Fig 1. Screen dumps of CAD test stimuli and template plot.

Section A shows stimulus colours that isolate the RG and YB axes. The mean threshold ellipse measured with respect to a daylight backgroud (CIE- (x, y): 0.305, 0.323) is also shown together with the RG and YB axes and the bespoke keypad employed to record the subject’s responses. Section B shows the standard CAD units for RG and YB colour vision (i.e., the ‘red’ and ‘blue’ arrows) which equal the minor and major axis of the mean threshold ellipse measured for 330 young, healthy normal trichromats [14]. The coloured sysmbols show typical thresholds measured in a subject with normal trichromatic colour vision. Section B, the grey shaded area shows the range of variation expected for young normal trichromats, the innermost and outermost ellipses corresponding to 2.5% and 97.5% confidence limits for normal population, respectively [18]. The dotted black ellipse represents the mean threshold values which define the young standard normal CAD observer. The corresponding mean thresholds are normalised to unity and all other CAD thresholds are expressed in these standard normal units (SNU). Upper normal CAD threshold limits have also been measured as a function of age in a separate study [20]. The smallest colour thresholds correspond to ~ 20 years of age and are followed by a gradual, linear increase of ~ 1% per year for RG and ~ 1.6% for YB, over the remaining life span. Each patient’s thresholds measured in this study were compared against the upper normal threshold limits for the corresponding age [20].