Using electrical signals from the monitor’s
raster circuits, an electron beam can trace across the entire CRT face. Another element of the CRT that you should understand is called the
shadow mask. A shadow mask is basically a thin metal sheet with a series of small holes punched into it. Some CRTs use a mask of rectangular openings referred to as an
aperture grille or
slot mask. Both types of mask perform the same purpose—to ensure that electron beams strike only the color phosphors of the intended pixel. This is a vital element of a color monitor.
Ina monochrome monitor, the CRT is coated with a single homogeneous layer of phosphor if stray electrons strike nearby phosphor particles, a letter or line might simply appear to be a bit out of focus. Fora color CRT, however, stray electrons can cause incorrect colors to appear on nearby pixels. Masks help to preserve color purity. Color
purity is also aided by a purity magnet, which helps correct fine beam positioning. A
convergence magnet helps ensure that all three electron beams meet (or converge) at the shadow mask. Of course, grids, heaters, and cathodes are all located inside the glass CRT vessel. Electrical connections are made through a circular arrangement of sealed pins in the neck. Table 27-2 explains the designations for each pin. Remember that the high-voltage anode is attached directly to the CRT in the top right part of the glass funnel. Also remember that some CRT designs might use additional pins.
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