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.

What the risks are?

• There are no risks associated with this test.

How to prepare for the test?

• No special preparation is required before having vision tests.
• If you or your child normally wears glasses, wear them during the test.
• Many medications may affect the results of vision tests. Be sure to tell your health professional about all the nonprescription and prescription medications you take.
• Talk to your health professional about any concerns you have regarding the need for vision tests, how they will be done, or what the results will indicate.

Why It Is Done?

• After an injury to the eye.
• To determine if you need glasses or contact lenses to improve your vision.
• To monitor an eye problem, such as diabetic retinopathy.
• In some circumstances, color vision testing is used to evaluate the function of the optic nerve.

What Affects the Test?

Factors that can interfere with your test or the accuracy of the results include:

• Failure to wear prescribed eyeglasses or contact lenses.
• Poor lighting.
• Your ability to stay alert and respond to questions.