Colour-blindness (colour vision deficiency)
Colour vision deficiency is often referred to as colour blindness, and means a person cannot see some colours clearly and accurately, or distinguish between certain colours.
Colour vision deficiency is usually an inherited condition, and affects around 1 in 12 men. Colour vision problems are less common among women.
How does the eye normally see colours?
Think of your eye as a camera. The front of the eye contains a lens that focuses images on the inside of the back of the eye. This area, called the retina, is covered with special nerve cells that react to light.
These retinal nerve cells include the rods and cones. The rods and cones react to light because they contain pigments that change when light strikes them.
The cones are responsible for colour vision. There are several kinds of pigments present in three types of cone cells. Some cones react to short-wavelength light, others react to medium wavelengths and others react to longer wavelengths.
There is only one kind of pigment in the rods and it reacts the same way to any wavelength of light. The rods do not have anything to do with colour vision. However, they are very sensitive to light and allow us to see at night.
When the rods and all the types of cones are working together, the eye sees all possible colours. It’s rather like the way a painter can mix just a few colours together and make paint of every possible colour.
What is colour-blindness?
If there is a problem with the pigments in the cones, the eye will not see colours in the usual way. This is called colour deficiency or colour-blindness.
If just one pigment is missing the eye might have trouble seeing certain colours. Red-green colour-blindness, where red and green might look the same, is the most common form of colour-blindness, followed by blue-yellow colour-blindness.
In some eyes, none of the pigments are present in the cones, so the eye does not see colour at all. This most severe form is known as achromatopsia.
Does colour-blindness cause other health problems?
The kind of colour-blindness that is present at birth does not lead to additional vision loss or total blindness. But because the cone cells of the retina are also used to see fine details, people who are colour-blind tend to have vision that is less sharp. The rod cells also tend to be "overloaded" by bright light, so tinted eyeglasses often help colour-blind people to see better.
If you think you have a problem with colour vision, you should make an appointment with an eye specialist. They will be able to tell you whether you are seeing colours properly and what to do if you are not.