were black-and-white; the beginning of color television was in the 1950s.[4]
A practical television system needs to take luminance, chrominance (in a color system), synchronization (horizontal and vertical), and audio signals, and broadcast them over a radio transmission. The transmission system must include a means of television channel selection.
Analog broadcast television systems come in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. The monochrome combinations still existing in the 1950s were standardized by the International Telecommunication Union (ITU) as capital letters A through N. When color television was introduced, the chrominance information was added to the monochrome signals in a way that black and white televisions ignore. In this way backward compatibility was achieved.
There are three standards for the way the additional color information can be encoded and transmitted. The first was the American NTSC system. The European and Australian PAL and the French and former Soviet Union SECAM standards were developed later and attempt to cure certain defects of the NTSC system. PAL's color encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC. PAL had a late evolution called PALplus, allowing widescreen broadcasts while remaining fully compatible with existing PAL equipment.
In principle, all three color encoding systems can be used with any scan line/frame rate combination. Therefore, in order to describe a given signal completely, it's necessary to quote the color system and the broadcast standard as a capital letter. For example, the United States, Canada, Mexico and South Korea use NTSC-M,[a] Japan uses NTSC-J,[b] the UK uses PAL-I,[c] France uses SECAM-L,[d] much of Western Europe and Australia use PAL-B/G,[e] most of Eastern Europe uses SECAM-D/K or PAL-D/K and so on.
However, not all of these possible combinations actually exist. NTSC is currently only used with system M, even though there were experiments with NTSC-A (405 line) in the UK and NTSC-N (625 line) in part of South America. PAL is used with a variety of 625-line standards (B, G, D, K, I, N) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.
For this reason, many people refer to any 625/25 type signal as PAL and to any 525/30 signal as NTSC, even when referring to digital signals; for example, on DVD-Video, which does not contain any analog color encoding, and thus no PAL or NTSC signals at all.
Although a number of different broadcast television systems were in use worldwide, the same principles of operation apply.[5]
Displaying an image
A cathode-ray tube (CRT) television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a raster. At the end of each line, the beam returns to the start of the next line; at the end of the last line, the beam returns to the beginning of the first line at the top of the screen. As it passes each point, the intensity of the beam is varied, varying the luminance of that point. A color television system is similar except there are three beams that scan together and an additional signal known as chrominance controls the color of the spot.
When analog television was developed, no affordable technology for storing video signals existed; the luminance signal had to be generated and transmitted at the same time at which it is displayed on the CRT. It was therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact synchronization with the scanning in the television.
The physics of the CRT require that a finite time interval be allowed for the spot to move back to the start of the next line (horizontal retrace) or the start of the screen (vertical retrace). The timing of the luminance signal must allow for this.
The human eye has a characteristic called phi phenomenon. Quickly displaying successive scan images creates the illusion of smooth motion. Flickering of the image can be partially solved using a long persistence phosphor coating on the CRT so that successive images fade slowly. However, slow phosphor has the negative side-effect of causing image smearing and blurring when there is rapid on-screen motion occurring.
The maximum frame rate depends on the bandwidth of the electronics and the transmission system, and the number of horizontal scan lines in the image. A frame rate of 25 or 30 hertz is a satisfactory compromise, while the process of interlacing two video fields of the picture per frame is used to build the image. This process doubles the apparent number of video frames per second and further reduces flicker and other defects in transmission.
Receiving signals
The television system for each country will specify a number of television channels within the UHF or VHF frequency ranges. A channel actually consists of two signals: the picture information is transmitted using amplitude modulation on one carrier frequency, and the sound is transmitted with frequency modulation at a frequency at a fixed offset (typically 4.5 to 6 MHz) from the picture signal.
The channel frequencies chosen represent a compromise between allowing enough bandwidth for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called vestigial sideband is used to reduce the channel spacing, which would be nearly twice the video bandwidth if pure AM was used.
Signal reception is invariably done via a superheterodyne receiver: the first stage is a tuner which selects a television channel and frequency-shifts it to a fixed intermediate frequency (IF). The signal amplifier performs amplification to the IF stages from the microvolt range to fractions of a volt.
Extracting the sound
At this point the IF signal consists of a video carrier signal at one frequency and the sound carrier at a fixed offset in frequency. A demodulator recovers the video signal. Also at the output of the same demodulator is a new frequency modulated sound carrier at the offset frequency. In some sets made before 1948, this was filtered out, and the sound IF of about 22 MHz was sent to an FM demodulator to recover the basic sound signal. In newer sets, this new carrier at the offset frequency was allowed to remain as intercarrier sound, and it was sent to an FM demodulator to recover the basic sound signal. One particular advantage of intercarrier sound is that when the front panel fine tuning knob is adjusted, the sound carrier frequency does not change with the tuning, but stays at the above-mentioned offset frequency. Consequently, it is easier to tune the picture without losing the sound.
So the FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of the NICAM and MTS systems, television sound transmissions were monophonic.
Structure of a video signal
The video carrier is demodulated to give a composite video signal[f] containing luminance, chrominance and synchronization signals.[6] The result is identical to the composite video format used by analog video devices such as VCRs or CCTV cameras. To ensure good linearity and thus fidelity, consistent with affordable manufacturing costs of transmitters and receivers, the video carrier is never modulated to the extent that it is shut off altogether. When intercarrier sound was introduced later in 1948, not completely shutting off the carrier had the side effect of allowing intercarrier sound to be economically implemented.
Each line of the displayed image is transmitted using a signal as shown above. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for PAL, NTSC, and SECAM television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the colorburst, and the chrominance signal) are not present.
The front porch is a brief (about 1.5 microsecond) period inserted between the end of each transmitted line of picture and the leading edge of the next line's sync pulse. Its purpose was to allow voltage levels to stabilise in older televisions, preventing interference between picture lines. The front porch is the first component of the horizontal blanking interval which also contains the horizontal sync pulse and the back porch.[7][8][9]
The back porch is the portion of each scan line between the end (rising edge) of the horizontal sync pulse and the start of active video. It is used to restore the black level (300 mV) reference in analog video. In signal processing terms, it compensates for the fall time and settling time following the sync pulse.[7][8]
In color television systems such as PAL and NTSC, this period also includes the colorburst signal. In the SECAM system, it contains the reference subcarrier for each consecutive color difference signal in order to set the zero-color reference.
In some professional systems, particularly satellite links between locations, the digital audio is embedded within the line sync pulses of the video signal, to save the cost of renting a second channel. The name for this proprietary system is Sound-in-Syncs.
Monochrome video signal extraction
The luminance component of a composite video signal varies between 0 V and approximately 0.7 V above the black level. In the NTSC system, there is a blanking signal level used during the front porch and back porch, and a black signal level 75 mV above it; in PAL and SECAM these are identical.
In a monochrome receiver, the luminance signal is amplified to drive the control grid in the electron gun of the CRT. This changes the intensity of the electron beam and therefore the brightness of the spot being scanned. Brightness and contrast controls determine the DC shift and amplification, respectively.
Color video signal extraction
U and V signals
A color signal conveys picture information for each of the red, green, and blue components of an image. However, these are not simply transmitted as three separate signals, because: such a signal would not be compatible with monochrome receivers, an important consideration when color broadcasting was first introduced. It would also occupy three times the bandwidth of existing television, requiring a decrease in the number of television channels available.
Instead, the RGB signals are converted into YUV form, where the Y signal represents the luminance of the colors in the image. Because the rendering of colors in this way is the goal of both monochrome film and television systems, the Y signal is ideal for transmission as the luminance signal. This ensures a monochrome receiver will display a correct picture in black and white, where a given color is reproduced by a shade of gray that correctly reflects how light or dark the original color is.
