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. This card depicts a line of orange dots on a background of blue dots. People with normal vision can trace the entire line. People with red-green colour blindness are unable to distinguish the line and therefore cannot complete the task. 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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