Investigating colour vision

People can have defective colour vision to a greater or lesser extent. Some people's eyes are only sensitive to two of the three colours, while others can discriminate all colour, but not as well as those with normal colour vision.

Defective colour vision results from abnormalities in the retina at the back of the eye. Most people's retinas contain three substances, each of which is predominantly sensitive to red, green or blue light. All normal colour vision results from the interaction of these three substances. Defective colour vision can be considered as being a result of a defect in one or more of the three substances. Defects in blue sensitivity are, however, extremely rare.