Contributed talks II: The caerulean line: Its relationship to the red-green category boundary of deuteranomalous observers

At any one time and place, natural illuminants - skylight, sunlight, and their mixtures - lie on a straight line in a chromaticity diagram. We have termed this locus the 'caerulean line'. In the MacLeod-Boynton diagram, it has a negative slope and is not aligned with either of the ordinates of the diagram. Three properties of normal colour perception exhibit a provocative (though approximate) alignment with the caerulean line: 1. The phenomenological category boundary between reddish and greenish hues. 2. The orientation of the discrimination ellipse at a neutral chromaticity (e.g. Boynton et al 1983). 3. The locus of minimal thresholds when chromatic thresholds are measured along +45° lines in the chromaticity diagram. These coincidences may suggest that human colour vision has evolved so that redness and greenness indicate departures from the caerulean line in opposite directions. Anomalous trichromats offer a means to explore the causal relationships (if any) between the caerulean line and (1) to (3) above. In deuteranomalous observers with good discrimination, we have measured the locus of the red-green category boundary; and the discrimination ellipse centred on a metamer (for the deuteranomal) of Illuminant D65. For most anomalous observers, the discrimination ellipse at the neutral point is not aligned with the caerulean line. However, their phenomenological category boundary between reddish and greenish hues does fall close to the caerulean line.