Receptor matrix

The cone receptors have an essentially logarithmic response to incoming light (in common with many other forms of light sensors), this logarithmic response being of a limited dynamic range under given stable lighting conditions (F.S. Werblin, Scientific American, January 1973).

The whole response function is, however, automatically adjustable over a wide range of lighting (adaptation), such that the response is optimised to suit prevailing lighting (which can vary very widely between summer & winter and between outdoor & indoor viewing).

Not only that but, in addition, for prolonged viewing of visual scenes with large variations of light over the visual field, adaptation can also take place differentially in different parts of the visual field.