00 to 300 lux. Note that cooler source CCTs would be preferred at illuminances near daylight levels of illumination (above 30,000 lux).
To my knowledge these effects have not been explained. The illuminances used in these studies (up to 10,000 lux in Kruithof's study) span the range where mesopic rod intrusion would be a likely explanation, but if low light adaptation shifts the white point from the photopic to scotopic sensitivity peaks (from 555nm to 510nm) then low (reddish) CCTs should appear warmer rather than perfectly "white". It may be that exposure to the diurnal cycle during development causes the visual system to associate the white balance with average illuminance levels, so that chromatic adaptation anticipates "warm" (sunset) spectral distributions to be relatively dim. In any case, I believe the baseline perceptual "white" comparison should not be a form of blackbody radiation but a true equal energy illuminant.
These variations explain why different CCTs are accepted as the best white standard for different lighting or colorimetric applications. Lighting with a CCT of 3500°K or less is considered "warm" and is used in the subdued lighting of restaurants or bars, while lighting at 4000°K is considered an attractive "white" for kitchen or office lighting, which is usually above 200 lux. The range of daylight that appears as "white" does not go much below 3500°K, but a photoflood or high wattage incandescent bulb, with a CCT below 3500°K, can appear perfectly "white", especially at night or when it is the only light source.
Daylight and the Warm/Cool Contrast. We now have ample evidence and context to link the variations in landscape light to the geometry of color vision.
"preferred" light intensity for different illuminants
yellow shows zone of preferred intensities for each CCT; adapted from Kruithof (1941), Weintraub (2004)
Just as light/dark adaptation is a visual adaptation to diurnal variations in light intensity, the y/b opponent function appears to provide the primary chromatic adaptation to diurnal changes in the yellow/blue balance of daylight. The r/g opponent function provides adjustments in the relative balance of the L and M contribution to the Y component of the y/b function, which restores the perception of "pure white" illumination as light shifts farther into the "red" wavelengths.
In the diagram above, the arrow indicating the direction of "yellow" wavelengths (around 575 nm) shows that the y/b opponent function can easily compensate for the daylight shifts in chromaticity down to about 5000°K. But under extreme "blue" or "red" (late afternoon) daylight phases the r/g function is also involved. This is easier to see if the chromaticities of daylight CCTs (disregarding their typically low saturation and huge differences in luminance) are shown as hue angles on the CIECAM chromaticity plane.
color analogs to daylight spectra chromaticities
the hue of blackbody temperatures illustrated as spectral locations on the CIECAM a*b* plane
This diagram allows comparison of the color shifts in natural light with the traditionally defined warm/cool contrast: the match is quite good. It also suggests the relative contributions of the r/g and y/b opponent functions in chromatic adaptation. The y/b function makes the major adjustments around the average solar "white", while the r/g function tracks hue changes at lower temperatures. In video production, there are analogous Y/B and R/Y controls to adjust the image white balance; digital artists use green/magenta, red/cyan and yellow/blue controls that change the balance between the three complementary contrasts that define the secondary color wheel.
Warm hues (matched by CCTs below 5000°K) are smeared across the hue range from yellow to red; cadmium pigments represent this range very well. The daylight CCTs above 6500°K start at a teal blue and shift to a middle blue at 10,000°K; this range is represented by cobalt pigments.
The color of the clear sky (skylight) varies substantially by geographic latitude, altitude, season, humidity, distance from the zenit