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 might 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 colours", after the realization of his own color blindness.
Some animals are (partially) color blind by nature (cattle, cats...) and this is not considered a defect or inability. However, some animals see in a different spectrum or wavelength range, or have excellent night-vision.
Color blindness is usually labelled as a disability; however, in select situations color blind people have advantages over people with a full color range. 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  (http://vision.psychol.cam.ac.uk/jdmollon/papers/hudibras.pdf).
This is a sample image. Anyone not color blind should be able to see the number 83. The pictures below should look similar (containing just different numbers) to people with normal vision, but they won't be visible to people with some color vision deficiency.
This image contains a two digit number similar to the sample above. If you can't see it on this image, you might be protanopic.
If you can't see this number, you might be deuteranopic.
If you can't see this number, you might be tritanopic.
Simple test for color blindness
We begin by demonstrating a simple test for color blindness. To the right are four pictures with 2-digit numbers. People who are not completely color blind should be able to read the number 83 in the first picture. The following images test for different kinds of color blindness. Bear in mind that these are not substitutes for proper testing conducted by a doctor in a controlled environment with fixed lighting conditions and standardized procedure. If you can't see some of these images, this may just mean that you use a different type of monitor or there are different light conditions in your room. In this case, we recommend to visit your doctor and do a real test.
Rates of occurrence
Although exact numbers vary in various populations, color blindness affects a significant proportion of people. Among Americans, approximately 10% of males suffer from some form of color perception deficiency. 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.
Causes of color blindness
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 occurs [very rarely] in the "aura" of some migraine sufferers.
In order to understand retinal color blindness, it is necessary to know that 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 in the blue-violet, green-yellow, and greenish-yellow regions of the spectrum, respectively). It is important to realise 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 greenish-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 we see as wavelength reduces is the result of the other two cone systems being increasingly stimulated as well.
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.
Red-green color blindness
Types of red-green color blindness
There are several types of red-green color blindness:
- Protanopia: lacking the long-wavelength sensitive retinal cones, those with this condition are unable to distinguish between colors in the green-yellow-red section of the spectrum. They have a neutral point at a wavelength of 492 nanometres, that is they cannot discriminate light of this wavelength from white. Their sensitivity to light in the orange and red part of the spectrum is also reduced. A very few people have been found who have one normal eye and one protanopic eye. These unilateral dichromats report that with only their protanopic eye open, they see wavelengths below the neutral point as blue and those above it as yellow. This is a rare form of colorblindness.
- Deuteranopia: lacking the medium-wavelength cones, those affected are again unable to distinguish between colors in the green-yellow-red section of the spectrum. Their neutral point is at a slightly longer wavelength, 498 nanometres. This is one of the rarer forms of colorblindness making up about 1% of the male population, also known as Daltonism after Dalton. (Dalton's diagnosis was confirmed as deuteranopia in 1995, some 150 years after his death, by DNA analysis of his preserved eyeball.) Deuteranopic unilateral dichromats report that with only their deuteranopic eye open, they see wavelengths below the neutral point as blue and those above it as yellow.
- Protanomaly: having a mutated form of the long-wavelength pigment, whose peak sensitivity is at a shorter wavelength than in the normal retina, protanomalous individuals are less sensitive to red light than normal. This means that they are less well able to discriminate colors, and they do not see mixed lights as having the same colors as normal observers. They also suffer from a darkening of the red end of the spectrum. This causes reds to reduce in intensity to the point where they can be mistaken for black. Protoanomaly is a fairly rare form of colorblindness making up about 1% of the male population.
- Deuteranomaly: having a mutated form of the medium-wavelength pigment. The medium-wavelength pigment is shifted towards the red end of the spectrum resulting in a reduction in sensitivity to the green area of the spectrum. Unlike protanomaly the intensity of colors is unchanged. This is the most common form of colorblindness making up about 8% of the male population.
Dichromacy and anomalous trichromacy
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.
Genetics of red-green color blindness
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 colorblind, 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.
There is a third, blue color opsin gene in humans that is present on one of the numbered chromosomes (autosomes), specifically chromosome 7. Defects in this gene cause tritanopia and are less frequently evident, since it requires mutations to be present in two chromosomes.
The gene traits 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 protanomalopia has a child with a deuteranomalopic 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 wavelength, medium wavelength 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.
Blue-yellow color blindness
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. It is equally distributed among males and females, because the gene coding for the short-wavelength receptor is not sex-linked (being located on chromosome 7).
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 maskun, 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, complete color blindness (maskun) is very common in Pohnpei: about 1/12 of the population there has maskun.
Tests for color blindness
Color blindness is most often tested using the Ishihara colour 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 slighly 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 colour test is often being criticized for containing only numbers and thus not be useful for small children in the preschool age. It's often stated that it's 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 draw 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 (for example (http://www.hubmed.org/display.cgi?uids=15192692)) to collect thorough datasets, identify copunctal points, and measure just noticeable differences.
Design implications of color blindness
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.
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 e.g. emergency brake handles or emergency phones.
- Congenital Colour Vision Deficiencies (http://psychology.ucalgary.ca/pace/VA-Lab/colourperceptionweb/congenital.htm), description from the University of Calgary
- Color Vision Testing Made Easy (http://colorvisiontesting.com/online%20test.htm), samples of this alternative test
- How the world looks to a color blind person (http://colorvisiontesting.com/what%20colorblind%20people%20see.htm), example images drawn using distorted colors