Color vision deficiency is the inability to distinguish certain shades of color or in more severe cases, see colors at all. The term "color blindness" is also used to describe this visual condition, but very few people are completely color blind. Most people with color vision deficiency can see colors, but they have difficulty differentiating between
- particular shades of reds and greens (most common) or
- blues and yellows (less common).
People who are totally color blind, a condition called achromatopsia, can only see things as black and white or in shades of gray. The severity of color vision deficiency can range from mild to severe depending on the cause. It will affect both eyes if it is inherited and usually just one if the cause for the deficiency is injury or illness. Color vision is possible due to photoreceptors in the retina of the eye known as cones. These cones have light sensitive pigments that enable us to recognize color. Found in the macula, the central portion of the retina, each cone is sensitive to either red, green or blue light, which the cones recognize based upon light wavelengths. Normally, the pigments inside the cones register differing colors and send that information through the optic nerve to the brain enabling you to distinguish countless shades of color. But if the cones lack one or more light sensitive pigments, you will be unable to see one or more of the three primary colors thereby causing a deficiency in your color perception. The most common form of color deficiency is red-green. This does not mean that people with this deficiency cannot see these colors at all; they simply have a harder time differentiating between them. The difficulty they have in correctly identifying them depends on how dark or light the colors are. Another form of color deficiency is blue-yellow. This is a rarer and more severe form of color vision loss than red-green since persons with blue-yellow deficiency frequently have red-green blindness too. In both cases, it is common for people with color vision deficiency to see neutral or gray areas where a particular color should appear.