This image is part of the standard test for color blindness. Individuals with normal color vision will see the number 57, while those with red-green deficiencies will see the number 35. Color blindness, an inability to distinguish between red and green and sometimes between blue and yellow, is caused by a defect in one of the three color-sensitive cells in the retina. Color blindness affects approximately one person in thirty.
Color Blindness, defect of vision affecting the ability to distinguish colors, occurring mostly in males. Color blindness is caused by a defect in the retina or in other nerve portions of the eye. The first detailed report on this condition was written by the British chemist John Dalton, who was himself afflicted with it. Total color blindness, in which all hues are perceived as variations of gray, is known as achromatopsia or monochromatism. This condition is congenital, extremely rare, and affects men and women almost equally. Partial color blindness, called dichromatism, consists generally of the inability to differentiate between the reds and the greens or to perceive either reds or greens; infrequently, the confusion may involve the blues or the yellows.
Dichromatism is the most common form of color blindness, affecting about 7 percent of men and less than 1 percent of women. Dichromatism is identified as a sex-linked hereditary characteristic. Color blindness also may occur as a temporary condition following a serious illness.
The vision of most color-blind people is normal in all other respects. In addition, color-blind people can generally learn by experience to associate certain colors with varying sensations of brightness. Consequently, many victims of the defect are unaware that they are color-blind. Several types of tests have been devised for the rapid diagnosis of color blindness and of the particular variations of the condition.
Modern safety regulations in the U.S. require the tests for all transportation workers. In some states, color-vision tests are also given to drivers of private automobiles and to such public employees as police officers and firefighters.