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.
Adam Hart-Davis/Science Source/Photo Researchers, Inc.
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