SPENCER SUTTON / SCIENCE PHOTO LIBRARY SPENCER SUTTON / SCIENCE PHOTO LIBRARY
Ishihara color diagram. This type of colored plate is used to test for red-green color deficiencies. The color plate is comprised of red and orange dots on a background of green dots. A person with normal vision will be able to see the word 'color'. A person with red deficiency (protanopia) will only see the orange parts of the word, while those with green deficiency (deuteranopia) will only see the red parts of the word. Color blindness is usually caused by an inherited genetic defect in the light-sensitive cells (cones) of the eye. Color blindness can also be caused by optic injury or nerve diseases.
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