DAVID NICHOLLS / SCIENCE PHOTO LIBRARY DAVID NICHOLLS / SCIENCE PHOTO LIBRARY
Colour blindness test. Computer artwork of an Ishihara colour test card used to check for red- green colour blindness. The card is comprised of red and orange dots on a background of green dots. A person with normal vision will be able to see an image of an eye. 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. Colour blindness is usually caused by an inherited genetic defect in the light-sensitive cells (cones) of the eye. Colour blindness can also be caused by optic injury or nerve diseases.
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