Colour contrast thresholds in congenital colour defectives.

The influence of congenital colour defects on a clinical computer test for equiluminous colour discrimination is studied. Differences in relative spectral sensitivity and changes in colour contrast discrimination are two distinct manifestations of the abnormal genes responsible for congenital red-green defects. The very simple and rapid method of the heterochromatic flicker brightness test acts like an anomaloscope and can be used to distinguish protan and deuteran defectives. The depth of the congenital colour defect can be quantified by the colour contrast threshold measured in equiluminous conditions along a single red-green axis identical for all types of red-green colour defectives. Colour contrast thresholds in tritan colour axes are not influenced by congenital red-green defects and therefore they are of extreme clinical interest to detect and quantify acquired colour defects, even in the presence of a previously unknown congenital red-green defect.