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.