Color Blindness

Color blindness, sometimes called daltonism, is the inability to tell all colors apart. The ability to see color originates in specific visual cells, called cones, in the retina of the eye. A person with normal color vision has three types of cones, and each type is sensitive to a different color. Color-blind people lack one, two, or all of these types of cones.

Most color-blind people have dichromatic vision. People with this kind of color blindness can see only yellows and blues. They confuse reds with greens, and some reds or greens with some yellows. Only a very few people are truly blind to all colors. They have achromatic vision. They see in shades of white, gray, and black--somewhat like a black-and-white photograph.

More men than women are color blind. About 8 of every 100 men are color blind, compared to about 1 of every 200 women. There is no cure for color blindness.

Many animals, including cats and horses, probably do not see colors as we do. But the condition is normal in their eyes, not defective.

Many color-blind people do not realize that their eyesight is defective. They have learned to use the color names that everyone else uses. These people may be hampered in their everyday activities, and their condition may place them in danger. If they confuse red and green, for example, they may only be able to tell traffic signals apart by their brightness. Many armed forces refuse to accept color-blind people for military service. In addition, color blindness can be a hindrance for airline pilots, fashion designers, and members of certain other professions.

Most people can be easily tested for color blindness. The Hardy-Rand-Rittler (H-R-R) and Ishihara tests indicate both the type and the degree of color blindness. In these and similar tests, colored triangles, squares, and other shapes lie in a jumble of dots. These dots vary in both color and intensity. As the person identifies the colored shapes, an examiner can determine the person's ability to see colors. Other tests, such as the Holmgren yarn-matching test and the Farnsworth-Munsell 100-hue disk-matching test, measure the ability to match colors.

Color blindness is inherited. If a color-blind man marries a woman who has no family history of color blindness, their children will have normal vision. Their daughters, however, will carry the gene for color blindness, and may pass it on to their children. If a woman whose father is color blind marries a man with normal vision, each of their sons has a 50-50 chance of inheriting the disorder. Injury to the retina or optic nerve and various diseases of the eye can also cause color blindness.

Contributor: Ramesh C. Tripathi, F.R.C.Path., Prof. and Chairman, Department of Ophthalmology, Univ. of South Carolina School of Medicine. and Brenda J. Tripathi

Color Blindness

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