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Thursday, May 11, 2023

05-11-2023-1615 - Laser-powered phosphor display (LPD)

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]

The rear of a 14-inch color cathode-ray tube showing its deflection coils and electron guns
Typical 1950s United States monochrome television set
Snapshot of a CRT television showing the line of light being drawn from left to right in a raster pattern
Animation of the image construction with interlacing method
Color computer monitor electron gun

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

Braun's original cold-cathode CRT, 1897

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

Small circular CRTs during manufacture in 1947 (screens being coated with phosphor)
A portable monochrome CRT TV
A Trinitron CRT computer monitor
A monochrome CRT as seen inside a TV. The CRT is the single largest component in a CRT TV.
A monochrome CRT as seen inside a Macintosh Plus computer

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]

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

An aluminized monochrome CRT. The black matte coating is aquadag.
The deflection yoke over the neck of a monochrome CRT. It has two pairs of deflection coils.

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]

Color CRTs

Magnified view of a delta-gun shadow mask color CRT
On the left: Magnified view of In-line phosphor triads (a slot mask) CRT. On the right: Magnified view of Delta-gun phosphor triads.
Magnified view of a Trinitron (aperture grille) color CRT. A thin horizontal support wire is visible.
CRT triad and mask types
Spectra of constituent blue, green and red phosphors in a common CRT
The in-line electron guns of a color CRT TV

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.

A magnet used on a CRT TV. Note the distortion of the image.

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

A degaussing in progress
Mu metal magnetic shields for oscilloscope CRTs

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

The front of a Sony Watchman monochrome CRT
A flat monochrome CRT assembly inside a 1984 Sinclair TV80 portable TV

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]

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

An oscilloscope showing a Lissajous curve
The electron gun of an oscilloscope. A pair of deflection plates is visible on the left.

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

The Tektronix Type 564: first mass-produced analog phosphor storage oscilloscope

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 tube BA0000-P31

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

A comparison between 21-inch Superslim and Ultraslim 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

A CRT during an 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

Datapoint 1500 terminal with exposed chassis, with its CRT suffering from a "cataract" due to aging PVA

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

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  • Selected patents

    External links

     

     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.

    Panasonic plasma TV of the last generation. 55 inch. Middle class ST60 series. (2013)

    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 standard computer case fitted with blue and green cold-cathode tubes
    Cold-cathode fluorescent lamp backlight

    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

    Macro image of a VFD digit with 3 horizontal tungsten wires and control grid

    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

    A cold-cathode fluorescent lamp from an emergency-exit sign. Operating at a much higher voltage than other fluorescents, the lamp produces a low-amperage glow discharge rather than an arc, similar to a neon light. Without direct connection to line voltage, current is limited by the transformer alone, negating the need for a ballast.

    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

    A preheat fluorescent lamp circuit using an automatic starting switch. A: Fluorescent tube, B: Power (+220 volts), C: Starter, D: Switch (bi-metallic thermostat), E: Capacitor, F: Filaments, G: Ballast
    0:04
    Starting a preheat lamp. The automatic starter switch flashes orange each time it attempts to start the lamp.

    Preheating, also called switchstart, uses a combination filamentcathode 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).

    A preheat fluorescent lamp "starter" (automatic starting switch)

    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]

    Electronic fluorescent lamp starters

    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

    T12 fluorescent tubes. The first two are rapid start, (for "tombstone" and socket holders respectively) while the third is an instant-start lamp. The instant-start has a characteristic, rounded, single pin, for plugging into the spring-loaded socket holders.

    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.

    A rapid-start "iron" (magnetic) ballast continually heats the cathodes at the ends of the lamps. This ballast runs two F40T12 lamps in series.

    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

    0:08
    A 65-watt fluorescent lamp starting on a semi-resonant start circuit
    A semi-resonant start circuit diagram

    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

    Fluorescent lamp with an electronic ballast.
    Electronic ballast for fluorescent lamp, 2×58 W
    Electronic ballast basic schematic
    Electronic ballasts and different compact fluorescent lamps

    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.

    This tube failed after it had been turned on many times. Too much of the thermionic emission mix had sputtered off the cathodes, instead sticking to and blackening the glass.
    The filament of a low-pressure mercury gas-discharge lamp, with white thermionic emission coating acting as hot cathode. A little of the coating is sputtered away at every start; the lamp ultimately fails.

    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.

    Compact fluorescent lamp that has reached end of life because of mercury adsorption. Light is produced only by the base argon fill.

    Phosphors and the spectrum of emitted light

    Light from a fluorescent tube lamp reflected by a CD shows the individual bands of color.

    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

    The color temperature of different electric lamps

    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

    A helical cool-white fluorescent lamp reflected in a diffraction grating reveals the various spectral lines which make up the light.
    Fluorescent spectra in comparison with other forms of lighting. Clockwise from upper left: Fluorescent lamp, incandescent bulb, candle flame and LED lighting.

    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.

    Fluorescent-lamp spectra
    Typical fluorescent lamp with rare-earth phosphor Fluorescent lighting spectrum peaks labeled with colored peaks added.png 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 Spectrum of halophosphate type fluorescent bulb (f30t12 ww rs).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 Spectra-Philips 32T8 natural sunshine fluorescent light.svg Peaks with stars are mercury lines.
    Yellow fluorescent lights Yellow fluorescent light spectrum.png 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 Fluorescent Black-Light spectrum with peaks labelled.gif 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

    Magnetic ballasts have a low power factor, when used without a capacitor, which increases current drawn by the lighting fixture.

    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]

    The "beat effect" problem created when shooting photos under standard fluorescent lighting

    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 

    0:16
    The "beat effect" problem created when shooting films under standard fluorescent lighting

    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

    Capacitive coupling with high-voltage power lines can light a lamp continuously at low intensity.
    Capacitive coupling with high-voltage power lines can light a lamp continuously at low intensity.

    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

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    Sources

    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

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