To understand colorblindness, you need to understand some things about color vision.
The part of the eye that senses light and deals with color vision is called the retina. There are structures in the retina shaped like rods and cones -- rods help to see in low light, cones help to see in color and also with seeing details. Rods and cones contain photosensitive chemicals. In rods, this chemical is rhodopsin. The chemicals in cones are called photopigments. There are three kinds of cones, and each cone has a different photopigment that's sensitive to a certain wavelength of light. Because most of us have all three kinds of cones, normal human vision is called trichromatic.
Colorblindness is a misleading term. It makes everything sound black and white. Color vision deficiency might describe the condition more clearly. There are different kinds of color vision problems and different degrees of severity. Red-green color vision defects are the most common.
People who have mild color vision defects have anomalous trichromacy, which means that they do have all three types of cones, but one of the cones is defective. Someone with deuteranomaly, the least severe kind of colorblindness and also the most common, has unusual red cones, while someone with protanomaly has unusual green. People with deuteranomolous vision may not even know they don't see colors normally. Tritanomaly, trouble distinguishing blue and yellow, is quite rare.
People who are missing one type of cone altogether have dichromatic vision, which is more serious than anomalous trichromacy. Within this category of dichromatic vision, we have three different types.
- Deuteranopia: no green cones (also referred to as L cones, as in sensitivity to long wavelengths of light)
- Protanopia: no red cones (M or medium wavelength cones)
- Tritanopia: no blue cones (S or short wavelength cones)
Monochromacy is the next step on the color vision ladder. Monochromats see life in black, white and shades of gray. There are two types of monochromacy: rod monochromacy and cone monochromacy. People with rod monochromacy, also called achromatopsia, also have very poor vision and a high sensitivity to light. They also have nystagmus, which makes your eyeballs look kind of wobbly.
In the next section, we'll take a look through the eyes of the colorblind.