The U and V signals are color difference signals. The U signal is the difference between the B signal and the Y signal, also known as B minus Y (B-Y), and the V signal is the difference between the R signal and the Y signal, also known as R minus Y (R-Y). The U signal then represents how purplish-blue or its complementary color, yellowish-green, the color is, and the V signal how purplish-red or it's complementary, greenish-cyan, it is. The advantage of this scheme is that the U and V signals are zero when the picture has no color content. Since the human eye is more sensitive to detail in luminance than in color, the U and V signals can be transmitted with reduced bandwidth with acceptable results.
In the receiver, a single demodulator can extract an additive combination of U plus V. An example is the X demodulator used in the X/Z demodulation system. In that same system, a second demodulator, the Z demodulator, also extracts an additive combination of U plus V, but in a different ratio. The X and Z color difference signals are further matrixed into three color difference signals, (R-Y), (B-Y), and (G-Y). The combinations of usually two, but sometimes three demodulators were:
- (I) / (Q), (as used in the 1954 RCA CTC-2 and the 1985 RCA "Colortrak" series, and the 1954 Arvin, and some professional color monitors in the 1990s),
- (R-Y) / (Q), as used in the 1955 RCA 21-inch color receiver,
- (R-Y) / (B-Y), used in the first color receiver on the market (Westinghouse, not RCA),
- (R-Y) / (G-Y), (as used in the RCA Victor CTC-4 chassis),
- (R-Y) / (B-Y) / (G-Y),
- (X) / (Z), as used in many receivers of the late '50s and throughout the '60s.
In the end, further matrixing of the above color-difference signals c through f yielded the three color-difference signals, (R-Y), (B-Y), and (G-Y).
The R, G, and B signals in the receiver needed for the display device (CRT, Plasma display, or LCD display) are electronically derived by matrixing as follows: R is the additive combination of (R-Y) with Y, G is the additive combination of (G-Y) with Y, and B is the additive combination of (B-Y) with Y. All of this is accomplished electronically. It can be seen that in the combining process, the low-resolution portion of the Y signals cancel out, leaving R, G, and B signals able to render a low-resolution image in full color. However, the higher resolution portions of the Y signals do not cancel out, and so are equally present in R, G, and B, producing the higher-resolution image detail in monochrome, although it appears to the human eye as a full-color and full-resolution picture.
NTSC and PAL systems
In the NTSC and PAL color systems, U and V are transmitted by using quadrature amplitude modulation of a subcarrier. This kind of modulation applies two independent signals to one subcarrier, with the idea that both signals will be recovered independently at the receiving end. For NTSC, the subcarrier is at 3.58 MHz.[g] For the PAL system it is at 4.43 MHz.[h] The subcarrier itself is not included in the modulated signal (suppressed carrier), it is the subcarrier sidebands that carry the U and V information. The usual reason for using suppressed carrier is that it saves on transmitter power. In this application a more important advantage is that the color signal disappears entirely in black and white scenes. The subcarrier is within the bandwidth of the main luminance signal and consequently can cause undesirable artifacts on the picture, all the more noticeable in black and white receivers.
A small sample of the subcarrier, the colorburst, is included in the horizontal blanking portion, which is not visible on the screen. This is necessary to give the receiver a phase reference for the modulated signal. Under quadrature amplitude modulation the modulated chrominance signal changes phase as compared to its subcarrier and also changes amplitude. The chrominance amplitude (when considered together with the Y signal) represents the approximate saturation of a color, and the chrominance phase against the subcarrier reference approximately represents the hue of the color. For particular test colors found in the test color bar pattern, exact amplitudes and phases are sometimes defined for test and troubleshooting purposes only.
Due to the nature of the quadrature amplitude modulation process that created the chrominance signal, at certain times, the signal represents only the U signal, and 70 nanoseconds (NTSC) later, it represents only the V signal. About 70 nanoseconds later still, -U, and another 70 nanoseconds, -V. So to extract U, a synchronous demodulator is utilized, which uses the subcarrier to briefly gate the chroma every 280 nanoseconds, so that the output is only a train of discrete pulses, each having an amplitude that is the same as the original U signal at the corresponding time. In effect, these pulses are discrete-time analog samples of the U signal. The pulses are then low-pass filtered so that the original analog continuous-time U signal is recovered. For V, a 90-degree shifted subcarrier briefly gates the chroma signal every 280 nanoseconds, and the rest of the process is identical to that used for the U signal.
Gating at any other time than those times mentioned above will yield an additive mixture of any two of U, V, -U, or -V. One of these off-axis (that is, of the U and V axis) gating methods is called I/Q demodulation. Another much more popular off-axis scheme was the X/Z demodulation system. Further matrixing[clarification needed] recovered the original U and V signals. This scheme was actually the most popular demodulator scheme throughout the 1960s.[clarification needed]
The above process uses the subcarrier. But as previously mentioned, it was deleted before transmission, and only the chroma is transmitted. Therefore, the receiver must reconstitute the subcarrier. For this purpose, a short burst of the subcarrier, known as the colorburst, is transmitted during the back porch (re-trace blanking period) of each scan line. A subcarrier oscillator in the receiver locks onto this signal (see phase-locked loop) to achieve a phase reference, resulting in the oscillator producing the reconstituted subcarrier.[i]
NTSC uses this process unmodified. Unfortunately, this often results in poor color reproduction due to phase errors in the received signal, caused sometimes by multipath, but mostly by poor implementation at the studio end. With the advent of solid-state receivers, cable TV, and digital studio equipment for conversion to an over-the-air analog signal, these NTSC problems have been largely fixed, leaving operator error at the studio end as the sole color rendition weakness of the NTSC system.[citation needed] In any case, the PAL D (delay) system mostly corrects these kinds of errors by reversing the phase of the signal on each successive line, and averaging the results over pairs of lines. This process is achieved by the use of a 1H (where H = horizontal scan frequency) duration delay line.[j] Phase shift errors between successive lines are therefore canceled out and the wanted signal amplitude is increased when the two in-phase (coincident) signals are re-combined.
NTSC is more spectrum efficient than PAL, giving more picture detail for a given bandwidth. This is because sophisticated comb filters in receivers are more effective with NTSC's 4 color frame sequence compared to PAL's 8-field sequence. However, in the end, the larger channel width of most PAL systems in Europe still gives PAL systems the edge in transmitting more picture detail.
SECAM system
In the SECAM television system, U and V are transmitted on alternate lines, using simple frequency modulation of two different color subcarriers.
In some analog color CRT displays, starting in 1956, the brightness control signal (luminance) is fed to the cathode connections of the electron guns, and the color difference signals (chrominance signals) are fed to the control grids connections. This simple CRT matrix mixing technique was replaced in later solid state designs of signal processing with the original matrixing method used in the 1954 and 1955 color TV receivers.
Synchronization
Synchronizing pulses added to the video signal at the end of every scan line and video frame ensure that the sweep oscillators in the receiver remain locked in step with the transmitted signal so that the image can be reconstructed on the receiver screen.[7][8][10]
A sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync.
Horizontal synchronization
The horizontal sync pulse, separates the scan lines. The horizontal sync signal is a single short pulse that indicates the start of every line. The rest of the scan line follows, with the signal ranging from 0.3 V (black) to 1 V (white), until the next horizontal or vertical synchronization pulse.
