A trichromat 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 three different pure spectral lights. The condition of being a trichromat is called trichromacy.
The normal explanation of trichromacy is that the organism's retina contains three 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 three, since different types may be active at different light intensities. In vertebrates with three types of cone cells, at low light intensities the rod cells may contribute to colour vision, giving a small region of tetrachromacy in the colour space.
Humans and other Old World primates are usually trichromats, as are female New World monkeys of most species, and both male and female howler monkeys. Carnivores, especially nocturnal species, tend to be anomalous trichromats: they possess three distinct types of color receptors, but which respond to different wavelengths than those of primates. An example is the domestic cat. (The domestic dog, however, is a dichromat, having color vision similar to humans with deuteranopia.) Some species of insects (such as honeybees) are also trichromats, being sensitive to ultraviolet, blue and green instead of blue, green and red.
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