Abstract
Little is known about human sensitivity to changes in illumination spectra over time. Illumination changes are usually represented as simplified stimuli with non-natural abrupt changes, and the few studies using uninterrupted naturalistic changes (e.g. Kong et al., 2019; Linnell & Foster, 1996) are not specifically concerned with measuring perceptual sensitivity to these. Yet temporal changes in illumination are ubiquitous; natural light, in particular, varies in color temperature and irradiance through the day. Is human vision tuned to these changes? Here we aimed to determine the minimum detectable velocity of chromaticity change of daylight metamers in a naturalistic immersive environment.
The main stimulus was a continuous change in global illumination chromaticity along the daylight locus in warmer or cooler directions, away from a base light to which the observer first adapted. Four base lights were tested (CCT: 13000 K, 6500 K, 4160 K, and 2000 K). All lights were generated by spectrally tunable overhead lamps as smoothest-possible metamers of the desired chromaticities. The duration of the change stimulus was fixed at 10 s while its magnitude varied across trials for threshold estimation.
Results: For 22 participants, mean change detection thresholds range from 1.5 to 0.2 ∆E/s, depending on base chromaticity and chromatic direction of change. There was a significant main effect of base illumination (p<0.001). For the most extreme base lights (13000 K and 2000 K) sensitivity to changes towards neutral was significantly lower than the opposite direction. This result supports the notion that the visual system encodes a neutral-daylight illumination prior.
The slowest change detected by the observers was about 10 times faster than the fastest changes of real daylight. We conclude that, although people are generally aware of color temperature changes in daylight, they are unable to sense these directly, and more likely build awareness through higher-level cognitive processes.