Dichromacy is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights. By comparison, trichromats can perceive colors made of up to three pure spectral lights, and tetrachromats can perceive colors made of four.
Dichromacy in humans is a color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, predominantly affecting males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions. The term is from di meaning "two" and chroma meaning "color".
There are various kinds of color blindness:
The three determining elements of a dichromatic opponent-colour space are the missing colour, the null-luminance plane, and the null-chrominance plane. The description of the phenomena itself does not indicate the colour that is impaired to the dichromat, however, it does provide enough information to identify the fundamental colour space, the colours that are seen by the dichromat. This is based on testing both the null-chrominance plane and null-luminance plane which intersect on the missing colour. The cones excited to a corresponding colour in the colour space are visible to the dichromat and those that are not excited are the missing colours.
According to color vision researchers at the Medical College of Wisconsin (including Jay Neitz), each of the three standard color-detecting cones in the retina of trichromats – blue, green and red – can pick up about 100 different gradations of color. If each detector is independent of the others, the total number of colors discernible by an average human is their product (100 × 100 × 100), i.e. about 1 million; Nevertheless, other researchers have put the number at upwards of 2.3 million. The same calculation suggests that a dichromat (such as a human with red-green color blindness) would be able to distinguish about 100 × 100 = 10,000 different colors, but no such calculation has been verified by psychophysical testing.
Furthermore, dichromats have a significantly higher threshold than trichromats for colored stimuli flickering at low (1 Hz) frequencies. At higher (10 or 16 Hz) frequencies, dichromats perform as well as or better than trichromats. This means such animals would still observe the flicker instead of a temporally fused visual perception as is the case in human movie watching at a high enough frame rate.
It is more informative to use situations where less than the total visual system is operating when studying about vision. For example, a system by which cones are the sole visual receptors could be used. This is rare in humans but certain animals possess this trait and this proves useful in understanding the concept of dichromacy.
While their Triassic ancestors were trichromatic, placental mammals are as a rule dichromatic; the ability to see long wavelengths (and thus separate green and red) was lost in the ancestor of placental mammals, though it is believed to have been retained in marsupials, where trichromatic vision is widespread. Recent genetic and behavioral evidence suggests the South American marsupial Didelphis albiventris is dichromatic, with only two classes of cone opsins having been found within the genus Didelphis. Dichromatic vision may improve an animal's ability to distinguish colours in dim light; the typically nocturnal nature of mammals, therefore, may have led to the evolution of dichromacy as the basal mode of vision in placental animals.
The exceptions to dichromatic vision in placental mammals are primates closely related to humans, which are usually trichromats, and sea mammals (both pinnipeds and cetaceans) which are cone 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 cone monochromats, and both sexes of howler monkeys are trichromats.