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Fig. 3. (A) Human-based u'v' chromaticities of daylight, sunset,
twilight and nocturnal irradiances. The upper starlight symbols for the Kitt
Peak and La Palma starlight data denote the chromaticities during a solar
maximum; the lower symbols denote the chromaticities during a solar minimum.
For comparison, the chromaticities of a 7° diameter patch of moonless sky
(zenith angle 45°) under thin clouds, clear skies and overcast conditions
are also shown (Höhn and
Büchtermann, 1973). The black line denotes sunset and
twilight data from North Carolina. Its symbols show data taken at solar
elevation intervals of about 2°. The colored circles next to Kitt Peak
starlight and `–11°' show the human-perceived colors at those two
chromaticity extremes. The Planckian locus shows the chromaticities of
blackbody radiators as a function of temperature. Data points for this locus
are every 500 K up to 5000 K, and every 1000 K up to 10000 K, after which each
point is labelled. (B) Deilephila-based relative quantum catches for
the data shown in A. The three corners depict illuminants that are absorbed by
one receptor only. The broken line shows the quantum catches of the spectral
colors, with points every 25 nm and numbers every 50 nm. Because 49 of the
civil twilight spectra and all 220 forest spectra were not taken at UV
wavelengths, their relative quantum catches could not be calculated.
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