The picture on a television screen is nothing but a pattern of glowing dots, or pixels. The pixels are made up of fluorescent chemicals called phosphors that are coated on the back of the screen. These glow on being hit by a beam of electrons. In a black and white television, the pixels are made of a single phosphor and are lit up by a single electron beam that rapidly sweeps across the screen. In a colour television, however, each pixel contains three phosphors each producing a different colour – green, red and yellow. Three electron beams produced by three electron guns are used to light up the different phosphors. The instructions specifying which of the phosphors in a pixel are to be lit are contained in the TV signal transmitted by the telecasting station. A perforated screen also called the shadow mask, placed behind the screen ensures that an electron beam reaches only the dot producing a specific colour. Different combinations of these three basic colours give illusion of different hues of colours in the final picture.
