Seeing and sensing temporal variations in natural daylight.

Natural illumination is a mixture of sunlight and skylight, modified by interactions with atmospheric particles and interreflections between physical surfaces. Unlike traditional artificial light sources, natural illumination is spectrally dynamic, changing over short and long timescales. Over the day, daylight's correlated color temperature typically ranges from cool (~12,000K) to warm (~2000K), following the well-defined daylight chromaticity locus.

Temporal dynamics of daylight perception: Detection thresholds.

Temporal changes in illumination are ubiquitous; natural light, for example, varies in color temperature and irradiance throughout the day. Yet little is known about human sensitivity to temporal changes in illumination spectra. Here, we aimed to determine the minimum detectable velocity of chromaticity change of daylight metamers in an immersive environment.

Neighboring chromaticity influences how white a surface looks.

To identify surface properties independently of the illumination the visual system must make assumptions about the statistics of scenes and their illumination. Are assumptions about the intensity of the illumination independent of assumptions about its chromaticity? To find out, we asked participants to judge whether test patches within three different sets of surrounding surfaces were white or grey. Two sets were matched in terms of their maximal luminance, their mean luminance and chromaticity, and the variability in their luminance and chromaticity, but differed in how luminance and chromaticity were associated: the highest luminance was either associated with colorful surfaces or with achromatic ones.

The colors of natural scenes benefit dichromats.

Dichromacy impairs color vision and impoverishes the discrimination of surface colors in natural scenes. Computational estimates based on hyperspectral imaging data from natural scenes suggest that dichromats can discriminate only about 10% of the colors discriminated by normal trichromats. These estimates, however, assume that the colors are equally frequent.