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. Analysis of existing spectral irradiance databases and new measurements reveal a characteristic tripartite pattern: for chromaticity, the periods of fastest change occur in early morning and late evening at the lowest irradiances, with an interim period of relative stability. Illuminance changes are governed systematically by changes in solar elevation and tend to be fastest at the day's extremities, but unsystematic largely weather-related factors perturb this smooth trajectory. The human visual and nonvisual responses to light evolved under these changes. Psychophysical studies demonstrate that the threshold visibility of temporal changes in global illumination chromaticity depends on the chromatic direction of change and adapting chromaticity, with the lowest sensitivity occurring for changes towards neutral. Comparisons with the measured systematic changes in natural illumination suggests that the latter are too slow to be directly detected. A speculation is that visual mechanisms dampen sensitivity to the largest natural changes in illumination chromaticity in order to maintain perceptual stability of object color. Nonvisual mechanisms appear tuned to chromaticity changes at dawn and dusk, and hence are critical for syncing the circadian clock with environmental conditions. The latter might also feed long-term memory of illumination conditions as well as subjective experiences of illumination atmosphere.