ent in a domestic tungsten bulb, appear "white": the feeble "yellow" or "yellow green" chromaticity is swamped by the light intensity.
The joint effect of adaptation and illuminance level explains why the same incandescent lighting appears "white" to someone inside a room at night, but appears "yellow" when the light is viewed on a window shade from outside. Similarly, a television screen or computer monitor, which typically has a white point approximating 6500°K, appears to produce a balanced light to the viewer in a darkened room indoors, but glows with a distinctly bluish tint when viewed on a window screen from outside.
chromaticities of CIE illuminants when eye is adapted to noon daylight (D65)
However, even when all the illumination comes from a single light source with a constant relative spectral power distribution, and chromatic adaptation is therefore minimal, illuminance changes produce contrasting mood effects. We commonly experience these variations in invigorating effect of a bright sunny day (high CCT, high illuminance) in contrast to the gloomy, cold demeanor of a heavily overcast day (high CCT, low illuminance), or the intimate or comforting effect of candlelight or a night campfire (low CCT, low illuminance) in contrast to the motivating or clinical effect of indoor task illumination (low CCT, high illuminance).
A study by A. Kruithof (graph, right) suggests that an incandescent (tungsten or halogen) source matching illuminant A provides "pleasing" illumination between 100 to 300 lux; sources that are rated cooler (higher CCT) should be used at proportionately higher illuminances (above 300 lux). Recently Steven Weintraub and colleagues determined that museum and gallery displays of many different styles of paintings were perceived as most attractive under a CCT of about 3700°K across 50 to 2000 lux, with the "white" appearing CCT values rising from about 3500°K to 3900°K across this illuminance range. A similar study by SoLux found preferred CCT values of around 3600°K at illuminances between 200 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 