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(CRT) An electrical device for displaying images byexciting phosphor dots with a scanned electron beam. CRTs arefound in computer VDUs and monitors, televisions andoscilloscopes. The first commercially practical CRT wasperfected on 29 January 1901 by Allen B DuMont.A large glass envelope containing a negative electrode (thecathode) emits electrons (formerly called "cathode rays") whenheated, as in a vacuum tube. The electrons are acceleratedacross a large voltage gradient toward the flat surface ofthe tube (the screen) which is covered with phosphor. When anelectron strikes the phosphor, light is emitted. The electronbeam is deflected by electromagnetic coils around the outsideof the tube so that it scans across the screen, usually inhorizontal stripes. This scan pattern is known as a raster.By controlling the current in the beam, the brightness at anyparticular point (roughly a "pixel") can be varied.Different phosphors have different "persistence" - thelength of time for which they glow after being struck byelectrons. If the scanning is done fast enough, the eye seesa steady image, due to both the persistence of the phospor andof the eye itself. CRTs also differ in their dot pitch,which determines their spatial resolution, and in whetherthey use interlace or not.