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second gun strikes only green phosphor areas and the beam from the
third gun strikes only blue phosphor areas. The use of a mask to
screen each beam from the areas it must not strike is known as the
shadowmask principle, and is used in all colour picture tubes
although there are many different forms of construction. Fig. 23.24
shows one arrangement known as the precision-in-line (p.i.l.) tube,
in which the phosphors are in the form of narrow vertical stripes on
the screen (arranged in the order R, G, B), the shadowmask consists
of vertical bars, and the electron guns are in a horizontal row, the
beams passing through the apertures in th ...