Dichromat
Encyclopedia
Dichromacy is the state of having two types of functioning color receptors, called cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

s, in the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

s. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

. By comparison, trichromat
Trichromat
Trichromacy or trichromaticism is the condition of possessing three independent channels for conveying color information, derived from the three different cone types...

s 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.

Research has shown that 5 - 8% of males and 0.8% of females have anomalous color vision.

There are three groups of inherited color vision defects: monochromacy, dichromacy and anomalous trichromacy. The last two groups are subdivided into red-green and blue-yellow types of defects. Inherited red-green color vision defects are more common in males (1 to 8 percent depending on race) than in females (about 0.4 percent). Inherited blue-yellow defects are rare in either sex.
Dichromacy is a less severe form of color defect than monochromacy. Dichromats can tell some hues apart. Dichromacy is divided into three types: protanopia, deuteranopia and tritanopia.

Classification

There are various kinds of color blindness
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...

:
  • 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. Protanomaly is a less severe version.X-linked (red-green) dichromats lack the function of either long-wavelength-sensitive (L) cones or middle sensitive (M) cones. This results in them being unable to discriminate within the red-green dimension of color space. This implies colorblindness, and concequently affects visual detection.

  • 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.
  • A more rare 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.


When one cone cell is dysfunctional a person can still recognize objects fairly well with the remaining hues of the two cones. Of course certain colors (eg. green and red) will be harder to distinguish.

Examples of how dichromats view the world:
A: Normal trichromatic view, B: Protanope simulation, C: Deuteranope simulation, D: Tritanope simulation.

Animals that are dichromats

It is currently believed that most mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s are dichromats. The straightforward exceptions are primates closely related to humans
Catarrhini
Catarrhini is one of the two subdivisions of the higher primates . It contains the Old World monkeys and the apes, which in turn are further divided into the lesser apes or gibbons and the great apes, consisting of the orangutans, gorillas, chimpanzees, bonobos, and humans...

, which are usually trichromats, and sea mammals (both pinniped
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

s and cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

ns) 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 monkey
Howler monkey
Howler monkeys are among the largest of the New World monkeys. Fifteen species are currently recognised. Previously classified in the family Cebidae, they are now placed in the family Atelidae. These monkeys are native to South and Central American forests...

s are trichromats.

Studying vision requires subjects with impairment in the visual system, preferably systems that only contain cone receptors. Pure-cone receptors are rare to find in humans, but common in ground squirrels which have become of particular interest when studying this subject dichromacy.
Recent research suggests that trichromacy may be widespread among marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

s.

Dichromats are capable of seeing 10,000 different colors

According to color vision researchers at the Medical College of Wisconsin
Medical College of Wisconsin
Medical College of Wisconsin is a private, freestanding medical school and graduate school located in Milwaukee, Wisconsin, United States. It was formerly affiliated with Marquette University....

 (including Jay Neitz
Jay Neitz
Jay Neitz is professor of ophthalmology and a color vision researcher at the University of Washington in Seattle, Washington in the United States.- Cone cells and the numbers of colors an organism can see :...

), each of the three standard color-detecting cones in the retina of trichromat
Trichromat
Trichromacy or trichromaticism is the condition of possessing three independent channels for conveying color information, derived from the three different cone types...

s – blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

, green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...

 and red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 – can pick up about 100 different gradations of color. Since each detector is independent of the others, simple exponentiation
Exponentiation
Exponentiation is a mathematical operation, written as an, involving two numbers, the base a and the exponent n...

 gives a total number of colors discernible by an average human as their product, or about 1 million. Similarly, a dichromat (such as a human with red-green color blindness
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...

) would be able to distinguish about 10,000 different colors. No such calculation has been verified by psychophysical
Psychophysics
Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, more completely, as "the analysis of perceptual...

 testing.

External links


http://jfly.iam.u-tokyo.ac.jp/color/- Color Universal Design
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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