The cathode ray tube (CRT) is a vacuum tube that contains one or more
electron guns and a phosphorescent screen, and is used to display
images. It modulates, accelerates, and deflects electron beam(s)
onto the screen to create the images. The images may represent
electrical waveforms (oscilloscope), pictures (television, computer
monitor), radar targets, or others. CRTs have also been used as memory
devices, in which case the visible light emitted from the fluorescent
material (if any) is not intended to have significant meaning to a
visual observer (though the visible pattern on the tube face may
cryptically represent the stored data).
In television sets and computer monitors, the entire front area of the
tube is scanned repetitively and systematically in a fixed pattern
called a raster. An image is produced by controlling the intensity of
each of the three electron beams, one for each additive primary color
(red, green, and blue) with a video signal as a reference. In all
modern CRT monitors and televisions, the beams are bent by magnetic
deflection, a varying magnetic field generated by coils and driven by
electronic circuits around the neck of the tube, although
electrostatic deflection is commonly used in oscilloscopes, a type of
electronic test instrument.
A 14-inch cathode ray tube showing its deflection coils and electron
Typical 1950s United States monochrome television set
A flat CRT assembly inside a 1984 Sinclair FTV1 pocket TV
A CRT is constructed from a glass envelope which is large, deep (i.e.,
long from front screen face to rear end), fairly heavy, and relatively
fragile. The interior of a CRT is evacuated to approximately 0.01
Pa to 133 nPa., evacuation being necessary to facilitate the
free flight of electrons from the gun(s) to the tube's face. That it
is evacuated makes handling an intact CRT potentially dangerous due to
the risk of breaking the tube and causing a violent implosion that can
hurl shards of glass at great velocity. As a matter of safety, the
face is typically made of thick lead glass so as to be highly
shatter-resistant and to block most
X-ray emissions, particularly if
the CRT is used in a consumer product.
Since the late 2000s, CRTs have been largely superseded by newer "flat
panel" display technologies such as LCD, plasma display, and OLED
displays, which in the case of LCD and
OLED displays have lower
manufacturing costs and power consumption, as well as significantly
less weight and bulk.
Flat panel displays can also be made in very
large sizes; whereas 38" to 40" was about the largest size of a CRT
television, flat panels are available in 60" and larger sizes.
2.2 Microchannel plate
2.4 Image storage tubes
2.5 Data storage tubes
3 Color CRTs
3.1 Convergence and purity in color CRTs
4 Vector monitors
5 CRT resolution
7 Other types
7.1 Cat's eye
7.4 Flood beam CRT
7.5 Print head CRT
7.6 Zeus thin CRT display
8 21st century usage
8.3 Slimmer CRT
9 Health concerns
9.1 Ionizing radiation
9.4 High-frequency audible noise
9.6 Electric shock
10 Security concerns
12 See also
14 Selected patents
15 External links
Braun's original cold-cathode CRT, 1897
Cathode rays were discovered by
Johann Hittorf in 1869 in primitive
Crookes tubes. He 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 mass of cathode rays, showing that they
consisted of negatively charged particles smaller than atoms, the
first "subatomic particles", which were later named electrons. The
earliest version of the CRT was known as the "Braun tube", invented by
the German physicist
Ferdinand Braun in 1897. It was a
cold-cathode diode, a modification of the
Crookes tube with a
The first cathode ray tube to use a hot cathode was developed by John
B. Johnson (who gave his name to the term Johnson noise) and Harry
Weiner Weinhart of Western Electric, and became a commercial product
Kenjiro Takayanagi demonstrated a CRT television that
received images with a 40-line resolution. By 1927, he improved the
resolution to 100 lines, which was unrivaled until 1931. By 1928,
he was the first to transmit human faces in half-tones on a CRT
display. By 1935, he had invented an early all-electronic CRT
It was named in 1929 by inventor Vladimir K. Zworykin, who was
influenced by Takayanagi's earlier work.
