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The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is
the display device used in most computer displays, video monitors,
televisions and oscilloscopes.
The earliest version of the CRT was a cold-cathode diode, a
modification of the Crookes tube (see X-ray) with a phosphor-coated
screen, sometimes called a Braun tube. The first version to use a
hot cathode was developed by J. B. Johnson (who gave his name to the
term Johnson noise) and H. W. Weinhart of Western Electric and
became a commercial product in 1922.
Early tubes were generally round in shape, this was due to the fact
that it was easier to make a round shape as apposed to a square one.
Today flat square edge tubes are quite common but are more expensive
due to the extra glass and work involved in manufacture.
Cathode rays are streams of high-speed electrons emitted from the
heated cathode of a vacuum tube. In a cathode ray tube, the
electrons are carefully directed into a beam, and this beam is
deflected by a magnetic field to scan the surface at the viewing end
(anode), which is lined with phosphorescent material (usually based
on transition metals or rare earths). When the electrons hit this
material, light is emitted.
In case of a television, the entire front area of the tube is
scanned in a fixed pattern called a raster, and a picture is created
by modulating the intensity of the electron beam according to the
programme's video signal. The beam in all modern TV sets is scanned
with a magnetic field applied to the neck of the tube with a
"magnetic yoke", a set of coils driven by electronic circuits.
In case of an oscilloscope, the intensity of the electron beam is
kept constant, and the picture is drawn by steering the beam along
an arbitrary path. Usually, the horizontal deflection is
proportional to time, and the vertical deflection is proportional to
the signal. The tube for this kind of use is longer and narrower,
and deflection is done by applying an electrical field via
deflection plates built into the tube's neck.
Color tubes use three different materials which specifically emit
red, green, and blue light, closely packed together in strips or
clusters. There are three electron guns, one for each color, and
each gun can reach only the dots of one color, as a metal mask with
small holes across it absorbs electrons that would otherwise hit the
wrong phosphor, this is why it is called a shadow mask. The other
design uses the same principal but uses a aperture grill (also
called a Trinitron tube by Sony) the grill is made up of fine
vertical wires which enables a more defined pixel.
The outer glass allows the light generated by the phosphor out of
the monitor, but (for color tubes) it must block dangerous X-rays
generated by the impact of the high-energy electron beam. For this
reason, the glass is made of lead crystal. Because of this and other
shielding, and protective circuits designed to prevent the anode
voltage rising too high, the X-ray emission of modern CRTs is well
within safety limits.
CRTs have a pronounced triode characteristic, which results in
significant gamma (a nonlinear relationship between beam current and
light intensity). In early televisions, screen gamma was an
advantage because it acted to compress the screen contrast. The
gamma characteristic exists today in all digital video systems.
However, in some systems where a linear response is required, as in
desktop publishing, gamma correction is applied.
It is likely that technologies such as plasma displays, liquid
crystal displays, and other newer technologies will eventually make
CRT based displays mostly obsolete, because the new designs are less
bulky and consume less power. As of mid-2003, LCDs are becoming
directly comparable in price to CRTs, with LCDs forming 30% of the
computer display market by value. However, color CRTs still find
adherents in computer gaming, due to their very quick response time,
and in the printing industry for their better color fidelity and
contrast.
Magnets should never be put next to a color CRT, as they may cause
magnetization of the shadow mask, which will cause incorrect colors
to appear in the magnetized area and may be expensive to have
corrected. Most modern television sets and nearly all newer computer
monitors have a built-in degaussing coil. This coil creates an brief
alternating magnetic field from standard 50- or 60-Hz household
power, upon power-up. The alternating magnetic field created is
sufficient enough to shake off most cases of shadow mask
magnetization. It is also possible to purchase or to build your own
external degaussing coil (often called a degaussing wand) which can
aid in demagnetizing older sets or in cases where the built-in coil
was not effective.
In extreme cases, high power magnets such as the now popular
neodymium iron boron, or NIB magnets, can actually deform the shadow
mask. This type of damage is considered permanent and will render
the CRT mostly useless. However, subjecting an old black and white
television or monochrome (green screen, amber screen) computer
monitor to magnets is generally harmless. Color tubes that have been
subject to a large fall can also be damaged if the shadow mask is
bent, this type of damage can not be repaired and caused the tube to
display a weird variety of patterns as the electron beams try in
vain to hit its corresponding phosphor position.
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