Colour vision deficiency: what it means, and what it does not

Colour vision deficiency is usually inherited, usually present from birth, and usually mild. Roughly eight per cent of men and under one per cent of women have some form of it — the inheritance is X-linked, which is why it is so much more common in boys.

"Colour blind" is a misleading phrase. Almost nobody sees in grey. The common forms involve difficulty distinguishing certain colours from each other, particularly reds from greens and both from browns and oranges.

How it works

Normal colour vision depends on three types of cone cell in the retina, responding to long, medium and short wavelengths. The brain compares their outputs. If one type is absent or its sensitivity is shifted, certain pairs of colours produce similar responses and become hard to tell apart.

Red-green deficiency is by far the commonest. Blue-yellow deficiency is rare. Complete absence of colour vision is very rare and is accompanied by other visual problems.

Acquired colour deficiency is different

Colour vision can also be lost rather than inherited, and that is a clinical finding worth taking seriously. Optic nerve disease — including glaucoma and optic neuritis — can reduce red-green discrimination. Some retinal conditions affect blue-yellow. Certain medications, including some used long-term for tuberculosis and some for cardiac rhythm, can affect colour vision and are monitored for exactly that reason.

The distinguishing feature is change. Inherited deficiency has always been there. If colour perception has altered, or is different between the two eyes, that needs examining.

Children and school

A child with inherited colour deficiency will not report it, for the same reason a child with amblyopia does not: they have nothing to compare it to. It is typically found when a teacher notices unusual colour choices, or at a screening.

The practical consequence in school is real but manageable. Colour-coded teaching materials, graphs distinguished only by colour, and coloured chalk on a board can all disadvantage the child invisibly. A teacher who knows can label rather than colour-code, use pattern as well as hue, and avoid red on green. Nothing else is needed — the child does not need extra support, only materials that do not depend on a distinction they cannot make.

Finding out early also avoids a much harder conversation later.

Careers

Some occupations set colour vision standards, and they are enforced at entry. Commercial and military piloting, some maritime roles, parts of the armed forces, certain electrical and rail work, and some laboratory and clinical roles are the common examples. Requirements differ, and some depend on which test is used and how severe the deficiency is.

Discovering this at nineteen, having planned a career around it, is avoidable. A colour vision test in childhood is quick and settles the question.

Testing

The familiar dotted plates — Ishihara — screen effectively for red-green deficiency and are quick. Where more detail is needed, arrangement tests classify the type and grade the severity, which is what occupational standards are usually written against.

Online tests are unreliable. Screen calibration, ambient light and image compression all affect the result, in both directions.

On the tinted lenses

Lenses marketed as correcting colour blindness do not restore normal colour vision. They filter particular wavelengths, which exaggerates the difference between colours that were previously confusable — some people find the effect striking and useful, others find it distorts colours they could see correctly before. They do not satisfy occupational colour vision standards, and it is worth being clear about that before spending on them.

For most people, inherited colour deficiency is a minor feature of life that requires knowing about and little else. The value of testing is in the small number of cases where it matters a great deal.