Above is an example of what a colour-blindness test may look like. The person being tested would try to read the letters in the test.
There are many types of partial color-blindness, the most frequent being red-green. Other forms of color blindness are much much rarer, like blue-yellow, and the rarest being complete color blindess (achromatopsia, or maskun), where one can see only shades of grey.
Types of red-green color blindness:
- Deuteranopia: lacking the green receptor cones in the retina; those affected are unable to distinguish between red and green. This is the commonest form of color blindness, 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.)
- Protanopia: lacking the retinal cones that detect red light, rendering those with this condition unable to see red.
- Protanomalopia: having a mutated form of the red-seeing pigment.
- Deuteranomalopia: having a mutated form of the green-seeing pigment.
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, while men are color blind as soon as their only X chromosome is defective.
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 five different primary colors, if, for example, a carrier of protanomalopia has a child with a deuteranomalopic man. 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. Half the cells in her body express her mother's chromosome pD; the other half express her father's Pd. Thus half of the red cones in her retina are anomalous, and half of the green cones are anomalous, so she has two shades of red, two shades of green, and blue as primary colors.
Color blindness involving blue (tritanopia) is equally distributed among males and females since the gene coding for the blue color receptor is located on chromosome 7.
Exact numbers vary in various populations. (Amongst Americans, approximately 10% of males suffer from some form of color perception deficiency. some more comparison values here)
While normally rare, complete color blindness (maskun) is very common in Pohnpei; about 1/12 of the population there has maskun.
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.
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 will perceive only in shades of grey, the extra information load in adding color is dropped.
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 colour scheme for color-blind readers.