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 lines of red dots on a background of green dots. People with normal vision can trace both lines. People with red deficiency (protanopia) can only trace the more orange of the two lines, whereas those with green deficiency (deuteranopia) can only trace the redder line. 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.
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