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.
The first commercially made electronic television sets with cathode
ray tubes were manufactured by
Telefunken in Germany in 1934.
Flat panel displays dropped in price and started significantly
displacing cathode ray tubes in the 2000s, with LCD screens exceeding
CRTs in 2008. The last known manufacturer of (in this case,
recycled) CRTs ceased in 2015.
Oscilloscope showing Lissajous curve
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.
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. 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.
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.
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. Oscilloscopes commonly provide a means for the
graticule to be illuminated from the side, which improves its
Image storage tubes
The Tektronix Type 564: First Mass Produced Analog
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
Data storage tubes
Main article: Williams tube
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.
Magnified view of a delta-gun shadow mask color CRT
Magnified view of a
Trinitron color CRT
Spectra of constituent blue, green and red phosphors in a common CRT
Color tubes 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). Color CRTs have three electron guns, one for each
primary color, arranged either in a straight line or in an equilateral
triangular configuration (the guns are usually constructed as a single
unit). (The triangular configuration is often called "delta-gun",
based on its relation to the shape of the Greek letter delta Δ.) A
grille or mask absorbs the electrons that would otherwise hit the
wrong phosphor. A shadow mask tube uses a metal plate with tiny
holes, placed so that the electron beam only illuminates the correct
phosphors on the face of the tube; the holes are tapered 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 uses an aperture
grille of tensioned vertical wires to achieve the same result.
Convergence and purity in color CRTs
Due to limitations in the dimensional precision with which CRTs can be
manufactured economically, it has not been practically possible to
build color CRTs in which three electron beams could be aligned to hit
phosphors of respective color in acceptable coordination, solely on
the basis of the geometric configuration of the electron gun axes and
gun aperture positions, shadow mask apertures, etc. The shadow mask
ensures that one beam will only hit spots of certain colors of
phosphors, but minute variations in physical alignment of the internal
parts among individual CRTs will cause variations in the exact
alignment of the beams through the shadow mask, allowing some
electrons from, for example, the red beam to hit, say, blue phosphors,
unless some individual compensation is made for the variance among
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. This is called convergence. 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 is called dynamic convergence. The
beams may converge at the center of the screen and yet stray from each
other as they are deflected toward the edges; such a CRT would be said
to have good static convergence but poor dynamic convergence.
Secondly, each beam must only strike the phosphors of the color it is
intended to strike and no others. This is called purity. Like
convergence, there is static purity and dynamic purity, with the same
meanings of "static" and "dynamic" as for convergence. Convergence and
purity are distinct parameters; a CRT could have good purity but poor
convergence, or vice versa. Poor convergence causes color "shadows" or
"ghosts" along displayed edges and contours, as if the image on the
screen were intaglio printed with poor registration. Poor purity
causes objects on the screen to appear off-color while their edges
remain sharp. Purity and convergence problems can occur at the same
time, in the same or different areas of the screen or both over the
whole screen, and either uniformly or to greater or lesser degrees
over different parts of the screen.
The solution to the static convergence and purity problems is a set of
color alignment magnets installed around the neck of the CRT. 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. 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.
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.
A degaussing in progress.
If the shadow mask becomes magnetized, its magnetic field deflects the
electron beams passing through it, causing color purity distortion as
the beams bend through the mask holes and hit some phosphors of a
color other than that which they are intended to strike; e.g. some
electrons from the red beam may hit blue phosphors, giving pure red
parts of the image a magenta tint. (Magenta is the additive
combination of red and blue.) This effect is localized to a specific
area of the screen if the magnetization of the shadow mask is
localized. Therefore, it is important that the shadow mask is
unmagnetized. (A magnetized aperture grille has a similar effect, and
everything stated in this subsection about shadow masks applies as
well to aperture grilles.)