The format of the horizontal sync pulse varies. In the 525-line NTSC system it is a 4.85 μs pulse at 0 V. In the 625-line PAL system the pulse is 4.7 μs at 0 V. This is lower than the amplitude of any video signal (blacker than black) so it can be detected by the level-sensitive sync separator circuit of the receiver.
Vertical synchronization
Vertical synchronization separates the video fields. In PAL and NTSC, the vertical sync pulse occurs within the vertical blanking interval. The vertical sync pulses are made by prolonging the length of horizontal sync pulses through almost the entire length of the scan line.
The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or midway through).
The format of such a signal in 525-line NTSC is:
- pre-equalizing pulses (6 to start scanning odd lines, 5 to start scanning even lines)
- long-sync pulses (5 pulses)
- post-equalizing pulses (5 to start scanning odd lines, 4 to start scanning even lines)
Each pre- or post-equalizing pulse consists of half a scan line of black signal: 2 μs at 0 V, followed by 30 μs at 0.3 V. Each long sync pulse consists of an equalizing pulse with timings inverted: 30 μs at 0 V, followed by 2 μs at 0.3 V.
In video production and computer graphics, changes to the image are often kept in step with the vertical synchronization pulse to avoid visible discontinuity of the image. Since the frame buffer of a computer graphics display imitates the dynamics of a cathode-ray display, if it is updated with a new image while the image is being transmitted to the display, the display shows a mishmash of both frames, producing a page tearing artifact partway down the image.
Vertical synchronization eliminates this by timing frame buffer fills to coincide with the vertical blanking interval, thus ensuring that only whole frames are seen on-screen. Software such as video games and computer-aided design (CAD) packages often allow vertical synchronization as an option, because it delays the image update until the vertical blanking interval. This produces a small penalty in latency because the program has to wait until the video controller has finished transmitting the image to the display before continuing. Triple buffering reduces this latency significantly.
Two-timing intervals are defined – the front porch between the end of the displayed video and the start of the sync pulse, and the back porch after the sync pulse and before the displayed video. These and the sync pulse itself are called the horizontal blanking (or retrace) interval and represent the time that the electron beam in the CRT is returning to the start of the next display line.
Horizontal and vertical hold
Analog television receivers and composite monitors often provide manual controls to adjust horizontal and vertical timing.
The sweep (or deflection) oscillators were designed to run without a signal from the television station (or VCR, computer, or other composite video source). This provides a blank canvas, similar to today's "CHECK SIGNAL CABLE" messages on monitors: it allows the television receiver to display a raster to confirm the basic operation of the set's most fundamental circuits, and to allow an image to be presented during antenna placement. With sufficient signal strength, the receiver's sync separator circuit would split timebase pulses from the incoming video and use them to reset the horizontal and vertical oscillators at the appropriate time to synchronize with the signal from the station.
The free-running oscillation of the horizontal circuit is especially critical, as the horizontal deflection circuits typically power the flyback transformer (which provides acceleration potential for the CRT) as well as the filaments for the high voltage rectifier tube and sometimes the filament(s) of the CRT itself. Without the operation of the horizontal oscillator and output stages, for virtually every analog television receiver since the 1940s, there will be absolutely no illumination of the CRT's face.
The lack of precision timing components in early television receivers meant that the timebase circuits occasionally needed manual adjustment. If their free-run frequencies were too far from the actual line and field rates, the circuits would not be able to follow the incoming sync signals. Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen.
The adjustment took the form of horizontal hold and vertical hold controls, usually on the front panel along with other common controls. These adjusted the free-run frequencies of the corresponding timebase oscillators.
Properly working, adjusting a horizontal or vertical hold should cause the picture to almost "snap" into place on the screen; this is called sync lock. A slowly rolling vertical picture demonstrates that the vertical oscillator is nearly synchronized with the television station but is not locking to it, often due to a weak signal or a failure in the sync separator stage not resetting the oscillator. Sometimes, the black interval bar will almost stop at the right place, again indicating a fault in sync separation is not properly resetting the vertical oscillator.
Horizontal sync errors cause the image to be torn diagonally and repeated across the screen as if it were wrapped around a screw or a barber's pole; the greater the error, the more "copies" of the image will be seen at once wrapped around the barber pole. Given the importance of the horizontal sync circuit as a power supply to many subcircuits in the receiver, they may begin to malfunction as well; and horizontal output components that were designed to work together in a resonant circuit may become damaged.
In the earliest electronic television receivers (1930s–1950s), the time base for the sweep oscillators was generally derived from RC circuits based on carbon resistors and paper capacitors. After turning on the receiver, the vacuum tubes in the set would warm up and the oscillators would begin to run, allowing a watchable picture. Resistors were generally simple pieces of carbon inside a Bakelite enclosure, and the capacitors were usually alternating layers of paper and aluminum foil inside cardboard tubes sealed with bee's wax. Moisture ingress (from ambient air humidity) as well as thermal instability of these components affected their electrical values. As the heat from the tubes and the electrical currents passing through the RC circuits warmed them up, the electrical properties of the RC timebase would shift, causing the oscillators to drift in frequency to a point that they could no longer be synchronized with the received pulses coming from the TV station via the sync separator circuit, causing tearing (horizontal) or rolling (vertical).
Hermetically sealed passive components and cooler-running semiconductors as active components gradually improved reliability to the point where the horizontal hold was moved to the rear of the set first, and the vertical hold control (due to the longer period in the RC constant) persisted as a front panel control well into the 1970s as the consistency of larger-value capacitors increased.
By the early 1980s the efficacy of the synchronization circuits, plus the inherent stability of the sets' oscillators, had been improved to the point where these controls were no longer necessary. Integrated Circuits which eliminated the horizontal hold control were starting to appear as early as 1969.[11]
The final generations of analog television receivers (most TV sets with internal on-screen displays to adjust brightness, color, tint, contrast) used "TV-set-on-a-chip" designs where the receiver's timebases were divided down from crystal oscillators, usually based on the 3.58 MHz NTSC colorburst reference. PAL and SECAM receivers were similar though operating at different frequencies. With these sets, adjustment of the free-running frequency of either sweep oscillator was either physically impossible (being derived inside the integrated circuit) or possibly through a hidden service mode typically offering only NTSC/PAL frequency switching, accessible through the On-Screen Display's menu system.
Horizontal and Vertical Hold controls were rarely used in CRT-based computer monitors, as the quality and consistency of components were quite high by the advent of the computer age, but might be found on some composite monitors used with the 1970s–1980s home or personal computers.
There is no equivalent in modern television systems.
Other technical information
Components of a television system
A typical analog monochrome television receiver is based around the block diagram shown below:
The tuner is the object which "plucks" the television signals out of the air, with the aid of an antenna. There are two types of tuners in analog television, VHF and UHF tuners. The VHF tuner selects the VHF television frequency. This consists of a 4 MHz video bandwidth and a 2 MHz audio bandwidth. It then amplifies the signal and converts it to a 45.75 MHz Intermediate Frequency (IF) amplitude-modulated picture and a 41.25 MHz IF frequency-modulated audio carrier.
The IF amplifiers are centered at 44 MHz for optimal frequency transference of the audio and frequency carriers. What centers this frequency is the IF transformer. They are designed for a certain amount of bandwidth to encompass the audio and video. It depends on the number of stages (the amplifier between the transformers). Most of the early television sets (1939–45) used 4 stages with specially designed video amplifier tubes (the type 1852/6AC7). In 1946 the RCA presented a new innovation in television; the RCA 630TS. Instead of using the 1852 octal tube, it uses the 6AG5 7-pin miniature tube. It still had 4 stages, but it was 1/2 the size. Soon all of the manufactures followed RCA and designed better IF stages. They developed higher amplification tubes, and lower stage counts with more amplification. When the tube era came to an end in the mid-70s, they had shrunk the IF stages down to 1-2 (depending on the set) and with the same amplification as the 4 stage, 1852 tube sets. Like radio, television has Automatic Gain Control (AGC). This controls the gain of the IF amplifier stages and the tuner. More of this will be discussed below.
