Temporal dynamics of daylight perception: Detection thresholds.

Ruben Pastilha,Gaurav Gupta,Anya Hurlbert,Naomi Gross

doi:10.1167/jov.20.13.18

Abstract

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. The main stimulus was a continuous, monotonic change in global illumination chromaticity along the daylight locus in warmer (toward lower correlated color temperatures [CCTs]) or cooler directions, away from an adapting base light (CCT: 13,000 K, 6500 K, 4160 K, or 2000 K). All lights were generated by spectrally tunable overhead lamps as smoothest-possible metamers of the desired chromaticities. Mean detection thresholds (for 22 participants) for a fixed duration of 10 seconds ranged from 15 to 2 CIELUV ΔE units, depending significantly on base light CCT and with a significant interaction between CCT and direction of change. Cool changes become less noticeable for progressively warmer base lights and vice versa. For the two extreme base lights, sensitivity to changes toward neutral is significantly lower than for the opposite direction. The results suggest a “neutral bias” in illumination change discriminability, and that typical temporal changes in daylight chromaticity are likely to be below threshold detectability, at least where there are no concomitant overall illuminance changes. These factors may contribute to perceptual stability of natural scenes and color constancy.

Highlights

Illumination makes objects visible by giving them light to reflect
These manifest as changes in both overall irradiance and correlated color temperature (CCT) occurring from dawn to dusk in the downwelling light that forms the global illumination at ground level (Judd et al, 1964; Lee, 1994; Spitschan et al, 2016)
The results suggest that when the change in surface chromaticities is sufficiently slow, the change is attributed to a global illumination change, even when spatial uniformity is violated

Summary

Introduction

Illumination makes objects visible by giving them light to reflect. Yet to what extent is the illumination itself visible to the human visual system? Here, we examine visual perception of the illumination through assessing sensitivity to temporal changes in illumination chromaticity.The illumination in its complete form may be defined as a complex light field, varying in spectral irradiance over three-dimensional space, and resulting from a mixture of emissive light sources and indirect mutual surface reflections (Bloj, Kersten, & Hurlbert, 1999; Forsyth & Zisserman, 1989; Koenderink et al, 2007). The light emitted from traditional manmade sources tends to be largely static (Thorington, 1985), natural light is characteristically dynamic, changing in both spectral shape and overall irradiance over both short and long-time scales. This dynamic behavior results from temporal variations in the geometrical and spectral properties of both the direct light sources and indirect mutual reflections. The primary changes in natural light are the massive, gradual spectral variations caused by changes in solar elevation These manifest as changes in both overall irradiance and correlated color temperature (CCT) occurring from dawn to dusk in the downwelling light that forms the global illumination at ground level (Judd et al, 1964; Lee, 1994; Spitschan et al, 2016)

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Conclusion