The term colour blindness is a common but misleading term that implies total loss of colour vision. In most cases the defect is usually only partial and the term “colour defective vision” would be more appropriate. The commonest defect is in the red/green part of the visual spectrum. Males are more commonly affected; about 8% of all males and 0.5% of females are affected to a varying degrees. There are other defects due to failure in the blue receptors, a total failure of all receptors and a failure of the other retinal receptor, the rod
All colours can be made by mixing the three primary colours; red, green and blue in varying amounts and the eye sees colours by detecting the amounts of these primary colours. This processing is done by special nerve endings in the retina of the eye. One type of nerve ending, the cone, has three special pigments which detect one colour, either red, green or blue. If one of these nerve types is impaired there is a colour defect.
Males are more affected because the genes that control the development of these colour sensitive nerve endings are carried on the x chromozone, which is the chromozone that determines male characteristics. Females are rarely affected, but may be carriers of the trait and up to one in seven are carriers.
Colour defects can also be acquired and a common cause in our community is cataracts. As the cataract becomes more dense it filters out all colours, but the bright reds, yellows, greens and blues are more affected. It has been said that the changing colour choices of many famous painters has been due to the affect cataracts have had on their colour vision. Other diseases affecting the optic nerve and the retina also affect colour vision but these are rare and often only affect one eye.
The defects are named after the Greek words for the three primary colours: Protos for red, Deutros for green and Tritos for blue. Someone who has a complete red defect is said to have Protanopia and if he only has partial defect he is said to have Protanomaly. The commonest defect involves the green receptors and it accounts for over half of the defects; 4% of all males have a partial defect and 1% have a complete defect. As the loss of colour vision is in the middle of the visual spectrum this defect causes the least awareness. The problem is the inability to distinguish red and green but they are sensitive to red light.
The next most common defect is due to failure in the red receptors, and it affects 1% of males. These people also confuse red and green but are not sensitive to red light.
There are other defects due to failure in the blue receptors, a total failure of all receptors and a failure of the other retinal receptor, the rod
Colours are constantly used to distinguish the difference between objects in everyday life. For many centuries colour has been an important part of many occupations and professions. Today the use of colour and colour coding has become much more widespread, a fact that undoubtedly influences the career choices of those with a colour vision deficiency. The use of colour extends to the work environment, and so it affects jobs and careers which require some degree of colour identification. These careers vary in the extent of reliance on colour vision, and so have been grouped into categories depending on if it is desirable or vital for operatives to have normal colour vision. This list can never be comprehensive and many jobs fall into several categories, as there are often different activities within a specific trade, profession or occupation.