The video amp and output amplifier consist of a low linear pentode or a high powered transistor. The video amp and output stage separate the 45.75 MHz from the 41.25 MHz. It simply uses a diode to detect the video signal. But the frequency-modulated audio is still in the video. Since the diode only detects AM signals, the FM audio signal is still in the video in the form of a 4.5 MHz signal. There are two ways to attach this problem, and both of them work. We can detect the signal before it enters into the video amplifier, or do it after the audio amplifier. Many television sets (1946 to late 1960s) used the after video amplification method, but of course, there is the occasional exception. Many of the later set late (1960s-now) use the before-the-video amplifier way. In some of the early television sets (1939–45) used its own separate tuner, so there was no need for a detection stage next to the amplifier. After the video detector, the video is amplified and sent to the sync separator and then to the picture tube.
The audio signal is detected by a 4.5 MHz traps coil/transformer. After that, it then goes to a 4.5 MHz amplifier. This amplifier prepares the signal for the 4.5Mhz detector. It then goes through a 4.5 MHz IF transformer to the detector. In television, there are 2 ways of detecting FM signals. One way is by the ratio detector. This is simple but very hard to align. The next is a relatively simple detector. This is the quadrature detector. It was invented in 1954. The first tube designed for this purpose was the 6BN6 type. It is easy to align and simple in circuitry. It was such a good design that it is still being used today in the Integrated circuit form. After the detector, it goes to the audio amplifier.
The next part is the sync separator/clipper. This also does more than what is in its name. It also forms the AGC voltage, as previously stated. This sync separator turns the video into a signal that the horizontal and vertical oscillators can use to keep in sync with the video.
The horizontal and vertical oscillators form the raster on the CRT. They are kept in sync by the sync separator. There are many ways to create these oscillators. The first one is the earliest of its kind is the thyratron oscillator. Although it is known to drift, it makes a perfect sawtooth wave. This sawtooth wave is so good that no linearity control is needed. This oscillator was for the electrostatic deflection CRTs. It found some purpose for the electromagnetically deflected CRTs. The next oscillator is the blocking oscillator. It uses a transformer to create a sawtooth wave. This was only used for a brief time period and never was very popular after the beginning. The next oscillator is the multivibrator. This oscillator was probably the most successful. It needed more adjustment than the other oscillators, but it is very simple and effective. This oscillator was so popular that it was used from the early 1950s until today.
The oscillator amplifier is sorted into two categories. The vertical amplifier directly drives the yoke. There is not much to this. It is similar to an audio amplifier. The horizontal oscillator is a different situation. The oscillator must supply the high voltage and the yoke power. This requires a high power flyback transformer, and a high powered tube or transistor. This is a problematic section for CRT televisions because it has to handle high power.
Sync separator
_and_line_(64%C2%B5s)_decoding.jpg)
Image synchronization is achieved by transmitting negative-going pulses; in a composite video signal of 1-volt amplitude, these are approximately 0.3 V below the "black level". The horizontal sync signal is a single short pulse which indicates the start of every line. Two-timing intervals are defined – the front porch between the end of the displayed video and the start of the sync pulse, and the back porch after the sync pulse and before the displayed video. These and the sync pulse itself are called the horizontal blanking (or retrace) interval and represent the time that the electron beam in the CRT is returning to the start of the next display line.
The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or midway through).
In the television receiver, a sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync.
Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen.
Counting sync pulses, a video line selector picks a selected line from a TV signal, used for teletext, on-screen displays, station identification logos as well as in the industry when cameras were used as a sensor.
Timebase circuits
In an analog receiver with a CRT display sync pulses are fed to horizontal and vertical timebase circuits (commonly called "sweep circuits" in the United States), each consisting of an oscillator and an amplifier. These generate modified sawtooth and parabola current waveforms to scan the electron beam in a linear way. The waveform shapes are necessary to make up for the distance variations from the electron beam source and the screen surface. The oscillators are designed to free-run at frequencies very close to the field and line rates, but the sync pulses cause them to reset at the beginning of each scan line or field, resulting in the necessary synchronization of the beam sweep with the originating signal. The output waveforms from the timebase amplifiers are fed to the horizontal and vertical deflection coils wrapped around the CRT tube. These coils produce magnetic fields proportional to the changing current, and these deflect the electron beam across the screen.
In the 1950s, the power for these circuits was derived directly from the mains supply. A simple circuit consisted of a series voltage dropper resistance and a rectifier valve (tube) or semiconductor diode. This avoided the cost of a large high voltage mains supply (50 or 60 Hz) transformer. This type of circuit was used for the thermionic valve (vacuum tube) technology. It was inefficient and produced a lot of heat which led to premature failures in the circuitry. Although failure was common, it was easily repairable.
In the 1960s, semiconductor technology was introduced into timebase circuits. During the late 1960s in the UK, synchronous (with the scan line rate) power generation was introduced into solid state receiver designs.[12] These had very complex circuits in which faults were difficult to trace, but had very efficient use of power.
In the early 1970s AC mains (50 or 60 Hz), and line timebase (15,625 Hz), thyristor based switching circuits were introduced. In the UK use of the simple (50 Hz) types of power, circuits were discontinued. The reason for design changes arose from the electricity supply contamination problems arising from EMI,[13] and supply loading issues due to energy being taken from only the positive half cycle of the mains supply waveform.[14]
CRT flyback power supply
Most of the receiver's circuitry (at least in transistor- or IC-based designs) operates from a comparatively low-voltage DC power supply. However, the anode connection for a cathode-ray tube requires a very high voltage (typically 10–30 kV) for correct operation.
This voltage is not directly produced by the main power supply circuitry; instead, the receiver makes use of the circuitry used for horizontal scanning. Direct current (DC), is switched through the line output transformer, and alternating current (AC) is induced into the scan coils. At the end of each horizontal scan line the magnetic field, which has built up in both transformer and scan coils by the current, is a source of latent electromagnetic energy. This stored collapsing magnetic field energy can be captured. The reverse flow, short duration, (about 10% of the line scan time) current from both the line output transformer and the horizontal scan coil is discharged again into the primary winding of the flyback transformer by the use of a rectifier which blocks this negative reverse emf. A small value capacitor is connected across the scan switching device. This tunes the circuit inductances to resonate at a much higher frequency. This slows down (lengthens) the flyback time from the extremely rapid decay rate that would result if they were electrically isolated during this short period. One of the secondary windings on the flyback transformer then feeds this brief high voltage pulse to a Cockcroft–Walton generator design voltage multiplier. This produces the required EHT supply. A flyback converter is a power supply circuit operating on similar principles.
A typical modern design incorporates the flyback transformer and rectifier circuitry into a single unit with a captive output lead, (known as a diode split line output transformer or an Integrated High Voltage Transformer (IHVT)),[15] so that all high-voltage parts are enclosed. Earlier designs used a separate line output transformer and a well-insulated high voltage multiplier unit. The high frequency (15 kHz or so) of the horizontal scanning allows reasonably small components to be used.
Transition to digital
In many countries, over-the-air broadcast television of analog audio and analog video signals has been discontinued, to allow the re-use of the television broadcast radio spectrum for other services such as datacasting and subchannels.
