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Cathode ray tube
The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the
display device used in most computer displays, video monitors, televisions
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 luminescent
material (usually phosphor). When the electrons hit this material, light is emitted.
Extra acceleration is added to the electrons by applying a high voltage to
the anode. The anode voltage in a color CRT is typically 20-26 kilovolts,
less for a monochrome CRT. A control grid is added to modulate the electron
beam; hence, electrically, a simple CRT is a triode. More complex CRTs
contain greater numbers of electrodes.
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.
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.
Color tubes use three different materials which specifically emit red,
green, and blue light, closely packed together in strips, (in aperture
grille designs) or clusters (in shadow mask CRTs). There are three electron
guns, one for each color, and each gun can reach only the dots of one
colour, as the grille or mask absorbs electrons that would otherwise hit the
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 obsolete, because the new designs are less bulky and consume less
power. As of mid-2003, LCD displays are becoming directly comparable in
price to CRTs, with LCD displays forming 30% of the computer display market
Magnets should never be put next to a CRT tube, as they may cause permanent
magnetization which will stop the tube from working properly and may be
impossible to correct.
WARNING: CRTs operate at very high voltages. These voltages can persist long
after the device containing the CRT has been switched off. Do not tamper
with devices containing CRT tubes unless you have proper engineering
training and have taken appropriate precautions.