Colour vision is made possible by cones on the retina of the eye, which respond to different colours. The cones are of three types sensitive to certain bands of light – either green, red or blue. The three responses combine so that, with normal vision, all other colours can be discerned. There is a wide variation in an individual’s receptor response to different colours but many tests have established an average response. Colour television adopts the same principle by using a prism behind the lens to split the light from a scene into three separate channels. The amplitude of these individual colour signals depends on the actual colour in the televised scene. Colours that are composed of two or more of these primary colours produce proportional signals in each channel. A fourth signal called the luminance signal is obtained by combining proportions of the red, green and blue signals. It is this signal which allows compatibility with a monochrome display. The amplitude of the signal at any moment is proportional to the brightness of the particular picture element being scanned.