Colour vision processing in the primate visual system is initiated by absorption of light by three different spectral classes of cones. Consequently, colour vision is described as being trivariant or trichromatic, and initial psychophysical studies demonstrated that colours could be matched by the use of three different primaries. In 1802, Thomas Young proposed a model that perception of colour can be coded by three principal colour receptors rather than thousands of colour receptors coding for individual colours.