Color blindness, sometimes called color vision deficiency or Daltonism, is the inability to tell certain colors apart. The ability to see color originates in specific visual cells, called cones, in the retina. The retina is the tissue lining the inside of the back of the eye. A person with normal color vision has three types of cones, each sensitive to a different color—red, green, and blue. Scientists call this trichromatic vision. People with color blindness lack the function of one or more of these types of cones.
People lacking the function of one type of cone are said to have dichromatic vision. Most color-blind people have lost the function of either the red or green cones, called red-green defect. People with this condition confuse reds with greens, and some reds or greens with yellows. Other people have lost the function of the blue cones, resulting in a blue-yellow defect. People with this defect confuse blues with yellows.
More men than women are color blind. About 1 in every 12 men has a red-green color vision defect compared to about 1 of every 200 women. Only about 1 in 30,000 people are truly blind to all colors. They have monochromatic vision and see everything in shades of gray. These people have lost function in all cones and their vision is limited to cells called rods. Rods provide vision in dim light and are not sensitive to colors.
Many color-blind people do not realize that they have abnormal color vision. They have learned to use the color names that everyone else uses. These people can be hampered in everyday activities. For example, they may have difficulty matching clothing by color. Many armed forces and local police and fire departments refuse to accept individuals with color vision defects. Color blindness can also be a hindrance for the many other professions in which discerning colors is important.
Most people can be easily tested for impaired color vision. The Hardy-Rand-Rittler (H-R-R) test and Ishihara test can indicate the type and severity of the color vision defect. These tests use plates that feature colored shapes, such as triangles and squares, in a jumble of background dots. The dots of both the shape and the background vary in color and intensity. On some plates, a person with a color vision defect will fail to see anything. On others, there may be shapes that are only visible to people with specific color vision defects.
Color blindness is usually inherited. It may also be acquired as a result of eye disease or an injury to the brain. Of the inherited forms, red-green color vision defects are the most common. If a color-blind man marries a woman with no family history of color blindness, their children will have normal color vision. If a woman whose father is color blind marries a man with normal color vision, each of the sons has a 50-50 chance of inheriting the defect.
