Television Colour Theory

Colour television reproduces a full range of hues by combining just three primary colours of light: red, green and blue. This is additive colour mixing, in which lights of different colours add together to form others; equal amounts of all three produce white. A colour display generates these three primaries at each point of the picture in varying proportions, and the eye blends them so that the viewer perceives a continuous colour image. The human eye's three types of colour-sensitive cells are the reason three primaries suffice.

To transmit colour while remaining compatible with the structure of a television signal, the picture information is separated into a luminance component, which carries brightness, and chrominance components, which carry the colour information. Luminance corresponds to the black-and-white picture, while the chrominance signals describe how the colour departs from grey. This separation was originally devised so that colour broadcasts could still be displayed in monochrome on older receivers, and it also exploits the fact that the eye is more sensitive to detail in brightness than in colour, allowing the colour information to be transmitted with less bandwidth.

Different television standards encode the chrominance information in different ways. PAL, NTSC and SECAM each modulate the colour signals onto a subcarrier within the channel, using methods that affect how robustly colour survives transmission imperfections. The way the three primaries are produced depends on the display: a cathode-ray tube uses three phosphors, while LCD and plasma panels use coloured sub-pixels, but the underlying principle of combining red, green and blue remains the same.

Frequently asked questions

Which primary colours does a television use?

Red, green and blue light, combined by additive mixing in varying proportions to reproduce the full range of colours seen on screen.

What is the difference between luminance and chrominance?

Luminance carries the brightness or black-and-white part of the picture, while chrominance carries the colour information describing how each point departs from grey.

Why is the colour signal separated from brightness?

It allowed colour broadcasts to remain viewable in monochrome and lets colour be sent with less bandwidth, since the eye notices brightness detail more than colour detail.