Most color CRT displays, i.e. television sets and computer monitors,
each have a built-in degaussing (demagnetizing) circuit, the primary
component of which is a degaussing coil which is mounted around the
perimeter of the CRT face inside the bezel. Upon power-up of the CRT
display, the degaussing circuit produces a brief, alternating current
through the degaussing coil which smoothly decays in strength (fades
out) to zero over a period of 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.
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
The degaussing circuit is often built of a thermo-electric (not
electronic) device containing a small ceramic heating element and a
positive thermal coefficient (PTC) resistor, connected directly to the
AC power line with the resistor in series with the degaussing
coil. When the power is switched on, the heating element heats the PTC
resistor, increasing its resistance to a point where degaussing
current is minimal, but not actually zero. In older CRT displays, this
low-level current (which produces no significant degaussing field) is
sustained along with the action of the heating element as long as the
display remains switched on. To repeat a degaussing cycle, the CRT
display must be switched off and left off for at least several seconds
to reset the degaussing circuit by allowing the PTC resistor to cool
to the ambient temperature; switching the display-off and immediately
back on will result in a weak degaussing cycle or effectively no
This simple design is effective and cheap to build, but it wastes some
power continuously. Later models, especially
Energy Star rated ones,
use a relay to switch the entire degaussing circuit on and off, so
that the degaussing circuit uses energy only when it is functionally
active and needed. The relay design also enables degaussing on user
demand through the unit's front panel controls, without switching the
unit off and on again. This relay can often be heard clicking off at
the end of the degaussing cycle a few seconds after the monitor is
turned on, and on and off during a manually initiated degaussing
Main article: Vector monitor
Vector monitors were used in early computer aided design systems and
are in some late-1970s to mid-1980s arcade games such as
Asteroids. 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.
Dot pitch defines the maximum resolution of the display, assuming
delta-gun CRTs. In these, as the scanned resolution approaches the dot
pitch resolution, moiré appears, as the detail being displayed is
finer than what the shadow mask can render. Aperture grille
monitors do not suffer from vertical moiré; however, because their
phosphor stripes have no vertical detail. In smaller CRTs, these
strips maintain position by themselves, but larger aperture-grille
CRTs require one or two crosswise (horizontal) support strips.
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).
Main article: Magic eye tube
In better quality old-fashioned tube radio sets, a tuning guide
consisting of a phosphor tube was used to aid the tuning adjustment.
This was also known as a "Magic Eye" or "Tuning Eye". 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. The same type of device
was used with tape recorders as a recording level meter, and for
various other applications including electrical test equipment.
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
Main article: Nimo tube
Nimo tube BA0000-P31
Nimo was the trademark of a family of small specialised CRTs
manufactured by Industrial Electronics 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).
Flood beam CRT
Main article: Electron-stimulated luminescence
Flood beam CRTs are small tubes that are arranged as pixels for large
screens like Jumbotrons. The first screen using this technology was
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. 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.
Print head CRT
CRTs with an unphosphored front glass but with fine wires embedded in
it were used as electrostatic print heads in the 1960s. The wires
would pass the electron beam current through the glass onto a sheet of
paper where the desired content was therefore deposited as an
electrical charge pattern. The paper was then passed near a pool of
liquid ink with the opposite charge. The charged areas of the paper
attract the ink and thus form the image.
Zeus thin CRT display
In the late 1990s and early 2000s
Philips Research Laboratories
experimented with a type of thin CRT known as the Zeus display which
contained CRT-like functionality in a flat panel
display. The devices were demonstrated but never
21st century usage
This article needs to be updated. Please update this article to
reflect recent events or newly available information. (May 2017)
Although a mainstay of display technology for decades, CRT-based
computer monitors and televisions constitute a dead technology. The
demand for CRT screens has dropped precipitously since 2007, and this
falloff had accelerated in the last two years of that decade. The
rapid advances and falling prices of LCD flat panel technology, first
for computer monitors and then for televisions, has been the key
factor in the demise of competing display technologies such as CRT,
rear-projection, and plasma display.
