cathode ray tube

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.

(1994-11-17)