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Cathode-ray tube
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Vacuum tube manipulated to display images on a phosphorescent screen
"Picture tube" redirects here. For the PaintShop Pro image type, see
PaintShop Pro S: Picture tubes.
Cathode-ray tube using electromagnetic focus and deflection. Parts
shown are not to scale.
A cathode-ray tube as found in an oscilloscope
Cutaway rendering of a color CRT:
1. Three electron emitters (for red, green, and blue phosphor dots)
2. Electron beams
3. Focusing coils
4. Deflection coils
5. Connection for final anodes (referred to as the "ultor"^[1] in some
receiving tube manuals)
6. Mask for separating beams for red, green, and blue part of the
displayed image
7. Phosphor layer (screen)with red, green, and blue zones
8. Close-up of the phosphor-coated inner side of the screen
Cutaway rendering of a monochrome CRT:
1. Deflection coils
2. Electron beam
3. Focusing coil
4. Phosphor layer on the inner side of the screen; emits light when
struck by the electron beam
5. Filament for heating the cathode
6. Graphite layer on the inner side of the tube
7. Rubber or silicone gasket where the anode voltage wire enters the
tube (anode cup)
8. Cathode
9. Air-tight glass body of the tube
10. Screen
11. Coils in yoke
12. Control electrode regulating the intensity of the electron beam and
thereby the light emitted from the phosphor
13. Contact pins for cathode, filament and control electrode
14. Wire for anode high voltage.
The only visible differences are the single electron gun, the uniform
white phosphor coating, and the lack of a shadow mask.
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]
[ ]
Contents
* 1 History
+ 1.1 Discoveries
+ 1.2 Development
+ 1.3 Decline
* 2 Construction
+ 2.1 Body
+ 2.2 Size and weight
+ 2.3 Anode
+ 2.4 Electron gun
o 2.4.1 Construction and method of operation
o 2.4.2 Gamma
+ 2.5 Deflection
o 2.5.1 Magnetic deflection
o 2.5.2 Electrostatic deflection
+ 2.6 Burn-in
+ 2.7 Evacuation
+ 2.8 Rebuilding
+ 2.9 Reactivation
+ 2.10 Phosphors
o 2.10.1 Phosphor persistence
+ 2.11 Limitations and workarounds
o 2.11.1 Blooming
o 2.11.2 Doming
o 2.11.3 High voltage
o 2.11.4 Size
o 2.11.5 Limits imposed by deflection
* 3 Comparison with other technologies
* 4 Types
+ 4.1 Monochrome CRTs
+ 4.2 Color CRTs
o 4.2.1 Shadow mask
o 4.2.2 Screen manufacture
o 4.2.3 Convergence and purity in color CRTs
o 4.2.4 Magnetic shielding and degaussing
o 4.2.5 Resolution
+ 4.3 Projection CRTs
+ 4.4 Beam-index tube
+ 4.5 Flat CRTs
+ 4.6 Radar CRTs
+ 4.7 Oscilloscope CRTs
o 4.7.1 Microchannel plate
o 4.7.2 Graticules
o 4.7.3 Image storage tubes
+ 4.8 Vector monitors
+ 4.9 Data storage tubes
+ 4.10 Cat's eye
+ 4.11 Charactrons
+ 4.12 Nimo
+ 4.13 Flood-beam CRT
+ 4.14 Print-head CRT
+ 4.15 Zeus - thin CRT display
+ 4.16 Slimmer CRT
* 5 Health concerns
+ 5.1 Ionizing radiation
+ 5.2 Toxicity
+ 5.3 Flicker
+ 5.4 High-frequency audible noise
+ 5.5 Implosion
o 5.5.1 Implosion protection
+ 5.6 Electric shock
* 6 Security concerns
* 7 Recycling
* 8 See also
* 9 References
* 10 Selected patents
* 11 External links
History[edit]
Discoveries[edit]
Braun's original cold-cathode CRT, 1897
Cathode rays were discovered by Julius Pluecker 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 Roentgen
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[edit]
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[edit]
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^[update], 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[edit]
Body[edit]
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 90DEG for computer monitor CRTs and small CRTs
and 110DEG which was the standard in larger TV CRTs, with 120 or 125DEG
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 50DEG in 1938 to 110DEG in
1959,^[22] and 125DEG in the 2000s. 140DEG deflection CRTs were
researched but never commercialized, as convergence problems were never
resolved.^[166]
* A monochrome CRT with 110DEG deflection
A monochrome CRT with 110DEG deflection
* A monochrome CRT with 90DEG deflection
A monochrome CRT with 90DEG deflection
Size and weight[edit]
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[edit]
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[edit]
Main article: 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[edit]
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 doesn't 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
800-1000DEGC, 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[edit]
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[edit]
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[edit]
Main article: 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[edit]
Main article: 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[edit]
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[edit]
CRTs are evacuated or exhausted (a vacuum is formed) inside an oven at
approx. 375-475 DEGC, 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 DEGC,
or, the CRT was kept at a temperature higher than 400 DEGC 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 DEGC, 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[edit]
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[edit]
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 either manually^[citation needed].
