Colorimetric and Spectroradiometric Characteristics of Narrow-Field-of-View Clear Skylight in Granada, Spain
GRANADA UNIV (SPAIN)
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As part of our ongoing research into the clear daytime skys visible structure, we analyze over 1500 skylight spectra measured during a seven-month period in Granada, Spain. We use spectral radiances measured within 3 deg fields of view FOVs to define colorimetric characteristics along four sky meridians the solar meridian and three meridians at azimuths of 45 deg, 90 deg, and 315 deg 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 as expected from theory, and 3 have correlated color temperatures CCTs 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.