The first country to make a wholesale switch to digital over-the-air (terrestrial television) broadcasting was Luxembourg in 2006, followed later in 2006 by the Netherlands; in 2007 by Finland, Andorra, Sweden and Switzerland; in 2008 by Belgium (Flanders) and Germany; in 2009 by the United States (high power stations), southern Canada, the Isle of Man, Norway, and Denmark. In 2010, Belgium (Wallonia), Spain, Wales, Latvia, Estonia, the Channel Islands, San Marino, Croatia, and Slovenia; in 2011 Israel, Austria, Monaco, Cyprus, Japan (excluding Miyagi, Iwate, and Fukushima prefectures), Malta and France; in 2012 the Czech Republic, Arab World, Taiwan, Portugal, Japan (including Miyagi, Iwate, and Fukushima prefectures), Serbia, Italy, Canada, Mauritius, the United Kingdom, the Republic of Ireland, Lithuania, Slovakia, Gibraltar, and South Korea; in 2013, the Republic of Macedonia, Poland, Bulgaria, Hungary, Australia, and New Zealand, completed the transition. The United Kingdom made the transition to digital television between 2008 and 2012, with the exception of Whitehaven, which made the switch over in 2007. The first digital TV-only area in the United Kingdom was Ferryside in Carmarthenshire.[citation needed]
The Digital television transition in the United States for high-powered transmission was completed on 12 June 2009, the date that the Federal Communications Commission (FCC) set. Almost two million households could no longer watch television because they had not prepared for the transition. The switchover had been delayed by the DTV Delay Act.[16] While the majority of the viewers of over-the-air broadcast television in the U.S. watch full-power stations (which number about 1800), there are three other categories of television stations in the U.S.: low-power broadcasting stations, class A stations, and television translator stations. They were given later deadlines. In broadcasting, whatever happens in the United States also influences southern Canada and northern Mexico because those areas are covered by television stations in the U.S.
In Japan, the switch to digital began in northeastern Ishikawa Prefecture on 24 July 2010 and ended in 43 of the country's 47 prefectures (including the rest of Ishikawa) on 24 July 2011, but in Fukushima, Iwate, and Miyagi prefectures, the conversion was delayed to 31 March 2012, due to complications from the 2011 Tōhoku earthquake and tsunami and its related nuclear accidents.
In Canada, most of the larger cities turned off analog broadcasts on 31 August 2011.[17]
China had scheduled to end analog broadcasting between 2015 and 2018.[citation needed]
Brazil switched to digital television on 2 December 2007 in its major cities. It is now estimated that Brazil will end analog broadcasting in 2023.[18]
In Malaysia, the Malaysian Communications & Multimedia Commission (MCMC) advertised for tender bids to be submitted in the third quarter of 2009 for the 470 through 742 MHz UHF allocation, to enable Malaysia's broadcast system to move into DTV. The new broadcast band allocation would result in Malaysia's having to build an infrastructure for all broadcasters, using a single digital terrestrial transmission/television broadcast (DTTB) channel.[citation needed] Large portions of Malaysia are covered by television broadcasts from Singapore, Thailand, Brunei, and Indonesia (from Borneo and Batam). Starting from 1 November 2019, all regions in Malaysia were no longer using the analog system after the states of Sabah and Sarawak finally turned it off on 31 October 2019.[19]
In Singapore, digital television under DVB-T2 began on 16 December 2013. The switchover was delayed many times until analog TV was switched off at midnight on 2 January 2019.[20]
In the Philippines, the National Telecommunications Commission required all broadcasting companies to end analog broadcasting on 31 December 2015 at 11:59 p.m. Due to delay of the release of the implementing rules and regulations for digital television broadcast, the target date was moved to 2020. Full digital broadcast is expected in 2021 and all of the analog TV services should be shut down by the end of 2023.[21]
In the Russian Federation, the Russian Television and Radio Broadcasting Network (RTRS) disabled analog broadcasting of federal channels in five stages, shutting down broadcasting in multiple federal subjects at each stage. The first region to have analog broadcasting disabled was Tver Oblast on 3 December 2018, and the switchover was completed on 14 October 2019.[22] During the transition, DVB-T2 receivers and monetary compensations for purchasing of terrestrial or satellite digital TV reception equipment were provided to disabled people, World War II veterans, certain categories of retirees and households with income per member below living wage.[23]
See also
Notes
- A typical circuit used with this device converts the low-frequency color signal to ultrasound and back again.
References
- Plotnikova, Elena (17 February 2019). "Compensation for digital TV. How to get 2000 rubles for buying a digital TV receiver". Argumenty i Fakty. Retrieved 14 October 2019.
External links
- Video signal measurement and generation
- Television synchronisation
- Video broadcast standard frequencies and country listings
- EDN magazine describing design of a 1958 transistorised television receiver
- Designing the color television signal in the early 1950s as described by two engineers working directly with the NTSC
https://en.wikipedia.org/wiki/Analog_television
https://en.wikipedia.org/wiki/Vertical_blank_interrupt
A framebuffer (frame buffer, or sometimes framestore) is a portion of random-access memory (RAM)[1] containing a bitmap that drives a video display. It is a memory buffer containing data representing all the pixels in a complete video frame.[2] Modern video cards contain framebuffer circuitry in their cores. This circuitry converts an in-memory bitmap into a video signal that can be displayed on a computer monitor.
In computing, a screen buffer is a part of computer memory used by a computer application for the representation of the content to be shown on the computer display.[3] The screen buffer may also be called the video buffer, the regeneration buffer, or regen buffer for short.[4] Screen buffers should be distinguished from video memory. To this end, the term off-screen buffer is also used.
The information in the buffer typically consists of color values for every pixel to be shown on the display. Color values are commonly stored in 1-bit binary (monochrome), 4-bit palettized, 8-bit palettized, 16-bit high color and 24-bit true color formats. An additional alpha channel is sometimes used to retain information about pixel transparency. The total amount of memory required for the framebuffer depends on the resolution of the output signal, and on the color depth or palette size.
History
Computer researchers[who?] had long discussed the theoretical advantages of a framebuffer, but were unable to produce a machine with sufficient memory at an economically practicable cost.[citation needed][5] In 1947, the Manchester Baby computer used a Williams tube, later the Williams-Kilburn tube, to store 1024 bits on a cathode-ray tube (CRT) memory and displayed on a second CRT.[6][7] Other research labs were exploring these techniques with MIT Lincoln Laboratory achieving a 4096 display in 1950.[5]
A color scanned display was implemented in the late 1960s, called the Brookhaven RAster Display (BRAD), which used a drum memory and a television monitor.[8] In 1969, A. Michael Noll of Bell Labs implemented a scanned display with a frame buffer, using magnetic-core memory.[9] Later on, the Bell Labs system was expanded to display an image with a color depth of three bits on a standard color TV monitor.
In the early 1970s, the development of MOS memory (metal–oxide–semiconductor memory) integrated-circuit chips, particularly high-density DRAM (dynamic random-access memory) chips with at least 1 kb memory, made it practical to create, for the first time, a digital memory system with framebuffers capable of holding a standard video image.[10][11] This led to the development of the SuperPaint system by Richard Shoup at Xerox PARC in 1972.[10] Shoup was able to use the SuperPaint framebuffer to create an early digital video-capture system. By synchronizing the output signal to the input signal, Shoup was able to overwrite each pixel of data as it shifted in. Shoup also experimented with modifying the output signal using color tables. These color tables allowed the SuperPaint system to produce a wide variety of colors outside the range of the limited 8-bit data it contained. This scheme would later become commonplace in computer framebuffers.
