Describe in brief about Pseudoisoehromatic Plates test

Commonly known as Ishihara's plates, these reveal one pattern to the normal eye and another to the colour deficient. Coloured numbers are seen against a background of dots. The advantage of performing this test is that it is easy, quick and sensitive for screening. Disadvantage is that it does not test for tritanope, it 128 does not differentiate protans and deutrans and it does not grade the degree of colour defect. Hardy Rand Ritter (HRR) is the only pseudoisochromatic test that can diagnose all three types of colour defect