What is color blindness?

Causes and Symptoms

The retina of the eye is a thin, fragile membrane that contains millions of photoreceptor
cells. They convert light energy into an electrical signal, which is transmitted to the brain via the optic nerve. On a microscopic scale, the structure of the retina is like a carpet with its many fibers sticking upward. There are two types of photoreceptor cells, called rods and cones because of their distinctive shapes. Only the cones are important for color vision. There are three varieties of cones with peak sensitivities for red, green, and blue, respectively. The shades and tints of all other colors are mixtures of these three.

Color blindness involves a deficiency in these photoreceptor cells. A deficiency of green photoreceptor cells is much more common than a deficiency of red photoreceptors. Some people are totally color-blind, which means that they are completely unable to distinguish among red, orange, yellow, and green. Color blindness is quite rare in females (less than 1 percent of the population) but is more prevalent in males (about 8 percent).

Diagnostic tests are available to determine the extent of color blindness. The Ishihara color test, named after a Japanese ophthalmologist, consists of a mosaic of colored dots containing a letter of the alphabet made up of dots of a different color—for example, yellow dots in a background of green ones. Color-blind individuals would be unable to distinguish the letter because yellow and green look the same to them.

A more precise diagnostic test makes use of the Nagel anomaloscope, which has two colored light sources whose brightness can be adjusted. The patient tries to match a given color by superimposing the two light beams while varying their intensities. For normal eyes, red and green lights of similar intensities can be superimposed to create yellow. However, a patient who requires a considerably larger green component to create yellow evidently has a deficiency of green photoreceptor cells.

Treatment and Therapy

Color blindness is a genetic defect from birth, not a disease. No procedure is known by which it can be corrected. Color-blind people must find ways to counter the effects of their condition. For example, they can obtain driver’s licenses because they learn that stoplights are always red on top, yellow in the middle, and green on the bottom. Color-blind individuals may need help, however, with tasks such as clothing selection. Good color discrimination is required for some occupations, such as interior decorating, graphic design, advertising, or airplane piloting. Fortunately, color blindness is not a deterrent for most jobs.

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