Applying the Ishihara test to a PC-based screening system

Aim: A commonly used method to examine the ability of the visual system to discriminate colours is based on isochromatic plates, such as the Ishihara plates. This article will introduce a computer-based method to determine red-green colour deficiencies based on presenting Ishihara plates using a CRT monitor. Method: The spectral emission of the reflected daylight of the plates was compared with the spectral emission of the plates presented on a CTR monitor. The screening efficiency of the Ishihara plates was assessed by testing 10 subjects with normal and 10 subjects with abnormal colour vision, using the Ishihara plates and using the computer-based approach. Results: It was experimentally shown that despite the differences between the spectral emission of the CRT monitor and the reflected daylight of the Ishihara plates, the computer-based method discriminates subjects with colour deficiencies from subjects without colour deficiencies. Conclusions: Using a CRT monitor for screening purposes, a reduced number of Ishihara plates is recommended to assess colour vision. The suggested method uses nine instead of 15 plates and a criteria of two not correctly detected plates to determine colour deficiency. (C) 1999 Elsevier Science B.V. All rights reserved.