A dichromat is an organism for which the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no more than two different pure spectral lights. The condition of being a dichromat is called dichromacy.

The normal explanation of dichromacy is that the organism's retina contains two types of color receptors (called cone cells in vertebrates) with different absorption spectra. In practice the number of such receptor types may be greater than two, since different types may be active at different light intensities. In vertebrates with two types of cone cells, at low light intensities the rod cells may contribute to colour vision, giving a small region of trichromacy in the colour space.

It is currently believed that most mammals are dichromats. The straightforward exceptions are Old World primates, including humans, which are usually trichromats, and sea mammals (both pinnipeds and cetaceans) which are monochromats. New World monkeys are a partial exception: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are monochromats, and both sexes of howler monkeys are trichromats.

The two best-known forms of color blindness in humans result in dichromacy, since one of the three cone systems is non-functional in these conditions. However many people who are described as color blind are in fact anomalous trichromats; in this condition, there are three functional cone systems but one of them has an unusual absorption spectrum so the person does not make the same color matches as the rest of the population.

References

• Jacobs, G. H., & Deegan, J. F. (2001). Photopigments and colour vision in New World monkeys from the family Atelidae. Proceedings of the Royal Society of London, Series B, 268, 695-702.
• Jacobs, G. H., Deegan, J. F., Neitz, J., Crognale, M. A., & Neitz, (1993). Photopigments and colour vision in the nocturnal monkey, Aotus. Vision Research, 33, 1773-1783.
• Mollon, J. D., Bowmaker, J. K., & Jacobs, G. H. (1984). Variations of colour vision in a New World primate can be explained by polymorphism of retinal photopigments. Proceedings of the Royal Society of London, Series B, 222, 373-399.