Anomaloscope colour matching (i.e., matching a spectral yellow with a mixture of red and green lights) and chromatic discrimination thresholds are the two most commonly used methods for assessing colour deficiency and loss of chromatic sensitivity. The purpose of this study was to examine the factors that cause variability in such measurements and to establish how performance on the anomaloscope correlates with the subject’s loss of chromatic sensitivity.

The CAD (Colour Assessment & Diagnosis) test was used to measure red-green and blue-yellow thresholds in 125 “normal” trichromats and 120 colour-deficient observers. In addition, the subjects investigated made Nagel matches and their colour vision was also assessed with a number of occupational colour vision tests.

The Nagel matches reveal a diversity of results that cannot be explained in terms of simple shifts in lmax. In order to account for these findings and some unusual, extreme matches, a model was developed to examine how cone photoreceptor density changes, pre-receptoral absorption differences, shifts in peak photoreceptor spectral responsivity and post-receptoral amplification of cone signals can affect the outcome of the yellow match.

Anomaloscope matches show poor correlation with measurements of colour discrimination sensitivity. Examination of model parameters reveals the limitations of anomaloscope matches and explains, at least in part, why the parameters of the yellow match are not a good measure of the subject’s ability to discriminate small colour differences.