Abstract
The achromatic point (surface chromaticity perceived as neutral) serves as a measure of both chromatic adaptation and colour constancy. Larger deviations of the achromatic point from the chromaticity of the adapting illumination indicate worse colour constancy and less complete adaptation. Previously reported deviations vary with adapting illumination chromaticity and experimental method (see e.g. Foster 2012). Here we obtain achromatic settings for a large range of adapting chromaticities in an immersive illumination setup. Participants sat in an (2 m3) enclosure with white walls, illuminated by four tuneable twelve-channel LED lamps, viewing a smart-phone OLED display (visible size approx. 8 degrees) embedded in a neutral cardboard frame and recessed to minimise incident illumination. Participants adjusted the display to appear neutral using four joystick button inputs, corresponding to the four cardinal colour directions in CIELAB (roughly blue, yellow, red, and green). Test illumination irradiance and display luminance were held at a constant mean level. An interstimulus mask of 30 seconds of equal-energy-white light preceded each 120 second test illumination adaptation period; during the following 5 trials, lasting approximately 240 seconds, the test illumination remained unchanged. At the start of each trial, the display was set to a random chromaticity within its gamut. Test lights were generated as smooth spectra, matching chromaticities in three sets: (1) even samples of the hue circle equidistant from D65 at 40 deltaEuv (6); (2) daylight locus values ranging from 2000K to 10000K (9); (3) extreme chromaticities (6). Results: Mean colour constancy indices ranged from ~0.3 to ~0.9 depending on test illumination chromaticity and individual participant, with lower values for more extreme chromaticities. Deviations in achromatic settings pointed towards the daylight locus for off-locus test lights, and towards neutral for on-locus lights. The results suggest a systematic bias towards the daylight locus in chromatic adaptation and colour constancy.
Acknowledgement: Supported by Huawei Device (Dongguan) Co., Ltd. and EC H2020-MSCA-ITN (DyViTo)