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Color samples grossly exaggerate the chromatic contrast and drastically reduce relative luminance for purposes of visual illustration. To facilitate the comparison of color samples within modern color modelsa series of CIE standard illuminants are commonly used to define the white point in a chromaticity diagram.
Changing the white point in a chromaticity diagram changes the predicted and actual distribution of colors and the calculation of color matches metamers. Notice the fact inconvenient for color theory and sometimes confusing when using CCT values in lighting and photography: To avoid confusion, photographers instead use the mired Mdefined as: How well do correlated color temperatures describe the chromaticities of actual landscape illumination?
This is not surprising, because the solar spectrum is one of many natural light sources that resemble a blackbody radiator. But the most essential point is that the blackbody locus describes the entire sequence of landscape illumination across diurnal and seasonal cycles. Compared to an "unbiased" or equal energy illuminant, the blackbody locus and the path of all daylight spectra are shifted toward green by a small, consistent amount.
This occurs because the equal energy illuminant is perfectly flat, while the daylight spectra are typically peaked near the center or short wavelength end of the spectrum. Sunlight is not yellow, it is a very pale green! Can the correlated color temperature describe lights that do not exactly match a blackbody chromaticity?
The answer is yes, provided the color difference is not large. We simply determine the closest matching CCT to the light we want to describe. In a nonuniform color space, such as the CIE Yxy chromaticity diagram used above, these lines shown in green are not perpendicular to the blackbody locus but slanting to it.
The subtle chromaticity differences between the blackbody curve and the aggregate swath of daylight spectra can be compared in the daylight color series shown below. This represents the actual colors of light that mix with and alter landscape surface colors. These colors appear because "green" light shifts all "red" wavelengths toward yellow and all "blue" wavelengths toward blue green cerulean.
Ceruleans and very subtle greens are often visible in the sky close to the horizon, especially around sunset when the "violet" component of daylight is heavily filtered.
At these times the low sky can appear to shade from a grayish deep yellow into scarlet photo, right. It is intriguing that green and rose hues shown in the diagram immediately around the white point appear in the afterimage colors of specular reflections, and commonly in the glint of iridescence or refraction colors in water and high altitude clouds near the sun, and the rose colors of dawn.
The artist can only marvel that the solar light and atmosphere can weave such a narrow chromaticity path and yet create so many landscape color changes and light contrasts.
The color variations between dawn and sunset, or the sky seen from a seagoing ship and a mountain peak, or a desert sky before and after a cleansing cloudburst, or the light of spring and fall, or noon and twilight, or the colors of rainbow and iridescent sundog, are a source of abiding pleasure and fascination for landscape painters.
Lighting "Mood" Depends on Luminance. 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.
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 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.
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 illuminanceor 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 to lux; sources that are rated cooler higher CCT should be used at proportionately higher illuminances above lux.
Note that cooler source CCTs would be preferred at illuminances near daylight levels of illumination above 30, lux. To my knowledge these effects have not been explained. 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.Similarities between the biblical account of the flood and the flood of Gilgamesh: In the Bible, Noah is told to build an ark, a type of boat, out of cypress wood.
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