Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. It is most often of genetic nature, but may also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals. The English chemist John Dalton in 1794 published the first scientific paper on the subject, "Extraordinary facts relating to the vision of colors", after the realization of his own color blindness; because of Dalton's work, the condition is sometimes called Daltonism, although this term is now used for a type of color blindness called deuteranopia (see below).
Color blindness is usually classed as a disability; however, in select situations color blind people have advantages over people with normal color vision. Color blind hunters are better at picking out prey against a confusing background, and the military have found that color blind soldiers can sometimes see through camouflage that fools everyone else. Monochromats may have a minor advantage in dark vision, but only in the first five minutes of dark adaptation.
| This is a sample image. Everyone should be able to see the number 83. The pictures below should look similar (containing just different numbers) to people with normal vision, but some of them will not be visible to people with a color vision deficiency. Note, however, that the contrast in these tests is much subtler than commonly seen in other similar tests. |
To the bottom are four pictures with 2-digit numbers designed to test for different kinds of color blindness. Note that monitor or lighting differences may affect whether or not the images can be seen.
Color blindness affects a significant number of people, although exact proportions vary among groups. In Australia, for example, approximately 4% of the population suffers from some deficiency in color perception. Isolated communities with a restricted gene pool sometimes produce high proportions of color blindness, including the less usual types. Examples include rural Finland and some of the Scottish islands.
| This image contains a two digit number similar to the sample above. Someone who is protanopic might not see this number. |
There are many types of color blindness. The most common varieties are hereditary (genetic) photoreceptor disorders, but it is also possible to acquire color blindness through damage to the retina, optic nerve, or higher brain areas. Higher brain areas implicated in color processing include the parvocellular pathway of the lateral geniculate nucleus of the thalamus, and visual area V4 of the visual cortex. Acquired color blindness is generally unlike the more typical genetic disorders. For example, it is possible to acquire color blindness only in a portion of the visual field but maintain normal color vision elsewhere. Some forms of acquired color blindness are reversible. Transient color blindness also occurs (very rarely) in the aura of some migraine sufferers.
| Someone who is deuteranopic might not see this number. Please note that the second digit in this number may be difficult to discern even by those with normal vision. |
The normal human retina contains two kinds of light sensitive cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cones, each containing a different pigment. The cones are activated when the pigments absorb light. The absorption spectra of the pigments differ; one is maximally sensitive to short wavelengths, one to medium wavelengths, and the third to long wavelengths (their peak sensitivities are in the blue, yellowish-green, and yellow regions of the spectrum, respectively). It is important to realize that the absorption spectra of all three systems cover much of the visible spectrum, so it is incorrect to refer to them as "blue", "green" and "red" receptors, especially because the "red" receptor actually has its peak sensitivity in the yellow. The sensitivity of normal color vision actually depends on the overlap between the absorption spectra of the three systems: different colors are recognized when the different types of cone are stimulated to different extents. For example, red light stimulates the long wavelength cones much more than either of the others, but the gradual change in hue seen as wavelength reduces is the result of the other two cone systems being increasingly stimulated as well.
| Someone who is tritanopic might not see this number. |
The different kinds of color blindness result from one or more of the different cone systems either not functioning at all, or functioning in an unusual way. When one cone system is compromised, dichromacy results. The most frequent forms of human color blindness result from problems with either the middle or long wavelength sensitive cone systems, and involve difficulties in discriminating reds, yellows, and greens from one another. They are collectively referred to as "red-green color blindness", though the term is an over-simplification and somewhat misleading. Other forms of color blindness are much rarer. They include problems in discriminating blues from yellows, and the rarest forms of all, complete color blindness or monochromacy, where one cannot distinguish any color from grey, as in a black-and-white movie or photograph.
There are several types of red-green color blindness:
Protanopes and deuteranopes are dichromats; that is, they can match any color they see with some mixture of just two spectral lights (whereas normally humans are trichromats and require three lights). Those having protanomaly or deuteranomaly are trichromats, but the color matches they make differ from the normal: In order to match a given spectral yellow light, protanomalous observers need more red light in a red/green mixture than a normal observer, and deuteranomalous observers need more green. They are called anomalous trichromats.
Protanomaly and deuteranomaly can be readily observed using an instrument called an anomaloscope, which mixes spectral red and green lights in variable proportions, for comparison with a fixed spectral yellow. If this is done in front of a large audience of men, as the proportion of red is increased from a low value, first a small proportion of people will declare a match, while most of the audience are seeing the mixed light as greenish. These are the deuteranomalous observers. Next, as more red is added the majority will say that a match has been achieved. Finally, as yet more red is added, the remaining, protanomalous, observers will declare a match at a point were everyone else is seeing the mixed light as definitely reddish.
