This tube is really three cathode ray tubes in one, showing pictures in red, green and blue. A combination of these colours can match almost any colour. The tube has three electron guns in the neck and a metal shadow mask near the screen, positioned so that any point on the screen receives electrons from only one gun. All the points that receive electrons from the red gun are coated with a chemical phosphor, which glows red when electrons hit it, and likewise for green and blue. The result is a full-colour picture.
© Science Museum / Science & Society Picture Library