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Computing (FOLDOC) dictionary
cathode ray tube
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hardware (CRT) An electrical device for displaying images by
exciting phosphor dots with a scanned electron beam. CRTs are
found in computer VDUs and monitors, televisions and
oscilloscopes. The first commercially practical CRT was
perfected on 29 January 1901 by Allen B DuMont.
A large glass envelope containing a negative electrode (the
cathode) emits electrons (formerly called "cathode rays") when
heated, as in a vacuum tube. The electrons are accelerated
across a large voltage gradient toward the flat surface of
the tube (the screen) which is covered with phosphor. When an
electron strikes the phosphor, light is emitted. The electron
beam is deflected by electromagnetic coils around the outside
of the tube so that it scans across the screen, usually in
horizontal stripes. This scan pattern is known as a raster.
By controlling the current in the beam, the brightness at any
particular point (roughly a "pixel") can be varied.
Different phosphors have different "persistence" - the
length of time for which they glow after being struck by
electrons. If the scanning is done fast enough, the eye sees
a steady image, due to both the persistence of the phospor and
of the eye itself. CRTs also differ in their dot pitch,
which determines their spatial resolution, and in whether
they use interlace or not.