By population, about 64% of the cones are red-sensitive, about 32% green sensitive, and about 2% are blue sensitive. The "blue" cones have the highest sensitivity and are mostly found outside the fovea. The shapes of the curves are obtained by measurement of the absorption by the cones, but the relative heights for
the three types are set equal for lack of detailed data. There are fewer blue cones, but the
blue sensitivity is comparable to the others, so
there must be some boosting mechanism. In the final visual perception, the three types seem to be comparable, but
the detailed process of achieving this is not known.
When light strikes a cone, it interacts with a visual pigment which consists of a protein called opsin and a small molecule called a chromophore which in humans is a derivative of vitamin A. Three different kinds of opsins respond to short, medium and long wavelengths of light and lead to the three response curves shown above.