Visual colour deficiencies

Colour deficiencies include colour blindness, a condition of incomplete colour vision, of which the most common type is Daltonism, the inability to distinguish between red and green. Colour blindness may be genetically determined (innate) in which case there is a sex linkage, which explains why men have about 16 times more chance to be afflicted or acquired by traumatic or toxic damage to the optic sensory path. Colour blindness can result in accidents if people are unable to identify the conventional identification colours used on electric cables, control buttons on machines, gas cylinders and various types of transport - railways, automobiles, aircraft and shipping. Many countries have changed the colour codes for domestic electrical wiring (from red/black/green to say brown/blue/green-yellow stripe) to make them safe for colour blind people. There is no effective substitute for high-intensity red/green lights used for long-distance signalling, and there is no alternative but to exclude colour deficient people from key occupations requiring such discrimination. In addition, colour blind persons may have difficulties (but not necessarily so) in paint or colour mixing, geology, chemistry, microscopic work or in the operation of nuclear reactors.

8% of men and 0.5% of women suffer from dichromatism, in which vision is based two primary colours rather than the normal three. Only about 3% of men have difficulty distinguishing the bright reds and greens of traffic lights, and only one percent cannot see red lights at all after dark.