Close-up of part of a television screen showing two colours; green and magenta. The red, green and blue phosphor strips which make up the image are clearly visible. The colour seen by the eye at a given spot on the screen is made up of varying proportions of red, green & blue light, the strips being close enough together to be perceived 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 scanning beams control the intensity of the three colours at different points on the screen.