The end of most high-end CRT production by around 2010 (including
high-end Sony and Mitsubishi product lines) means an erosion of the
CRT's capability. In Canada and the United States, the sale
and production of high-end CRT TVs (30-inch screens) in these markets
had all but ended by 2007. Just a couple of years later, inexpensive
combo CRT TVs (20-inch screens with an integrated VHS player)
disappeared from discount stores. It has been common to replace
CRT-based televisions and monitors in as little as 5–6 years,
although they generally are capable of satisfactory performance for a
much longer time.
Companies are responding to this trend. Electronics retailers such as
Best Buy have been steadily reducing store spaces for CRTs. In 2005,
Sony announced that they would stop the production of CRT computer
displays. Samsung did not introduce any CRT models for the 2008 model
year at the 2008 Consumer Electronics Show, and on 4 February 2008
Samsung removed their 30" wide screen CRTs from their North American
website and has not replaced them with new models.
However, the demise of CRTs has been happening more slowly in the
developing world. According to iSupply, production in units of CRTs
was not surpassed by LCDs production until 4Q 2007, owing largely to
CRT production at factories in China. In the United
Kingdom, DSG (Dixons), the largest retailer of domestic electronic
equipment, reported that CRT models made up 80–90% of the volume of
televisions sold at Christmas 2004 and 15–20% a year later, and that
they were expected to be less than 5% at the end of 2006. Dixons
ceased selling CRT televisions in 2006.
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CRTs, despite advances, have remained relatively heavy and bulky and
take up a lot of space in comparison to other display technologies.
CRT screens have much deeper cabinets compared to flat panels and
rear-projection displays for a given screen size, and so it becomes
impractical to have CRTs larger than 40 inches (102 cm). The CRT
disadvantages became especially significant in light of rapid
technological advancements in LCD and plasma flat-panels which allow
them to easily surpass 40 inches (102 cm) as well as being thin
and wall-mountable, two key features that were increasingly being
demanded by consumers.
A comparison between 21-inch Superslim and Ultraslim CRT
Some CRT manufacturers, both LG Display and Samsung Display, have
innovated CRT technology by creating a slimmer tube. Slimmer CRT has a
trade name Superslim and Ultraslim. A 21-inch flat CRT has 447.2
millimeter depth. The depth of Superslim is 352 millimeters and
Ultraslim is 295.7 millimeters.
CRTs can emit a small amount of
X-ray radiation as a result of the
electron beam's bombardment of the shadow mask/aperture grille and
phosphors. 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 C.F.R. 1020.10 are used to strictly
limit, for instance, television receivers to 0.5 milliroentgens per
hour (mR/h) (0.13 µC/(kg·h) or 36 pA/kg) at a distance of
5 cm (2 in) from any external surface; since 2007, most CRTs
have emissions that fall well below this limit.
Older color and monochrome CRTs may contain toxic substances, such as
cadmium, in the phosphors. The rear glass tube of modern
CRTs may be made from leaded glass, which represent an environmental
hazard if disposed of improperly. By the time personal computers
were produced, glass in the front panel (the viewable portion of the
CRT) used barium rather than lead, though the rear of
the CRT was still produced from leaded glass.
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 leache (The lead is vitrified, contained inside the glass
itself, similar to leaded glass crystalware).
In October 2001, the United States Environmental Protection Agency
created rules stating that CRTs must be brought to special 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
In Europe, disposal of CRT televisions and monitors is covered by the
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
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.
Non-computer CRTs or CRT for sonar or radar may have long persistence
phosphor and are thus flicker free. If the persistence is too long on
a video display, moving images will be blurred.
High-frequency audible noise
50 Hz/60 Hz CRTs used for television operate with horizontal
scanning frequencies of 15,734 Hz (for
NTSC systems) or
15,625 Hz (for
PAL systems). 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 television CRT. The sound is
due to magnetostriction in the magnetic core and periodic movement of
windings of the flyback transformer.
