chapter  14
8 Pages

Colour

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 (see Figure 14.1). Colour television adopts the same principle by using a prism

behind the lens to split the light from a scene into three separate channels. Colour analysis in the camera will give the appropriate red, green and blue signals according to the spectral energy distribution of the colour being observed. A fourth signal, called the luminance signal, is obtained by combining proportions of the red, green and blue signals. It is this signal that allows compatibility with a monochrome display. The amplitude of the luminance signal at any moment is proportional to the brightness of the particular picture element being scanned. Colour film negative uses a similar filter technique to expose different layers of emulsion to the different colours of the spectrum. A TV colour signal is an electrical representation of the original

scene processed and reproduced on a TV display monitor. The fidelity of the displayed colour picture to the original colours will depend on the analysis characteristics of the light splitting block and the linear matrix of the video camera, which are designed and adjusted to be displayed on the appropriate phosphor characteristics of the display tube, all of which collectively take into account, and accurately reproduce the average human perceptual response to colour. In practice, the available phosphor compounds that are employed in tube manufacture determine the selection and handling of the television primary colour signals needed to provide accurate perceptual response to a displayed colour picture.