In 1974, Evans & Sutherland released the first commercial framebuffer, the Picture System,[12] costing about $15,000. It was capable of producing resolutions of up to 512 by 512 pixels in 8-bit grayscale, and became a boon for graphics researchers who did not have the resources to build their own framebuffer. The New York Institute of Technology would later create the first 24-bit color system using three of the Evans & Sutherland framebuffers.[13] Each framebuffer was connected to an RGB color output (one for red, one for green and one for blue), with a Digital Equipment Corporation PDP 11/04 minicomputer controlling the three devices as one.
In 1975, the UK company Quantel produced the first commercial full-color broadcast framebuffer, the Quantel DFS 3000. It was first used in TV coverage of the 1976 Montreal Olympics to generate a picture-in-picture inset of the Olympic flaming torch while the rest of the picture featured the runner entering the stadium.
The rapid improvement of integrated-circuit technology made it possible for many of the home computers of the late 1970s to contain low-color-depth framebuffers. Today, nearly all computers with graphical capabilities utilize a framebuffer for generating the video signal. Amiga computers, created in the 1980s, featured special design attention to graphics performance and included a unique Hold-And-Modify framebuffer capable of displaying 4096 colors.
Framebuffers also became popular in high-end workstations and arcade system boards throughout the 1980s. SGI, Sun Microsystems, HP, DEC and IBM all released framebuffers for their workstation computers in this period. These framebuffers were usually of a much higher quality than could be found in most home computers, and were regularly used in television, printing, computer modeling and 3D graphics. Framebuffers were also used by Sega for its high-end arcade boards, which were also of a higher quality than on home computers.
Display modes
Framebuffers used in personal and home computing often had sets of defined modes under which the framebuffer can operate. These modes reconfigure the hardware to output different resolutions, color depths, memory layouts and refresh rate timings.
In the world of Unix machines and operating systems, such conveniences were usually eschewed in favor of directly manipulating the hardware settings. This manipulation was far more flexible in that any resolution, color depth and refresh rate was attainable – limited only by the memory available to the framebuffer.
An unfortunate side-effect of this method was that the display device could be driven beyond its capabilities. In some cases, this resulted in hardware damage to the display.[14] More commonly, it simply produced garbled and unusable output. Modern CRT monitors fix this problem through the introduction of protection circuitry. When the display mode is changed, the monitor attempts to obtain a signal lock on the new refresh frequency. If the monitor is unable to obtain a signal lock, or if the signal is outside the range of its design limitations, the monitor will ignore the framebuffer signal and possibly present the user with an error message.
LCD monitors tend to contain similar protection circuitry, but for different reasons. Since the LCD must digitally sample the display signal (thereby emulating an electron beam), any signal that is out of range cannot be physically displayed on the monitor.
https://en.wikipedia.org/wiki/Framebuffer
A vertical blank interrupt (or VBI) is a hardware feature found in some legacy computer systems that generate a video signal. Cathode-ray tube based video display circuits generate vertical blanking and vertical sync pulses when the display picture has completed and the raster is being returned to the start of the display. With VBI, the vertical blank pulse is also used to generate an interrupt request for the computer's microprocessor.
The interrupt service routine can then run specific software to modify data in the video display memory while it is not being read to avoid screen tearing effects. This was particularly useful in simple home computers and video game consoles that relied upon a central microprocessor to generate text or graphic displays. More advanced home computers featuring hardware sprites often supported the more flexible horizontal blank interrupt instead in order to allow them to be multiplexed.
As the VBI will be generated at the start of every displayed frame (50 Hz for PAL, 60 Hz for NTSC), it is a useful timebase in systems lacking other timing sources. VBIs are used in some home computers to perform regular functions like scanning the keyboard and joystick ports. It can also be used to implement a basic form of multitasking as well as a buffered graphics screen via page flipping, if hardware permits.
Modern protected mode operating systems generally do not support VBIs as access to hardware interrupts for unprivileged user programs could compromise the system stability. Instead, various APIs like DirectX provide efficient and safe ways to present graphics free of tear and flicker.
For computers that support VBIs see the page about raster interrupts.
https://en.wikipedia.org/wiki/Vertical_blank_interrupt
https://en.wikipedia.org/wiki/Screen_tearing
A display device is an output device for presentation of information in visual[1] or tactile form (the latter used for example in tactile electronic displays for blind people).[2] When the input information that is supplied has an electrical signal the display is called an electronic display.
Common applications for electronic visual displays are television sets or computer monitors.
https://en.wikipedia.org/wiki/Display_device
https://en.wikipedia.org/wiki/Holographic_display
https://en.wikipedia.org/wiki/Field-emission_display
https://en.wikipedia.org/wiki/Vergence-accommodation_conflict
https://en.wikipedia.org/wiki/Thin-film_transistor
https://en.wikipedia.org/wiki/Volumetric_display#Swept-volume_display
Static volume
So-called "static-volume" volumetric 3D displays create imagery without any macroscopic moving parts in the image volume.[7] It is unclear whether the rest of the system must remain stationary for membership in this display class to be viable.
This is probably the most "direct" form of volumetric display. In the simplest case, an addressable volume of space is created out of active elements that are transparent in the off state but are either opaque or luminous in the on state. When the elements (called voxels) are activated, they show a solid pattern within the space of the display.
Several static-volume volumetric 3D displays use laser light to encourage visible radiation in a solid, liquid, or gas. For example, some researchers have relied on two-step upconversion within a rare-earth-doped material when illuminated by intersecting infrared laser beams of the appropriate frequencies.[8][9]
Recent advances have focused on non-tangible (free-space) implementations of the static-volume category, which might eventually allow direct interaction with the display. For instance, a fog display using multiple projectors can render a 3D image in a volume of space, resulting in a static-volume volumetric display.[10][11]
A technique presented in 2006 does away with the display medium altogether, using a focused pulsed infrared laser (about 100 pulses per second; each lasting a nanosecond) to create balls of glowing plasma at the focal point in normal air. The focal point is directed by two moving mirrors and a sliding lens, allowing it to draw shapes in the air. Each pulse creates a popping sound, so the device crackles as it runs. Currently it can generate dots anywhere within a cubic metre. It is thought that the device could be scaled up to any size, allowing 3D images to be generated in the sky.[12][13]
Later modifications such as the use of an neon/argon/xenon/helium gas mix similar to a plasma globe and a rapid gas recycling system employing a hood and vacuum pumps could allow this technology to achieve two-colour (R/W) and possibly RGB imagery by changing the pulse width and intensity of each pulse to tune the emission spectra of the luminous plasma body.
In 2017, a new display known as the "3D Light PAD" was published.[14] The display's medium consists of a class of photoactivatable molecules (known as spirhodamines) and digital light-processing (DLP) technology to generate structured light in three dimensions. The technique bypasses the need to use high-powered lasers and the generation of plasma, which alleviates concerns for safety and dramatically improves the accessibility of the three-dimensional displays. UV-light and green-light patterns are aimed at the dye solution, which initiates photoactivation and thus creates the "on" voxel. The device is capable of displaying a minimal voxel size of 0.68 mm3, with 200 μm resolution, and good stability over hundreds of on–off cycles.
