|Classification and external resources|
|OMIM||303900 303800 190900|
|DiseasesDB||30021 30023 30022|
Dichromacy (di- meaning 'two' and chromo meaning 'color') 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 require three pure spectral lights to match all colors that they can perceive.
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.
There are various kinds of color blindness:
- Protanopia is a severe form of red-green color-blindness, in which there is impairment in perception of very long wavelengths, such as reds. To these individuals, reds are perceived as beige or grey and greens tend to look beige or grey like reds. It is also the most common type of dichromacy today. The reason why this problem occurs is because patients do not have the red cone cells in the retina. Protanomaly is a less severe version.
- Deuteranopia consists of an impairment in perceiving medium wavelengths, such as greens. Deuteranomaly is a less severe form of deuteranopia. Those with deuteranomaly cannot see reds and greens like those without this condition; however, they can still distinguish them in most cases. It is very similar to Protanopia. In this form patients do not have green cone cells in the retina, which makes it hard to see the green color.
- A rarer form of color blindness is tritanopia, where there exists an inability to perceive short wavelengths, such as blues. Sufferers have trouble distinguishing between yellow and blue. They tend to confuse greens and blues, and yellow can appear pink. This is the rarest of all dichromacy, having a ratio of 10 in 1 million people. Patients do not have the blue cone cells in the retina.
Testing for dichromacy 
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 provides enough information to identity 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. 
Animals that are dichromats 
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 dicromatic; 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 basically nocturnal nature of mammals, therefore, may have lead 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.
In recent investigations, it was found that some ground squirrels possess dichromatic vision. It is beneficial to use situations in which less than the total visual system is functional when studying vision. This is present in systems in which cones are the sole visual receptors such as the dichromatic colour vision in squirrels.
Dichromats may be capable of seeing 10,000 different colours 
According to colour vision researchers at the Medical College of Wisconsin (including Jay Neitz), each of the three standard colour-detecting cones in the retina of trichromats – blue, green and red – can pick up about 100 different gradations of colour. If each detector is independent of the others, simple exponentiation gives a total number of colours discernible by an average human as their product, or about 1 million; nevertheless, other researchers have put the number at upwards of 2.3 million. Exponentiation suggests that a dichromat (such as a human with red-green color blindness) would be able to distinguish about 10,000 different colours,  but no such calculation has been verified by psychophysical testing.
Furthermore, research conducted in 2006 suggests that dichromats have a significantly higher threshold for coloured stimuli than trichromats at low (1 Hz) frequencies. At higher(100 Hz) frequencies, dichromats perform as well as trichromats - if not better. 
See also 
- Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
- "Guidelines: Colour Blindness." Tiresias.org. Accessed September 29, 2006.
- Scheibner, H., & Cleveland, S. (1998). Dichromacy characterized by chrominance planes. Vision Research, 38(21), 3403–3407.
- Scheibner, H., & Cleveland, S. (1997). Dichromacy characterized by chrominance planes. Vision Research, 38(1), 3403–3407.
- Jacobs, G. H., & Yolton, R. L. (1969). Dichromacy in a ground squirrel. letters to nature, 223, 414–415.
- Jacobs, G. H. (2009). "Evolution of colour vision in mammals". Philosophical Transactions of the Royal Society Series B 364 (1531): 2957–67. doi:10.1098/rstb.2009.0039. PMC 2781854. PMID 19720656.
- Bowmaker, JK (1998). "Evolution of colour vision in vertebrates.". Eye (London, England). 12 ( Pt 3b): 541–7. PMID 9775215. Retrieved 7 January 2013.
- Arrese, C. A., Oddy, A. Y., Runham, P. B., Hart, N. S., Shand, J., Hunt, D. M., * Beazley, L. D. (2005). Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (Setonix brachyurus) and quenda (Isoodon obesulus). Proceedings of the Royal Society of London Series B, 272: 791-796.
- Gutierrez, E.A., Pegoraro, B.M., Magalhães-Castro, B., Pessoa, V.F. (2011).Behavioural evidence of dichromacy in a species of South American marsupial. Animal Behaviour 81: 1049-1054
- Vorobyev, M. (2006). "Evolution of colour vision: The story of lost visual pigments". Perception (ECVP Abstract Supplement) 35.
- Jacobs, GH; Neitz, M; Neitz, J (1996). "Mutations in S-cone pigment genes and the absence of colour vision in two species of nocturnal primate.". Proceedings of the Royal Society of London Series B 263 (1371): 705–10. PMID 8763792. Retrieved 19 January 2013.
- Vorobyev, M (Jul). "Ecology and evolution of primate colour vision.". Clinical & experimental optometry : journal of the Australian Optometrical Association 87 (4-5): 230–8. PMID 15312027. Retrieved 7 January 2013.
- 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.
- Sternberg, Robert J. (2006): Cognitive Psychology. 4th Ed. Thomson Wadsworth.
- Jacobs, G. H.; Yolton R. L. (July 1969). "Dichromacy in the ground squirrel". Nature 223: 414–415. doi:10.1038/223414a0.
- Mark Roth (September 13, 2006]). "Some women who are tetrachromats may see 100,000,000 colors, thanks to their genes". Pittsburgh Post-Gazette.
- "Color Vision:Almost Reason for Having Eyes" by Jay Neitz, Joseph Carroll, and Maureen Neitz Optics & Photonics News January 2001 1047-6938/01/01/0026/8- Optical Society of America
- Sharpe Lindsay T. ; de Luca Emanuela ; Hansen Thorsten ; et al. "Advantages and disadvantages of human dichromacy" JOURNAL OF VISION Volume: 6 Issue: 3 Pages: 213-223 DOI: 10.1167/6.3.3 Published: 2006
Scheibner, H., & Cleveland, S. (1997). Dichromacy characterized by chrominance planes. Vision Research, 38(1), 3403–3407.
- Visual comparisons of various types of color vision impairments by Cal Henderson
- Color Vision, Color Deficiency at Firelily Designs
- Colorblindness at handprint.com
- Colblindor -- Color Blindness Viewed Through Colorblind Eyes by Daniel Flück