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The test consists of a number of coloured plates, called ishihara test plates, each of which contains a circle of dots appearing randomized in colour and size. Within the pattern are dots which form a number or shape clearly visible to those with normal colouur vision, and invisible, or difficult to see, to those with a red-green colour vision defect, or the other way around. The full test consists of 38 plates, but the existence of a deficiency is usually clear after a few plates. There is also the smaller test consisting only 14, 24 plates.