Color Blindness

Color blindness occurs when a single type of color-receptor cone is missing from the retina, due to a genetic defect, rendering the person unable to distinguish some colors from others. There are three main types of color blindness-green, red and rarely, blue. In this test card, both the fork and spade can be seen by people with normal color vision. The spade is purple, a color which is received by the receptors for blue-green in the retina. Since green-blind people have no green receptor cones they cannot distinguish the spade from the background. Similarly, red-blind people have no red-receptor cones and so fail to see the fork, which is red and should, therefore, stimulate the cones which receive red. Using a series of such cards, the nature of color blindness can be determined.