Colour vision deficiencies (colour blindness)

How do we see colour?

The light-sensitive layer inside the eye (the ‘retina’) has special cells that are responsible for converting light into signals that are sent to the brain. These cells are called ‘photoreceptors’. There are two types of photoreceptors.

• ‘Rods’ are the most common type. They are used in peripheral (side) vision and help us see when light levels are low
• ‘Cones’ are sensitive to different wavelengths of light, which allows us to see in colour.

When light enters the eye, it stimulates the photoreceptors, which convert it into signals that are processed in the brain.

What causes colour vision deficiencies?

There are three types of cone photoreceptors at the back of the eye. They are sensitive to different wavelengths of light in the visible spectrum (long, medium and short). Each cone photoreceptor corresponds to a different part of the visible light spectrum (red, green and blue). Colour vision deficiencies occur when one (or more) of the cone photoreceptors isn’t working properly.

• ‘Anomalous trichromacy’ is when one of the cone photoreceptors isn’t tuned to the correct wavelength of light. Small tuning errors tend to cause mild deficiencies, while large tuning errors lead to more severe deficiencies
• ‘Dichromacy’ is when only two types of cone cells are working properly
• ‘Achromatopsia’ happens when none of the cone photoreceptors are working. This condition is rare and may cause poor vision and total ‘colour blindness’
• ‘Monochromacy’ is when only one type of cone photoreceptor is working, making it difficult to see certain colours.

What are the different types of inherited colour vision deficiencies?

People with colour vision deficiencies typically see a reduced range of colours and may find it difficult to distinguish between certain colours. Red-green deficiencies are the most common, while inherited blue-yellow deficiencies are relatively rare.

There are three types of deficiencies, depending on which cone is affected.

Why are colour vision deficiencies more common in males?

Colour vision deficiency affects around one in 12 males, but only around one in 200 females. This is because colour vision deficiency is a genetic condition associated with mutations in genes on the X chromosome.

Females have two X chromosomes, whereas males have one X chromosome, which they inherit from their mother. A female needs to inherit the deficiency from both parents to have colour vision deficiency, whereas males will experience the deficiency if it’s present on their single X chromosome.

How do eye-care professionals test for colour vision deficiency?

There are various tests to detect and classify colour vision deficiency. The most common test involves using a book with a series of numbers made up of coloured dots, hidden within dots of a different colour. Optometrists sometimes check for colour vision deficiencies as part of a child’s first sight test, or if a patient is wanting to start a career which requires normal colour vision.

Will colour vision deficiencies get worse over time?

Most colour vision deficiencies are present from birth and don’t change over time. However, deficiencies that are caused by eye disease or medication can change, and you should speak to an eye specialist if you are concerned about this.

What can help if someone has a colour vision deficiency?

Colour vision deficiency is very common, especially in males, and is not generally considered to be a disability or an eye disease.

It can be helpful for teachers to know which students have a colour vision deficiency, because they may find certain tasks more difficult than their classmates. For examples, they may struggle to:

• Read coloured text from a whiteboard
• Choose which colours to use for colouring in (for example, for maps in geography lessons), or
• Detecting changes in colour (for example, colour changes in chemistry lessons).

Using additional non-colour cues, such as shape, size and brightness, will make it easier for these students to complete certain tasks.

Adults can also experience difficulties, such as when assessing whether food is cooked or fruit is ripe, or when choosing which clothes to wear. Certain careers, such as electricians, pilots, police or train drivers may not accept applicants with colour vision deficiencies, so it’s important to have this checked before applying for these careers.

There is currently no cure for colour vision deficiencies. Some people find that specially tinted contact lenses or glasses can make it easier to differentiate certain colours, but these aren’t usually allowed for use in careers with colour vision requirements. If you have any concerns or questions about colour vision deficiencies, you should talk to your optometrist.

Resources for your practice

Download our leaflet on Colour vision deficiencies. 

For more information on eye health and eye conditions, explore our For patients section.