 a source of abiding pleasure and fascination for landscape painters.
Lighting "Mood" Depends on Luminance. By focusing on the chromaticity of light we have ignored what is its most important attribute the intensity (illuminance or luminance) of the light source. The actual light color depends on light intensity, especially for chromaticities close to white, which means lights with the same CCT can have a very different visual impact depending on how bright they are.
The CCT does not literally stand for a specific color, only for a color tendency or color bias; the apparent color of a light will not necessarily match the "color" of its CCT. In addition, luminance affects our visual response. For objects at low to moderately high luminances, the blackbody color sequence seems to correspond to our experience of heated metals from the feeble red to yellow glow of heated iron. But metals heated above 1700°K appear to be a brilliant white, even though this "white" might be equivalent to an "orange" CCT of 2000°K or little more.
natural display of the
looking east during a Maine sunset
Solar light does not appear to be green, but either white or faintly yellow, for three reasons. First, our eyes easily adapt to the total daylight illumination as the standard for "white" light, and this adaptation is to a white point that resembles one of the CIE daylight illuminants, usually D65, that are slightly cooler or bluer than direct sunlight, which by contrast appears a pale amber or orange. (The effect is enhanced if a shaft of sunlight is observed within a darkened room.) Second, illuminating green, orange or red surface colors with more intense light appears to shift the color toward yellow, and to shift yellow surface colors toward white, as a result of changes in the proportional response of the S cones relative to the L and M cones at different luminance levels, and these shifts cause the illuminant to appear warm. Finally, the "white" appearance occurs for the same reason that very luminous hot metals, such as the filament 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 2