People with colour blindness cannot tell the difference between certain colours, or they may not see colours at all. Most colour blindness is inherited as a genetic condition.
There are far more males who are colour blind than there are females. Between five and eight percent of males, but less than one percent of females, are colour blind.
The most common form of colour blindness is red-green colour blindness. The Ishihara Color Test is the test most often used to diagnose red-green colour deficiencies.
The test consists of a number of coloured plates, called Ishihara plates, each of which contain a circle of dots appearing randomized in colour and size. Within the pattern are dots which form a number visible to those with normal colour vision and invisible, or difficult to see, for those with a red-green color vision defect.
For example, a person with normal colour vision would see the number 74 below, while a person with a red-green color vision defect would see only spots.
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