This problem does not occur on 100/120 Hz TVs and on non-CGA
computer displays, because they use much higher horizontal scanning
frequencies (22 kHz to over 100 kHz).
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. The implosion energy is
proportional to the evacuated volume of the CRT.
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
To accelerate the electrons from the cathode to the screen with
sufficient velocity, a very high voltage (EHT or Extra High Tension)
is required, from a few thousand volts for a small oscilloscope
CRT to tens of kV 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.
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.
TEMPEST shielding can
mitigate this effect. Such radiation of a potentially exploitable
signal, however, occurs also with other display technologies and
with electronics in general.
As electronic waste, CRTs are considered one of the hardest types to
recycle. CRTs have relatively high concentration of lead and
phosphors (not phosphorus), 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" 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.
Leaded CRT glass is sold to be remelted into other CRTs, or even
broken down and used in road construction.
Basics of cathode rays and discharge in low-pressure gas:
Light production by cathode rays:
Manipulating the electron beam:
Horizontal blanking interval
Vertical blanking interval
Electron beam processing
Applying CRT in different display-purpose:
Comparison of CRT, LCD, plasma, and OLED
Comparison of display technology
Cathode ray oscilloscpe
Direct-view bistable storage tube
Flat panel display
History of display technology
LCD television, LED-backlit LCD, LED display
Surface-conduction electron-emitter display
Safety and precautions:
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Wikimedia Commons has media related to
Cathode ray tube.
Cathode Ray Tube site
Cathode Ray Tubes
CRTs at the Virtual Valve Museum
Samuel M. Goldwasser TV and Monitor CRT (Picture Tube) Information
Electroluminescent display (ELD)
Light emitting diode
Light emitting diode display (LED)
Cathode ray tube (CRT) (Monoscope)
Liquid-crystal display (LCD)
Plasma display panel (PDP)
Digital Light Processing
Digital Light Processing (DLP)
Liquid crystal on silicon
Liquid crystal on silicon (LCoS)
Organic light-emitting diode (OLED)
Organic light-emitting transistor (OLET)
Surface-conduction electron-emitter display
Surface-conduction electron-emitter display (SED)
Field emission display (FED)
Quantum dot display
Quantum dot display (QD-LED)
Ferro liquid crystal display (FLCD)
Thick-film dielectric electroluminescent technology (TDEL)
Telescopic pixel display (TPD)
Laser-powered phosphor display (LPD)
Vacuum fluorescent display (VFD)
Light-emitting electrochemical cell (LEC)
Seven-segment display (SSD)
Fourteen-segment display (FSD)
Sixteen-segment display (SISD)
History of display technology
Large-screen television technology
Optimum HDTV viewing distance
High-dynamic-range imaging (HDRI)
Color Light Output
Comparison of display technology
Bipolar junction transistor
Bipolar junction transistor (BJT)
Field-effect transistor (FET)
Constant-current diode (CLD, CRD)
Heterostructure barrier varactor
Insulated-gate bipolar transistor
Insulated-gate bipolar transistor (IGBT)
Integrated circuit (IC)
Light-emitting diode (LED)
Silicon controlled rectifier
Silicon controlled rectifier (SCR)
Unijunction transistor (UJT)
Pentagrid (Hexode, Heptode, Octode)
Vacuum tubes (RF)
Backward-wave oscillator (BWO)
Crossed-field amplifier (CFA)
Inductive output tube
Inductive output tube (IOT)
Traveling-wave tube (TWT)
Cathode ray tubes
Beam deflection tube
Magic eye tube
Video camera tube
audio and video
Pentagrid (Hexode, Heptode, Octode)
Cathode ray tube
Beam deflection tube
Inductive output tube
Video camera tube
List of vacuum tubes
List of tube sockets