Close-up of part of a television screen, showing the red, green & blue phosphor strips which make up the image. The colour perceived by the eye at a given spot on a TV screen is made up of varying proportions of red, green & blue light, the strips being close enough together to be seen by the eye as a single colour. The three colours are controlled by three separate electron beams, which scan across the screen line by line, producing a complete picture twenty five times each second. Variations in the intensity of the electron beams as they scan across the screen control the intensity of the three colours at different points on the screen.