Chromatic illumination discrimination ability reveals that human colour constancy is optimised for blue daylight illuminations

Abstract

The phenomenon of colour constancy in human visual perception keeps surface colours constant, despite changes in their reflected light due to changing illumination. Although colour constancy has evolved under a constrained subset of illuminations, it is unknown whether its underlying mechanisms, thought to involve multiple components from retina to cortex, are optimised for particular environmental variations. Here we demonstrate a new method for investigating colour constancy using illumination matching in real scenes which, unlike previous methods using surface matching and simulated scenes, allows testing of multiple, real illuminations. We use real scenes consisting of solid familiar or unfamiliar objects against uniform or variegated backgrounds and compare discrimination performance for typical illuminations from the daylight chromaticity locus (approximately blue-yellow) and atypical spectra from an orthogonal locus (approximately red-green, at correlated colour temperature 6700 K), all produced in real time by a 10-channel LED illuminator. We find that discrimination of illumination changes is poorer along the daylight locus than the atypical locus, and is poorest particularly for bluer illumination changes, demonstrating conversely that surface colour constancy is best for blue daylight illuminations. Illumination discrimination is also enhanced, and therefore colour constancy diminished, for uniform backgrounds, irrespective of the object type. These results are not explained by statistical properties of the scene signal changes at the retinal level. We conclude that high-level mechanisms of colour constancy are biased for the blue daylight illuminations and variegated backgrounds to which the human visual system has typically been exposed.

Figure 1. Photographs of the illuminator equipment and the scene backgrounds, with a plot of the chromaticity coordinates of illuminations used in the experiment.

A. Photograph of illuminator and the viewing box (with front wall removed) under extreme blue illumination, with fake pear, banana and chromatically matched novel objects. B. The Mondrian background used for the variegated scene condition, under D67 illumination. C. The grey background used for the grey scene condition. D. Chromaticities of generated metamers atop daylight measurements taken and digitised from Hernandez-Andres et al. , in CIE 1931 colour space; green markers show chromaticities of Ugandan forest canopy illuminations measured by Sumner and Mollon .

Figure 2. Mean discrimination accuracy for various conditions.

A. Mean discrimination accuracy for illuminations by their chromatic direction, for conditions using the grey or Mondrian background; for significant differences see main text. B. Mean accuracy across all conditions and participants for each chromatic direction as a function of perceptual distance from the target chromaticity ΔE_uv. C. Computed ΔE_uv mean thresholds at 75% accuracy for each chromatic direction, plotted in CIE u*v* colour space, with a spline forming the just-noticeable-difference discrimination contour (bold line) from D67 (black marker); just-noticeable-difference MacAdam ellipse boundary for D65 (dashed line) plotted around D67 point.

Figure 3. Histograms of changes in cone-opponent channel excitations of 95 surfaces between D67 and the bluer, redder, greener and yellower illuminations ±18ΔE_uv away in the Mondrian background condition, in modified MacLeod-Boynton (McB) coordinates.

B. Mean scene chromaticities under each comparison illumination for one target illumination (grey symbol), for the daylight (blue symbols) and orthogonal (green symbols) loci, in CIE 1931 xy chromaticity coordinates. Illumination chromaticities are also shown (black symbols). Left: grey background condition. Right: Mondrian condition. Note that targets (lighter markers) are asymmetrically placed with respect to the crossing position.