People with colour vision deficiency are unable to see some colours clearly and accurately.
They may find it difficult to distinguish between different colours.
For example, reds, oranges, yellows, browns and greens may all appear to be a similar colour to someone with a red-green deficiency.
Other types of colour vision deficiency may make it difficult to identify pale or deep colours, particularly if the light is poor.
In many cases, completely different colours may appear to be the same – for example, people often confuse red with black.
Colour vision deficiency is often referred to as colour blindness. However, in true colour blindness, no colour can be seen at all. As this is very rare, colour vision deficiency is a more appropriate description.
Colour vision deficiency can vary in severity. Some people are unaware they have a colour deficiency until they have a colour vision test.
Read more about the symptoms of colour vision deficiency.
What causes colour vision deficiency?
In most cases, colour vision deficiency is an inherited condition (passed on from your parents). However, it can also sometimes develop as the result of a pre-existing health condition or as a side effect of a medicine.
Inherited colour vision deficiency occurs as the result of an abnormality in the retina (the light-sensitive layer of cells that line the back of the eye).
The retina is made of rod and cone cells, which respond to light and other neural cells that process the incoming signals.
There are three types of cone cells, and each type has a different sensitivity to light wavelengths.
One type of cone is most sensitive to light in the yellow-green part of the spectrum, one type is most sensitive to light in the green part of the spectrum, and one type is most sensitive to light in the violet part. They're known as long-wavelength (L), middle-wavelength (M) and short-wavelength (S) cones respectively.
As you look at an object, your brain compares the signals from the three cones, enabling you to see the colour of the object. If the L, M and S cones work together, you'll be able to see the full spectrum of colours. Normal colour perception is known as trichromacy.
If you have a colour vision deficiency, one or more cones will be missing or won't function normally, which means you'll be unable to see the full spectrum of colours.
Read more about the causes of colour vision deficiency.
Types of colour vision deficiency
The three main types of colour vision deficiency are often known as red-green colour deficiencies (two separate conditions) and blue-yellow colour deficiency (one condition).
Red-green colour deficiencies
Red-green colour deficiencies are the most common types of colour deficiency, affecting significantly more men than women (1 in 12 men have red-green colour vision deficiencies, but only 1 in 200 women).
The deficiencies arise from problems either with the L-cones or M-cones. People with these types of colour deficiencies confuse colours in the red-green spectrum.
A loss of M-cones is known as deuteranopia. If you're diagnosed with deuteranopia, it's likely you'll confuse colours in the red-yellow-green spectrum.
Deuteranomaly is a less severe form of red-green colour deficiency. If you're diagnosed with deuteranomaly, it's likely you'll have problems distinguishing different shades of the same colour in the red-yellow-green spectrum. It can vary in severity from mild to severe.
Protanopia is a loss of L-cones. Like deuteranopia, if you're diagnosed with protanopia it means you're likely to confuse colours in the red-yellow-green spectrum.
Protanomaly is a less severe form, where it's likely you'll have problems distinguishing different shades of the same colour in the red-yellow-green spectrum. It can vary in severity from mild to severe.
People with protanopia or protanomaly are likely to see red colours as darker than normal.
Blue-yellow colour deficiency
Blue-yellow colour deficiency is very rare, and usually associated with a deficit in the S-cones.
People with this type of colour deficiency have difficulty distinguishing between shades of blue and green. It's also known as tritanomaly or tritanopia.
Blue-yellow colour deficiency isn't linked to the sex chromosomes and occurs equally in men and women.
Recognising colour vision deficiency
Many people first become aware they have a colour vision deficiency when they have a problem identifying colours correctly. For example, a child may have difficulty naming colours or may struggle to read a coloured map or document.
It's important to identify a colour vision problem early. Your child's learning experience can be adapted if they are diagnosed at an early age and their teachers are made aware that they have the deficiency.
Colour vision testing isn't usually part of a standard NHS eye test, so some people with a colour vision deficiency may not realise they have it.
Read more about diagnosing colour vision deficiency.
Treating colour vision deficiency
There's currently no cure for inherited colour vision deficiency because it's not possible to repair or replace the cone cells in the retina.
However, studies being carried out in America may lead to a cure of some types of colour vision deficiency through gene therapy.
Colour vision deficiency doesn't cause any long-term health problems, so treatment isn't essential for you to be able to lead a normal, healthy life.
If your colour vision deficiency is caused by a pre-existing health condition or from taking a certain type of medication, it may be possible to improve your symptoms by treating the underlying condition or by using an alternative medication.
Most people with colour vision deficiency learn to adapt to the condition, and it's usually possible to find ways to compensate for your difficulty with colours. For example, it's possible to learn to recognise the position of the lights on a traffic light, rather than the different colours.
Read more about treating colour vision deficiency.
In most cases, having colour vision deficiency is unlikely to lead to long-term health problems.
However, if you have a red-green colour deficiency, it may be difficult for you to spot blood in body fluids, which can sometimes be an early sign of a medical condition.
Having colour vision deficiency could also affect your career choice. This is because certain jobs, such as pilots, train drivers, electricians and air traffic controllers, require accurate colour recognition.
If your child has colour vision deficiency, they may struggle at school unless the teacher is made aware of the problem. Many learning materials are colour coded and your child may have difficulty if their learning environment isn't adapted to their specific needs.
Read more about the potential complications of colour vision deficiency.