Color blindness
Can you see both red and green in the pattern above?
Color blindness 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 use of some chemical substances. 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.
Rates of occurrence
While exact numbers vary in various populations, color blindness affects a significant proportion of people. Amongst 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.
Retinal origins of color blindness
There are many types of color-blindness. In order to understand them, 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 are in the blue, green and 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 since 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 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. 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.
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.
Red-green color blindness is "handed down" from a color blind male through his daughters (who are unaffected carriers) to his male grandchildren. His sons are unaffected, since they receive his Y chromosome and not his (defective) X chromosome.
Since 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.
While normally rare, complete color blindness (maskun) is very common in Pohnpei; about 1/12 of the population there has maskun.
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 emergency situations 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.
Red-green color blindness
Types of red-green color blindness
There are several types of red-green color blindness:
Dichromacy and anomalous trichromacy
Protanopes and Deuteranopes are dichromats, that is, they can match any colour 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 colour 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.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. Women are red-green colorblind if both their X chromosomes are defective with the exact same deficiency, while men are color blind as soon as their only X chromosome is defective.Blue-yellow color blindness
Color blindness involving the inactivation of the short-wavelength sensitive cone system (whose absorption spectrum peaks in the blue) is called tritanopia or, loosely, blue-yellow color blindness. It is equally distributed among males and females, since the gene coding for the short-wavelength receptor is located on chromosome 7.Monochromacy
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, 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.Tests for color blindness
Color blindness is 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.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.External links