Image the world around you totally devoid of colour. Thankfully, the total absence of colour vision is an extremely rare condition. But millions of Americans do suffer from varying degrees of vision that is colour deficient.

Colour blindness or colour deficiency affects about eight percent of the entire population. Men make up the vast majority of people with colour deficient vision. Less than one percent of females are affected, but they certainly have an important role to play. Although women seldom suffer symptoms themselves, they pass colour deficient vision on to their male offspring. As a result, the condition gives the appearance of skipping generations. Colour deficient grandfathers pass the trait on to non-affected daughters who pass it on to grandsons.

To understand colour deficient vision it helps to know something about how our eyes defect colour. The back of the eye, or retina, contains sensors known as cones and rods. The cones come in three types; cones that pick up red, ones that defect green, and cones sensitive to blue light. Rods, on the other hand pick up brightness. They are the sensors of  black and white, or dark and light. The rods are much more sensitive to light than the cones which explains why we have poor colour perception in the dark.

Colour perception is the process of distinguishing varying ways in which points or homogeneous patches of light appear to a subject. The appearance may be described in terms of hue (red, orange, green, blue, yellow, and violet being the spectral colours); saturation (from pure, through pastels or brown, to unsaturated, hueless grays); and lightness or brightness (blacks, through grays, to white).

Colour blindness or deficiency results from the malfunction of one or more types of cones. The most common colour vision deficiency involves the cones than sense red and/or green. Red-green colour blindness results from a deficiency in the red receptors. The second most common type of colour blindness is yellow-blue, although this is quite rare compared to red-green colour blindness. Absolute colour blindness is even rarer and results when the colour receptors are missing entirely. Complete colour blindness affects fewer than three people in a million and is usually associated with a variety of other serious problems with vision.

Most people with colour deficient vision get along just fine, although they are usually banned from certain professions such as police work or piloting airplanes. They may also be teased when they show up at school or at work wearing colours that seem to clash. There are some advantages however. Since colour deficient people tend to look more at outlines than colours, they are not easily confused by camouflage. In fact, according to one source, colour blind people were often used in World War II spy planes to spot camouflaged German camps.

There is another problem, however, for some people with colour deficient vision. They may suffer from poor vision at the center of their visual field. This is because the most sensitive portion of the back of the eye contains a small area of normal retina that only has cones. It is here that our vision is most acute. If the cones don’t work as they should, an object may disappear from view when the colour deficient person looks at it directly, and reappear only when he uses his peripheral vision.

You might think that people who are red-green colour blind would have a terrible time driving a car, since they wouldn’t know whether to stop or go at a traffic signal. However, it turns out that traffic engineers have addressed this problem fairly well. You will notice that the coloured lenses on stop lights are very bright. Even people with colour deficiencies can often tell the difference. The green light is also really a shade of blue-green, that appears mostly blue to people with red-green colour blindness. The colour deficient person simply learns to “go” on blue. The position of the traffic light lenses is also a cue, with red on top and green at the bottom. Two of my uncles who have tested colour blindness, can attest to this fact.