ANNABELLA BLUESKY / SCIENCE PHOTO LIBRARY ANNABELLA BLUESKY / SCIENCE PHOTO LIBRARY
Colour blindness test. Card being used to check for red-green colour blindness. The Ishihara test, which uses pseudoisochromatic plates, is the most widely used. Each card depicts a number formed from red dots on a background of green dots, or vice-versa. People suffering from red- green colour blindness cannot distinguish the numbers. 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. The shape seen here is a circle.
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