Journal
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS IMAGE SCIENCE AND VISION
Volume 18, Issue 2, Pages 412-420Publisher
OPTICAL SOC AMER
DOI: 10.1364/JOSAA.18.000412
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As part of our ongoing research into the clear daytime sky's visible structure, we analyze over 1500 skylight spectra measured during a seven-month period in Granada, Spain. We use spectral radiances measured within 3 degrees fields of view (FOV's) to define colorimetric characteristics along four sky meridians: the solar meridian and three meridians at azimuths of 45 degrees, 90 degrees, and 315 degrees relative to it. The resulting clear-sky chromaticities in 44 different view directions (1) are close to but do not coincide with the CIE daylight locus, (2) form V-shaped meridional chromaticity curves along it las expected from theory), and (3) have correlated color temperatures (CCT's) ranging from 3800 K to infinity K. We also routinely observe that sky color and luminance are asymmetric about the solar meridian, usually perceptibly so. A principal-components analysis shows that three vectors are required for accurate clear-sky colorimetry, whereas six are needed for spectral analyses. (C) 2001 Optical Society of America OCIS codes: 010.1290, 330.1710, 330.1730.
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