Genetic red-green color blindness affects men much more often than women, because the genes for the red and green color receptors are located on the X chromosome, of which men have only one and women have two. Such a trait is called sex-linked. Genetic females (46, XX) are red-green color blind only if both their X chromosomes are defective with a similar deficiency, whereas genetic males (46, XY) are color blind if their only X chromosome is defective.
The gene for red-green color blindness is transmitted from a color blind male to all his daughters who are heterozygote carriers and are perceptually unaffected. In turn, a carrier woman passes on a mutated X chromosome region to only half her male offspring. The sons of an affected male will not inherit the trait, since they receive his Y chromosome and not his (defective) X chromosome.
Because one X chromosome is inactivated at random in each cell during a woman's development, it is possible for her to have four different cone types, if, for example, a carrier of protanomaly has a child with a deuteranomalic man. The deficiencies can combine to form a fourth receptor whose absorption spectrum peaks in the yellow-green area. Denoting the normal vision alleles by P and D and the anomalous by p and d, the carrier is PD pD and the man is Pd. The daughter is either PD Pd or pD Pd. Suppose she is pD Pd. The cells in her body express her mother's chromosome pD and her father's Pd. Thus some of the cones are anomalous with both deficiencies and some are normal. As a result she has the normal short, medium and long wavelength-sensitive types of cone, with an additional category of receptor that combines the deficiencies. Such women are tetrachromats, since with their four-cone systems, they require a mixture of four spectral lights to match an arbitrary light.
Color blindness involving the inactivation of the short-wavelength sensitive cone system (whose absorption spectrum peaks in the bluish-violet) is called tritanopia or, loosely, blue-yellow color blindness. Mutation of the short-wavelength sensitive cones is called tritanomaly. Tritanopia is equally distributed among males and females, because the gene coding for the short-wavelength receptor is located on chromosome 7 and therefore not sex-linked, but since mutations in both copies are required, it is less frequently apparent.
Complete inability to distinguish any colors is called monochromacy. It occurs in two forms: cone monochromacy, where only a single cone system appears to be functioning, so that no colors can be distinguished, but vision is otherwise more or less normal; and achromatopsia or rod monochromacy, where the retina contains no cone cells, so that in addition to the absence of color discrimination, vision in lights of normal intensity is difficult.
While normally rare, achromatopsia is very common on the island of Pingelap, a part of the Pohnpei state, where it is called maskun: about 1/12 of the population there has it. The island was devastated by a storm in the eighteenth century, and one of the few male survivors carried a gene for achromatopsia; the population is now several thousand, of whom about 30% carry this gene.
Color blindness is most often tested using the Ishihara color test, which consists of a series of pictures of colored spots. A figure (usually a number) is embedded in the picture as a number of spots in a slightly different color, and can be seen with normal color vision, but not with a particular color defect. The full set of tests has a variety of figure/background color combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described above, is also used in diagnosing anomalous trichromacy.
However, the Ishihara color test is criticized for containing only numbers and thus not being useful for young children. It is often stated that it is important to identify these problems as soon as possible and explain them to the children to prevent possible problems and psychological traumas ("Why did you color the sky purple, Johnny?!"). For this reason, alternative color vision tests were developed using only symbols (square, circle, car).
Most clinical tests are designed to be fast, simple, and effective at identifying broad categories of color blindness. In academic studies of color blindness, on the other hand, there is more interest in developing flexible tests ([1], for example) to collect thorough datasets, identify copunctal points, and measure just noticeable differences.
Color codes present particular problems for color blind people as they are often difficult or impossible for color blind people to understand.
Good graphic design avoids using color coding or color contrasts alone to express information, as this not only helps color blind people, but also aids understanding by normally sighted people. The use of Cascading Style Sheets on the world wide web allows pages to be given an alternative color scheme for color-blind readers. This color scheme generator helps a graphic designer see color schemes as seen by eight types of color blindness.
It is sometimes claimed that in extreme emergencies everyone is color blind. When the need to process visual information as rapidly as possible arises, for example in a train or aircraft crash, the visual system may operate only in shades of grey, with the extra information load in adding color being dropped. This is an important possibility to consider when designing, for example, emergency brake handles or emergency phones.