Human–computer interfaces
The unique properties of volumetric displays, which may include 360-degree viewing, agreement of vergence and accommodation cues, and their inherent "three-dimensionality", enable new user interface techniques. There is recent work investigating the speed and accuracy benefits of volumetric displays,[15] new graphical user interfaces,[16] and medical applications enhanced by volumetric displays.[17][18]
Also, software platforms exist that deliver native and legacy 2D and 3D content to volumetric displays.[19]
https://en.wikipedia.org/wiki/Volumetric_display#Swept-volume_display
https://en.wikipedia.org/wiki/Multiplexed_display
https://en.wikipedia.org/wiki/Persistence_of_vision
https://en.wikipedia.org/wiki/Afterimage#Positive_afterimages
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Field electron emission, also known as field emission (FE) and electron field emission, is emission of electrons induced by an electrostatic field. The most common context is field emission from a solid surface into a vacuum. However, field emission can take place from solid or liquid surfaces, into a vacuum, a fluid (e.g. air), or any non-conducting or weakly conducting dielectric. The field-induced promotion of electrons from the valence to conduction band of semiconductors (the Zener effect) can also be regarded as a form of field emission. The terminology is historical because related phenomena of surface photoeffect, thermionic emission (or Richardson–Dushman effect) and "cold electronic emission", i.e. the emission of electrons in strong static (or quasi-static) electric fields, were discovered and studied independently from the 1880s to 1930s. When field emission is used without qualifiers it typically means "cold emission".
Field emission in pure metals occurs in high electric fields: the gradients are typically higher than 1 gigavolt per metre and strongly dependent upon the work function. While electron sources based on field emission have a number of applications, field emission is most commonly an undesirable primary source of vacuum breakdown and electrical discharge phenomena, which engineers work to prevent. Examples of applications for surface field emission include the construction of bright electron sources for high-resolution electron microscopes or the discharge of induced charges from spacecraft. Devices which eliminate induced charges are termed charge-neutralizers.
Field emission was explained by quantum tunneling of electrons in the late 1920s. This was one of the triumphs of the nascent quantum mechanics. The theory of field emission from bulk metals was proposed by Ralph H. Fowler and Lothar Wolfgang Nordheim.[1] A family of approximate equations, Fowler–Nordheim equations, is named after them. Strictly, Fowler–Nordheim equations apply only to field emission from bulk metals and (with suitable modification) to other bulk crystalline solids, but they are often used – as a rough approximation – to describe field emission from other materials.
https://en.wikipedia.org/wiki/Field_electron_emission
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A traveling-wave tube (TWT, pronounced "twit"[1]) or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range.[2] The TWT belongs to a category of "linear beam" tubes, such as the klystron, in which the radio wave is amplified by absorbing power from a beam of electrons as it passes down the tube.[2] Although there are various types of TWT, two major categories are:[2]
- Helix TWT - in which the radio waves interact with the electron beam while traveling down a wire helix which surrounds the beam. These have wide bandwidth, but output power is limited to a few hundred watts.[3]
- Coupled cavity TWT - in which the radio wave interacts with the beam in a series of cavity resonators through which the beam passes. These function as narrowband power amplifiers.
A major advantage of the TWT over some other microwave tubes is its ability to amplify a wide range of frequencies i.e. a large bandwidth. The bandwidth of the helix TWT can be as high as two octaves, while the cavity versions have bandwidths of 10–20%.[2][3] Operating frequencies range from 300 MHz to 50 GHz.[2][3] The power gain of the tube is on the order of 40 to 70 decibels,[3] and output power ranges from a few watts to megawatts.[2][3]
TWTs account for over 50% of the sales volume of all microwave vacuum tubes.[2] They are widely used as the power amplifiers and oscillators in radar systems, communication satellite and spacecraft transmitters, and electronic warfare systems.[2]
Description
A Basic TWT
The TWT is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A voltage applied across the cathode and anode accelerates the electrons towards the far end of the tube, and an external magnetic field around the tube focuses the electrons into a beam. At the other end of the tube the electrons strike the "collector", which returns them to the circuit.
Wrapped around the inside of the tube, just outside the beam path, is a helix of wire, typically oxygen-free copper. The RF signal to be amplified is fed into the helix at a point near the emitter end of the tube. The signal is normally fed into the helix via a waveguide or electromagnetic coil placed at one end, forming a one-way signal path, a directional coupler.
By controlling the accelerating voltage, the speed of the electrons flowing down the tube is set to be similar to the speed of the RF signal running down the helix. The signal in the wire causes a magnetic field to be induced in the center of the helix, where the electrons are flowing. Depending on the phase of the signal, the electrons will either be sped up or slowed down as they pass the windings. This causes the electron beam to "bunch up", known technically as "velocity modulation". The resulting pattern of electron density in the beam is an analog of the original RF signal.
Because the beam is passing the helix as it travels, and that signal varies, it causes induction in the helix, amplifying the original signal. By the time it reaches the other end of the tube, this process has had time to deposit considerable energy back into the helix. A second directional coupler, positioned near the collector, receives an amplified version of the input signal from the far end of the RF circuit. Attenuators placed along the RF circuit prevent the reflected wave from traveling back to the cathode.
Higher powered helix TWTs usually contain beryllium oxide ceramic as both a helix support rod and in some cases, as an electron collector for the TWT because of its special electrical, mechanical, and thermal properties.[4][5]
Comparison
There are a number of RF amplifier tubes that operate in a similar fashion to the TWT, known collectively as velocity-modulated tubes. The best known example is the klystron. All of these tubes use the same basic "bunching" of electrons to provide the amplification process, and differ largely in what process causes the velocity modulation to occur.
In the klystron, the electron beam passes through a hole in a resonant cavity which is connected to the source RF signal. The signal at the instant the electrons pass through the hole causes them to be accelerated (or decelerated). The electrons enter a "drift tube" in which faster electrons overtake the slower ones, creating the bunches, after which the electrons pass through another resonant cavity from which the output power is taken. Since the velocity sorting process takes time, the drift tube must often be several feet long.
In comparison, in the TWT the acceleration is caused by the interactions with the helix along the entire length of the tube. This allows the TWT to have a very low noise output, a major advantage of the design. More usefully, this process is much less sensitive to the physical arrangement of the tube, which allows the TWT to operate over a wider variety of frequencies. TWT's are generally at an advantage when low noise and frequency variability are useful.[6][7]
Coupled-cavity TWT
Helix TWTs are limited in peak RF power by the current handling (and therefore thickness) of the helix wire. As power level increases, the wire can overheat and cause the helix geometry to warp. Wire thickness can be increased to improve matters, but if the wire is too thick it becomes impossible to obtain the required helix pitch for proper operation. Typically helix TWTs achieve less than 2.5 kW output power.
The coupled-cavity TWT overcomes this limit by replacing the helix with a series of coupled cavities arranged axially along the beam. This structure provides a helical waveguide, and hence amplification can occur via velocity modulation. Helical waveguides have very nonlinear dispersion and thus are only narrowband (but wider than klystron). A coupled-cavity TWT can achieve 60 kW output power.
Operation is similar to that of a klystron, except that coupled-cavity TWTs are designed with attenuation between the slow-wave structure instead of a drift tube. The slow-wave structure gives the TWT its wide bandwidth. A free electron laser allows higher frequencies.
Traveling-wave-tube amplifier
A TWT integrated with a regulated power supply and protection circuits is referred to as a traveling-wave-tube amplifier[8] (abbreviated TWTA and often pronounced "TWEET-uh"). It is used to produce high-power radio frequency signals. The bandwidth of a broadband TWTA can be as high as one octave,[citation needed] although tuned (narrowband) versions exist; operating frequencies range from 300 MHz to 50 GHz.