Some rejuvenators can also fix heater-to-cathode shorts by running a
capacitive discharge through the short.^[222]
Phosphors[edit]
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[edit]
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[edit]
Blooming[edit]
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[edit]
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 DEGC.^[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
don't 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[edit]
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[edit]
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[edit]
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 120DEG, power consumption of the yoke must also go up
from 40 watts to 80 watts, and to increase it further from 120 to
150DEG, 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[edit]
Main article: Comparison of CRT, LCD, plasma, and OLED displays
* 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 refesh 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[edit]
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[edit]
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]
* Older monochrome CRT[381] without aluminum, only aquadag
Older monochrome CRT^[381] without aluminum, only aquadag
* The electron gun of a monochrome CRT
The electron gun of a monochrome CRT
Color CRTs[edit]
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 D.) 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[edit]
Main article: 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 DEGC 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[edit]
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 stricking 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 DEGC^[418] (other sources may state different temperatures, such as
440 DEGC)^[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[edit]
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
120DEG from one another. Dynamic convergence is necessary because the
front of the CRT and the shadow mask aren't spherical, compensating for
electron beam defocusing and astigmatism. The fact that the CRT screen
isn't 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] 90DEG 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 110DEG 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[edit]
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[edit]
Dot pitch defines the maximum resolution of the display, assuming
delta-gun CRTs. In these, as the scanned resolution approaches the dot
pitch resolution, moire appears, as the detail being displayed is finer
than what the shadow mask can render.^[443] Aperture grille monitors do
not suffer from vertical moire, 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] Moire is reduced in shadow mask
CRTs by arranging the holes in the shadow mask in a honeycomb-like
pattern.^[110]
Projection CRTs[edit]
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[edit]
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
didn't 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 wasn't 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[edit]
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]
* The side of a Sony Watchman monochrome CRT. One of the pairs of
deflection coils is easily noticeable.
The side of a Sony Watchman monochrome CRT. One of the pairs of
deflection coils is easily noticeable.
Radar CRTs[edit]
Radar CRTs such as the 7JP4 had a circular screen and scanned the beam
from the center outwards. 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.^[476]^[477] The deflection yoke rotated, causing the beam to
rotate in a circular fashion.^[478]
Oscilloscope CRTs[edit]
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[edit]
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[edit]
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[edit]
Main article: Storage tube
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[edit]
Main article: Vector monitor
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[edit]
Main article: Williams tube
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[edit]
Main article: Magic eye tube
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[edit]
Main article: Charactron
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[edit]
Main article: Nimo tube
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[edit]
Main article: Electron-stimulated luminescence
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[edit]
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[edit]
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[edit]
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[edit]
Ionizing radiation[edit]
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 not to be 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[edit]
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[edit]
Main article: Flicker (screen)
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[edit]
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[edit]
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[edit]
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 which puts the glass under compression,^[526]^[139]^[527]
strengthening the glass reducing 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[edit]
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[edit]
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[edit]
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[edit]
* icon Electronics portal
* Cathodoluminescence
* Crookes tube
* Scintillation (physics)
Applying CRT in different display-purpose:
* Analog television
* Image displaying
* Comparison of CRT, LCD, plasma, and OLED
* Overscan
* Raster scan
* Scan line
Historical aspects:
* Direct-view bistable storage tube
* Flat-panel display
* Geer tube
* History of display technology
* Image dissector
* LCD television, LED-backlit LCD, LED display
* Penetron
* Surface-conduction electron-emitter display
* Trinitron
Safety and precautions:
* Monitor filter
* Photosensitive epilepsy
* TCO Certification
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*
Mehmet Ali Recai Oenal (1 December 2018). "Recovering rare earths from
old TVs and computer screens". Solvomet.
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^ Ledwaba, Pontsho; Sosibo, Ndabenhle (16 February 2017). "Cathode
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^ "Waste CRT (Cathode Ray Tube) glass disassembling and recycling
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^ "With demand dwindling, questions swirl around Videocon". 1
February 2018.
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^ "Panasonic Glass Recycling". 9 November 2011.
^ "California CRT glass heads to disposal sites amid downstream
challenges". 22 September 2016.
Selected patents[edit]
* U.S. Patent 1,691,324: Zworykin Television System
External links[edit]
Wikimedia Commons has media related to Cathode ray tube.
*
"Cathode Ray Tube Monitors". PCTechGuide.
"CRTs". Virtual Valve Museum. Archived from the original on 10
October 2011. Retrieved 31 December 2006.
Goldwasser, Samuel M. (28 February 2006). "TV and Monitor CRT
(Picture Tube) Information". repairfaq.org. Archived from the original
on 26 September 2006.
"The Cathode Ray Tube site". crtsite.com.
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