Cite as:
McCann J J, 2006, "Do we discount or ignore illumination?" Perception 35 ECVP Abstract Supplement

Do we discount or ignore illumination?

J J McCann

Changes in the spectral composition of illuminants produce small, but consistent, departures from perfect colour constancy. These departures from perfect constancy provide a signature of the mechanism. We used a constant set of coloured papers in 27 different spectral illuminants. [All possible combinations of 3 intensities (1, 2, and 4 LEDs) for wavelengths 455, 530, and 625 nm]. Observers matched a gray, a purple, and an orange paper in each illuminant to chips in a Munsell book in constant spectral illumination. If we discount illumination, we need first to identify it. Departures from perfect constancy are held to be incomplete adaptations to each illuminant. The pattern in colour space of these departures should follow the pattern of illuminants. If, instead, we ignore the illumination and build the colour appearance from spatial comparisons (ratios), then changes of illumination affect equally both the numerator and the denominator of these spatial ratios. The overlap of cone spectral sensitivities creates chromatic crosstalk, which, in turn, affects spatial ratios for colourful papers, but not for grays. If we ignore illumination, then the signature of departures from constancy should vary with the reflectances of the papers, not the illuminants. Observer matches confirm the second, spatial-reflectance, hypothesis.

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