A TWTA consists of a traveling-wave tube coupled with its protection circuits (as in klystron) and regulated power supply electronic power conditioner (EPC), which may be supplied and integrated by a different manufacturer. The main difference between most power supplies and those for vacuum tubes is that efficient vacuum tubes have depressed collectors to recycle kinetic energy of the electrons, so the secondary winding of the power supply needs up to 6 taps of which the helix voltage needs precise regulation. The subsequent addition of a linearizer (as for inductive output tube) can, by complementary compensation, improve the gain compression and other characteristics of the TWTA; this combination is called a linearized TWTA (LTWTA, "EL-tweet-uh").
Broadband TWTAs generally use a helix TWT and achieve less than 2.5 kW output power. TWTAs using a coupled cavity TWT can achieve 15 kW output power, but at the expense of narrower bandwidth.
Invention, development and early use
The original design and prototype of the TWT was done by Andrei "Andy" Haeff c. 1931 while he was working as a doctoral student at the Kellogg Radiation Laboratory at Caltech. His original patent, "Device for and Method of Controlling High Frequency Currents", was filed in 1933 and granted in 1936.[9][10]
The invention of the TWT is often attributed to Rudolf Kompfner in 1942–1943. In addition, Nils Lindenblad, working at RCA (Radio Corporation of America) in the USA also filed a patent for a device in May 1940[11] that was remarkably similar to Kompfner's TWT.[12]: 2 Both of these devices were improvements over Haeff's original design as they both used the then newly invented precision electron gun as the source of the electron beam and they both directed the beam down the center of the helix instead of outside of it. These configuration changes resulted in much greater wave amplification than Haeff's design as they relied on the physical principles of velocity modulation and electron bunching.[10] Kompfner developed his TWT in a British Admiralty radar laboratory during World War II.[13] His first sketch of his TWT is dated November 12, 1942, and he built his first TWT in early 1943.[12]: 3 [14] The TWT was later refined by Kompfner,[14] John R. Pierce,[15] and Lester M. Winslow at Bell Labs. Note that Kompfner's US patent, granted in 1953, does cite Haeff's previous work.[10]
By the 1950s, after further development at the Electron Tube Laboratory at Hughes Aircraft Company in Culver City, California, TWTs went into production there, and by the 1960s TWTs were also produced by such companies as the English Electric Valve Company, followed by Ferranti in the 1970s.[16][17][18]
On July 10, 1962, the first communications satellite, Telstar 1, was launched with a 2 W, 4 GHz RCA-designed TWT transponder used for transmitting RF signals to Earth stations. Syncom 2 was successfully launched into geosynchronous orbit on July 26, 1963 with two 2 W, 1850 MHz Hughes-designed TWT transponders — one active and one spare.[19][20]
Uses
TWTAs are commonly used as amplifiers in satellite transponders, where the input signal is very weak and the output needs to be high power.[21]
A TWTA whose output drives an antenna is a type of transmitter. TWTA transmitters are used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems.[22] In such applications, a control grid is typically introduced between the TWT's electron gun and slow-wave structure to allow pulsed operation. The circuit that drives the control grid is usually referred to as a grid modulator.
Dual redundant 12-watt TWTAs were mounted on the body under the dish of the New Horizons spacecraft, which visited Pluto in 2015, then Kuiper belt object 486958 Arrokoth in 2019 to return data at a distance of 43.4 AU from the Sun.
Historical notes
A TWT has sometimes been referred to as a "traveling-wave amplifier tube" (TWAT),[23] although this term was never widely adopted. "TWT" has been pronounced by engineers as "twit",[24] and "TWTA" as "tweeta".[25]
See also
- Distributed amplifier
- Magnetron
- Klystron tube
- Crossed-field amplifier
- Backward wave oscillator
- Inductive output tube
- Extended interaction oscillator
References
- Mark Williamson (1990). Dictionary of Space Technology. A. Hilger. ISBN 0852743394.
Further reading
- Copeland, Jack; Haeff, Andre A. (September 2015). "The True History of the Traveling Wave Tube".
- Anderson, Carter M; (November 2015). "The Quest for the Ultimate Vacuum Tube". IEEE Spectrum; [2]
External links
- Memorial page, with photo of John Pierce holding a TWT
- Nyquist page, with photo of Pierce, Kompfner, and Nyquist in front of TWT calculations on blackboard
- TMD Travelling Wave Tubes, Information & PDF data sheets.
- Flash animation showing the operation of a traveling wave tube (TWT) and its internal construction
https://en.wikipedia.org/wiki/Travelling_wave_tube
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,[1] which is used as an amplifier for high radio frequencies, from UHF up into the microwave range. Low-power klystrons are used as oscillators in terrestrial microwave relay communications links, while high-power klystrons are used as output tubes in UHF television transmitters, satellite communication, radar transmitters, and to generate the drive power for modern particle accelerators.
In a klystron, an electron beam interacts with radio waves as it passes through resonant cavities, metal boxes along the length of a tube.[2] The electron beam first passes through a cavity to which the input signal is applied. The energy of the electron beam amplifies the signal, and the amplified signal is taken from a cavity at the other end of the tube. The output signal can be coupled back into the input cavity to make an electronic oscillator to generate radio waves. The gain of klystrons can be high, 60 dB (an increase in signal power by a factor of one million) or more, with output power up to tens of megawatts, but the bandwidth is narrow, usually a few percent although it can be up to 10% in some devices.[2]
A reflex klystron is an obsolete type in which the electron beam was reflected back along its path by a high potential electrode, used as an oscillator.
The name klystron comes from the Greek verb κλύζω (klyzo) referring to the action of waves breaking against a shore, and the suffix -τρον ("tron") meaning the place where the action happens.[3] The name "klystron" was suggested by Hermann Fränkel, a professor in the classics department at Stanford University when the klystron was under development.[4]
https://en.wikipedia.org/wiki/Klystron
https://en.wikipedia.org/wiki/Plasma_(physics)
Field electron emission, also known as field emission (FE) and electron field emission, is emission of electrons induced by an electrostatic field. The most common context is field emission from a solid surface into a vacuum. However, field emission can take place from solid or liquid surfaces, into a vacuum, a fluid (e.g. air), or any non-conducting or weakly conducting dielectric. The field-induced promotion of electrons from the valence to conduction band of semiconductors (the Zener effect) can also be regarded as a form of field emission. The terminology is historical because related phenomena of surface photoeffect, thermionic emission (or Richardson–Dushman effect) and "cold electronic emission", i.e. the emission of electrons in strong static (or quasi-static) electric fields, were discovered and studied independently from the 1880s to 1930s. When field emission is used without qualifiers it typically means "cold emission".
Field emission in pure metals occurs in high electric fields: the gradients are typically higher than 1 gigavolt per metre and strongly dependent upon the work function. While electron sources based on field emission have a number of applications, field emission is most commonly an undesirable primary source of vacuum breakdown and electrical discharge phenomena, which engineers work to prevent. Examples of applications for surface field emission include the construction of bright electron sources for high-resolution electron microscopes or the discharge of induced charges from spacecraft. Devices which eliminate induced charges are termed charge-neutralizers.
Field emission was explained by quantum tunneling of electrons in the late 1920s. This was one of the triumphs of the nascent quantum mechanics. The theory of field emission from bulk metals was proposed by Ralph H. Fowler and Lothar Wolfgang Nordheim.[1] A family of approximate equations, Fowler–Nordheim equations, is named after them. Strictly, Fowler–Nordheim equations apply only to field emission from bulk metals and (with suitable modification) to other bulk crystalline solids, but they are often used – as a rough approximation – to describe field emission from other materials.
https://en.wikipedia.org/wiki/Field_electron_emission
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