What is Colour Vision Deficiency (CVD)?
CVD is a defect in ‘colour perception’ by the eyes. This condition is more common in men compared to women. About 8% of males (1 in 12 males) and 0.4 % of females (1 in 200 females) are affected by various types of colour vision deficiencies (CVD). The red-green colour defect is very common, where the person confuses with the spectrum of red and green colours. CVD can be inherited (the defective gene is passed on by the parents) or acquired (as a result of other diseases or an injury that damages the parts of eye).
People with colour vision deficiency (CVD) can only see a limited number of colours. This means mistakes can be made in colour identification and some colours are confused. People with significant colour defect confuse pale or dark colours. The range of colour confusions increases in low level of lighting and if the coloured areas are small.
What causes CVD?Colour perception by our eye is similar to a television. A television contains three electron guns (red, green and blue) through which an image with varied colours is created. Similarly, in the retina (light sensitive tissue) of our eyes, there are two types of cells - rods and cones, of which cones are responsible for colour vision. There are three types of cones (L-cone, M -cone and S-cone), each containing a specific pigment called opsin which is sensitive to a particular part of the light spectrum.
A defect (mutation or rearrangement of the gene) in any one or two of the gene producing these cones will result in the production an abnormal cone causing CVD in a person.
This mutation passes from one generation to another( i.e. a genetic condition which is inherited from parents to their children) known as ‘congenital’ colour vision deficiency. Most commonly the inheritance takes place from mother to her child. “In some cases, if a single pigment is missing, the eye might have problem seeing the specific colour (Colour Vision Deficiency). But if all the pigments are missing in the cones, the eye will not see any colour at all. This severe condition is known as ‘total colour blindness’ and it is very rare to occur.”
Normal human colour vision uses all three types of light cones correctly and is known as trichromacy. People with normal colour vision are known as trichromats.