Most people with colour blindness are born with it, but it can arise as a result of illness or medical treatments.
Men are more likely to be colour blind than women - approximately 8% of men are affected by colour blindness compared to only 0.4% of women.
About colour blindness
Colour vision
In your eyes there are two types of light-sensitive cells, called rods and cones. They are both found in the retina, the layer at the back of the eye which processes images. Rods allow you to see in dim light or at night, whereas cones allow you to see in detail and distinguish colours.
There are three types of cone cells: red cones, green cones and blue cones. Each cone has a different level of sensitivity to light. When you look at an object, light enters your eyes and stimulates the cone cells. Your brain then interprets the cone cells allowing you to see the colour of the object. The red, green and blue cones all work together to allow you to see the whole spectrum of colours, for example when the red and green cones are stimulated to a certain level you will see the colour yellow.
Colour blindness
Colour blindness occurs when one or more of the cone types are faulty or missing. This means that you will have difficulty seeing the cone colour that is missing or faulty. For example, if the red cone is missing you will be unable to see colours containing red as clearly.
There are several different types of colour blindness.
Dichromatism
Dichromatism is when one of the cones is missing. There are three types of dichromatic colour blindness:
Anomalous trichomatsis
Anomalous trichomatsis is when you have all three cones but there is a deficiency in one of them making you less sensitive to certain colours. Again, depending on which cone is missing, this will cause:
Often red cone (protan) and green cone (deutan) defects are collectively referred to as red-green colour blindness.
Monochromatism
Monochromatism is a rare type of colour blindness, where only one type or no functioning cone cells are present. This results in no colour vision - the world is seen in black, white and shades of grey.
The main symptom of colour blindness is a difficulty distinguishing colours or making mistakes when identifying them. In particular situations this may be made worse, for example:
Colour blindness is either inherited (you are born with it) or it is acquired as a result of a disease, treatment, exposure to chemicals or an injury.
Inherited colour blindness
Red-green colour blindness, the most common form, is passed down on the X chromosome. Chromosomes are structures that contain genes - these contain the blueprint for your body and are inherited from your parents. The X chromosome contains instructions for the development of your cone cells. For people with colour blindness, these instructions are wrong and cause cones to be missing or less sensitive.
The X chromosome is a sex chromosome; males have an X and a Y chromosome whereas females have two X chromosomes. For a male to be affected, colour blindness only has to appear on his single X chromosome, whereas for a female to inherit colour blindness it must be on both X chromosomes. This is why red-green colour blindness is far more common in men than women. If you are male and have a brother who is affected by red-green colour blindness, you have a 50 percent chance of being colour blind yourself.
Blue colour blindness affects both men and women equally. This is because it's passed down on a non-sex chromosome.
If you have inherited colour blindness, it will stay the same throughout your life - it won't get any better or any worse.
Acquired colour blindness
You can get colour blindness as a result of a disease, treatment, exposure to certain chemicals or a head injury.
Eye diseases like glaucoma, cataracts, age-related macular degeneration and optic nerve disease are all causes of acquired colour blindness. Other diseases, such as multiple sclerosis, liver disease and diabetes, can also cause colour blindness.
Colour blindness can be a side-effect of some therapeutic drugs, such as anti-inflammatory treatments for rheumatoid arthritis, the contraceptive pill and some medicines for heart conditions. However, this is quite rare.
In recent years, there has been some evidence to suggest that exposure to industrial chemicals, such as solvents, may cause colour blindness. Alcohol abuse and cigarettes have also been found to contribute.
The ageing process can be a factor. As you get older, you may experience subtle changes to your vision in general, including your colour vision.
Unlike inherited colour blindness, acquired colour blindness can vary over time. Symptoms may be mild and remain stable, or they can be severe and progress to more serious forms of colour blindness, such as monochromatism, very quickly. Often the cause of the colour blindness determines how severe your symptoms will be.
Colour blindness can be difficult to detect. Inherited colour blindness may not be picked up in children as they may be unaware that they have any problems with their colour vision. For people with acquired colour blindness, colours of an object are often named from memory rather than the colour they see.
It's important to get problems with colour vision diagnosed as quickly and at as young an age as possible. This is because it can make everyday living difficult.
To test your colour vision, your doctor or optician may use the Ishihara test. This test is widely used to diagnose red-green colour blindness; it's not suitable for testing blue colour blindness.
Colour plates will be put in front of you and you will be asked what number you can see on the plate. Each plate will contain a number or a coloured path that you will be asked to run your finger along. Some of the plates contain information that people with normal colour vision can see, while others will contain information that only people with colour blindness can see. If a certain amount of errors are made, colour blindness can be diagnosed. More tests by a specialist may be needed to establish which type of colour blindness you have.
Another test used to diagnose colour blindness is the City University Test. It detects colour blindness using four charts with vertical columns of coloured spots on them. You will be asked to identify the presence and position of different coloured spots in the columns. The number of errors you make indicates which type of colour blindness you have and the severity of the condition. This test can be used for both red-green and blue colour blindness.
There is currently no treatment for inherited colour blindness. Colour filters can be used in some situations to enhance the brightness between some colours; these are occasionally used in the workplace.
For acquired colour blindness, once the cause has been established and treated, vision may return to normal.
Living with colour blindness
Colour blindness can affect many everyday tasks; from re-wiring an electrical plug, to reading a map, to knowing when the traffic lights have changed colour. Being aware that you are colour blind will help prevent accidents and allow you to develop strategies to deal with any difficulties it causes.
Colour blindness may affect your career choices. For some professions, such as a pilot or coastguard, good colour vision is essential and you may be tested if you apply for a position. If you are concerned about your colour vision, it's important to get a diagnosis and talk to your employer to ensure that your workplace remains safe.
Further information
RNIB (Royal National Institute of Blind People)