How we see colour

How our eyes register colour

The human eye does not detect frequencies directly. So when we see red, we do not necessarily see a narrow range of frequencies corresponding to a fraction of a white light beam, whether this has been split by a prism or a fraction removed by filters. Both of these could result in the same fraction of the spectrum being detected by the eye. As this is a fraction of the spectrum we call this a spectral colour.

To keep what we record as red separate we call this perceptual red. The frequencies falling on the eye when we say yes, that's red may not be only from a narrow part of the spectrum. A large number of combinations of frequencies can be recorded as red by the average eye. How does this come about?

It turns out that the eye has only 3 colour detectors. Stimulating these three to different degrees produces the full range of sensations of colour. Maxwell spent a good few years spinning disks with sectors of varying size and of varying hue in front of a wide range of eyes to prove just this point.

These days the television, in almost every home, gives everyday evidence that mixtures of three colours are enough to reproduce the whole range of colour sensation in us. Inspect one with a magnifying glass, or just sneeze on the computer monitor as you work, and you'll see small spots of red, green and blue. From these are synthesised the full range of visual sensations enjoyed during a broadcast.