Most colour vision deficiency (colour blindness) results from an inherited defect of the pigment in one or more types of the colour-sensitive (cone) cellsin the retina (lining of the back of the eye), or anabnormality in or reduced number of cone cells. The inherited defect is usually X-linked (transferred frommother to son), so colour vision deficiency mainlyoccurs in males: about 1 in 5 is affected to somedegree. In the most common form of colour blindness, the defect makes it difficult to distinguishreds from greens.
A series of specially designed coloured plates made up of dots of slightly different colour and size are used to detect colour vision deficiencies. Hidden within the dots of each plate is a number or letterpicked out in a colour that makes it visible to people with normal colour vision but not to those with colour vision deficiencies
The severity of the deficiency can be measured with more complex apparatus, which beams forth variable combinations of green (a mixture of yellow andblue) and red light. The degree of colour blind-nessis measured by the amount of adjustment needed to match the known proportions of red, yellowand blue with what the person sees.
Factors that can interfere with your test or the accuracy of the results include: