ANNABELLA BLUESKY / SCIENCE PHOTO LIBRARY ANNABELLA BLUESKY / SCIENCE PHOTO LIBRARY
Colour blindness test. Cards that are used to check for red-green colour blindness. The Ishihara test, which uses pseudoisochromatic plates, is the most widely used. These cards depict a pattern of red and green dots. People with normal vision or total colour blindness should not be able to distinguish any number. People with red-green deficiencies should read a number on each card. Colour blindness is usually caused by an inherited genetic defect in the light-sensitive pigments of the eye, but it can be caused by optic injury or nerve diseases. Up to 8% of the male population is affected, and ten times more men have colour blindness than women.
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