picture tube

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picture tube

[′pik·chər ‚tüb]
A cathode-ray tube used in television receivers to produce an image by varying the electron-beam intensity as the beam is deflected from side to side and up and down to scan a raster on the fluorescent screen at the large end of the tube. Also known as kinescope; television picture tube.

Picture tube

A cathode-ray tube used as a television picture tube. Television picture tubes use large glass envelopes that have a light-emitting layer of luminescent material deposited on the inner face. A modulated stream of high-velocity electrons scans this luminescent layer in a series of horizontal lines so that the picture elements (light and dark areas) are recreated.

In a color picture tube (see illustration), the glass bulb is made in two pieces, the face panel and the funnel-neck region. The separate face panel allows the fabrication of the segmented phosphor screen and the mounting of the shadow mask. The two glass pieces are sealed together by a special frit to provide a strong vacuum-tight seal.

The light-emitting colored phosphors on the segmented screen can be either in dot arrays or, now more commonly, in line arrays. Typically, the trios of vertical phosphor lines are spaced 0.6– 0.8 mm apart. Most tubes use a black matrix screen in which the phosphor lines are separated by opaque black lines. This black matrix reduces reflected light, thereby giving better contrast, and also provides a tolerance for the registration of the electron beam with the phosphor lines.

The shadow mask is made of a thin (0.10–0.17 mm) steel sheet in which elongated slits (one row of slits for each phosphor-line trio) have been photoetched. It is formed to a contour similar to that of the glass panel and is mounted at a precise distance from the glass. The width of the slits and their relative position to the phosphor lines are such that the electron beam from one of the three electron guns can strike only one of the sets of color phosphor lines. The shadow mask “shadows” the beam from the other two sets of phosphor lines.

The electron gun for color is similar to that for monochrome except that there are three guns, usually arranged side by side, or in-line. This triple gun has common structural elements, but uses three independent cathodes with separate beam forming and focusing for each beam.

The electromagnetic deflection yoke deflects or bends the beams, as in a monochrome tube, to scan the screen in a television raster. In addition, the yoke's magnetic field is shaped so that the three beams will be deflected in such a way that they land at the same phosphor trio on the screen at the same time. This convergence of the beams produces three images, one in red, one in green, and one in blue, that are superimposed to give a full-color picture. See Cathode-ray tube, Television