Approximately 8% of men and 1% of women have a form of colour vision deficiency, which can be genetic or acquired through illness, accident or poisoning. It is often a life-long condition, though deficiencies which are acquired may fluctuate.
Colour vision deficiency is most common in industrial nations, with the most common form being inherited red-green weakness. The deficiency is caused by a single gene, and it is males who are more prone to the condition.
There are different types of colour vision deficiency and these can be inherited or acquired. Rods and cones are the receptors in the back of the eye. Rods respond to brightness and cones respond to colour, therefore colour blindness is often caused when there is a problem with the cones. There are three types of cone receptors: red, blue and green.
Dichromatism
Dichromatism occurs when one of the three cone receptors is missing. There are three forms of dichromatism: protanopia, deuteranopia and tritanopia:
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Protanopia is the absence of red sensitivity resulting in confusion between red, green, brown and some purple hues.
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Deuteranopia is the absence of green sensitivity, causing confusion between red, green, blue and purple.
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Tritanopia is the absence of blue sensitivity, therefore blue and green hues cannot be discriminated. This condition is rare.
Anomalous trichromatism
Anomalous trichromatism occurs when there is altered sensitivity in one of the three cone receptors as opposed to the absence of cone receptors. There are three forms: protanomaly, deuteranomaly and tritanomaly:
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Protanomaly is a red sensitivity.
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Deuteranomaly is a green sensitivity. This deficiency is the most common.
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Tritanomaly is a blue sensitivity. This condition is rare.
Achromatopsia
Achromatopsia (or monochromatism), is the inability to distinguish any colour.
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Rod monochromatism is the absence of cone receptors, which results in perception of black, white and greyscale only.
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Cone monochromatism is very rare. This is possibly a problem in the brain rather than the eye as the cones appear normal.
For more information about how to create images and documents which are accessible to people with colour vision deficiency, see the page about Colour contrast for guidelines in using colour effectively, Testing for colour accessibility for tools which check colour contrast and colour ratio online, Tips for effective colour use for general advice, and What is colour?
Visit the Eye conditions page to find out about other sight conditions.