Color vision is the ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect, emit, or transmit. Colors can be measured and quantified in various ways; indeed, a person's perception of colors is a subjective process whereby the brain responds to the stimuli that are produced when incoming light reacts with the several types of cone cells in the eye. In essence, different people see the same illuminated object or light source in different ways.
Color vision deficiency is the inability to distinguish certain shades of color. 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 the following colors:
People who are totally color blind, a condition called achromatopsia, can only see things as black and white or in shades of gray.
Color vision deficiency can range from mild to severe, depending on the cause. It affects both eyes if it is inherited and usually just one if it is caused by 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 part of the retina), each cone is sensitive to either red, green or blue light. The cones recognize these lights based on their wavelengths.
Normally, the pigments inside the cones register different colors and send that information through the optic nerve to the brain. This enables you to distinguish countless shades of color. But if the cones don't have one or more light-sensitive pigments, you will be unable to see one or more of the three primary colors.
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, which can depend on the darkness or lightness of the colors.
Another form of color deficiency is blue-yellow. This is a rarer and more severe form of color vision loss than red-green, because people with blue-yellow deficiency frequently have red-green blindness too.
In both cases, people with color vision deficiency often see neutral or gray areas where a particular color should appear.
Usually, color deficiency is an inherited condition caused by a common X-linked recessive gene, which is passed from a mother to her son. But disease or injury that damages the optic nerve or retina can also cause loss of color recognition. Some diseases that can cause color deficits are:
Other causes for color vision deficiency include:
Color deficiency can be diagnosed through a comprehensive eye examination. The patient is shown a series of specially designed pictures composed of colored dots, called pseudoisochromatic plates. The patient is asked to look for numbers among the various colored dots. Individuals with normal color vision see a number, while those with a deficiency do not see it. On some plates, a person with normal color vision sees one number, while a person with a deficiency sees a different number.
Pseudoisochromatic plate testing can determine if a color vision deficiency exists and the type of deficiency. However, additional testing may be needed to determine the exact nature and degree of color deficiency.
A person could have poor color vision and not know it. Quite often, people with red-green deficiency aren't aware of their problem because they've learned to see the "right" color. For example, tree leaves are green, so they call the color they see green.
Also, parents may not suspect their children have the condition until a situation causes confusion or misunderstanding. Early detection of color deficiency is vital since many learning materials rely heavily on color perception or color coding. That is one reason the American Optometric Association recommends that all children have a comprehensive optometric examination before they begin school.
There is no cure for inherited color deficiency. But if the cause is an illness or eye injury, treating these conditions may improve color vision.
Using special tinted eye glasses or wearing a red-tinted contact lens on one eye can increase some people's ability to differentiate between colors, though nothing can make them truly see the deficient color.
Most people with color vision deficiency find ways to work around the inability to see certain colors by:
Color vision deficiency can be frustrating and may limit participation in some occupations, but in most cases it is not a serious threat to vision. With time, patience and practice, people can adapt. Although in the very early stages, several gene therapies that have restored color vision in animal models are being developed for humans.