The Ishihara plates provide a popular and effective method for screening for colour vision defects (Figure 3.4). The patient is presented with a series of plates on which are printed numerous coloured dots. The normal-sighted subject will see numbers on the majority of the plates, whereas the colour-defective patient will fail to see many of the numbers. The test is easy to do and will effectively screen out the more common red-green deficiency found in 8% of the male population. There are other tests available that will measure blue-green defects, for example, the City University test. Other tests, such as the Farnsworth 100 Hue test, are available for the more detailed analysis of colour vision.
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