Why does your camera sensor record only one colour per pixel?

A digital camera sensor cannot see colour. Each of its light-sensing spots, a pixel, only counts how much light reached it. To get colour, a tiny filter sits over every pixel, letting through red, green or blue. This grid of filters is a Bayer filter mosaic, named after Bryce Bayer of Eastman Kodak, and it is used in most single-chip digital image sensors. The pattern is half green, one quarter red and one quarter blue.

So each pixel records only one of three colours. The data from a single pixel cannot specify its red, green and blue values on its own. To obtain a full-colour image, the camera uses demosaicing algorithms that interpolate a complete set of red, green and blue values for every pixel from its neighbours. A green pixel measures green exactly, and borrows its red and blue from the pixels around it.

Why twice as many greens? Bayer's 1976 patent used twice as many green elements as red or blue to mimic the human eye, whose daylight brightness perception relies on cone cells that are most sensitive to green light.

The trick works well in smooth areas, but near sharp edges the borrowed colours can bleed, which is why smarter algorithms interpolate along edges rather than across them. It is the same idea as the Autochrome plate of 1907, whose coloured potato-starch grains it resembles.