"White light" is commonly described by its color temperature . A traditional incandescent light source's color temperature is determined by comparing its hue with a theoretical, heated black-body radiator . The lamp's color temperature is the temperature in kelvin at which the heated black-body radiator matches the hue of the lamp.



Because it is the standard against which other light sources are compared, the color temperature of a black-body radiator of course matches its theoretical surface temperature in kelvin, using the temperature scale named after the 19th-century British physicist William Thomson, 1st Baron Kelvin . The color temperature of an incandescent light , which plots mathematically according to theory, may be referred to directly as the kelvin value of a black-body radiator's heat when emitting the same hue. (Despite the comparison, it should not be construed that the color temperature in kelvin refers to the thermal temperature of anything other than the black-body radiator.) Other light sources, such as fluorescent lamps , are assigned a mathematical approximation known as the correlated color temperature (CCT). Because such an approximation is not required for incandescent light, the CCT for an incandescent light is simply its unadjusted kelvin value derived from the comparison to a heated black-body radiator.

As the sun crosses the sky it may appear to be red, orange, yellow, white, or blue, depending on position. The changing colors of the sun and sky as the day passes also match colors produced by a black-body radiator at certain temperatures in kelvin.



Some common examples:

•  1200 K: a candle

•  2800 K: tungsten lamp (ordinary household bulb), sunrise and sunset

•  3000 K: studio lamps, photofloods,

•  5000 K: electronic flash, average daylight. A designation of D50 stands for "Daylight 5000K" and is the most common standard for professional light booths for photography , graphic arts , and other purposes.

•  6000 K: bright midday sun

•  7000 K: lightly overcast sky

•  8000 K: hazy sky

•  10,000 K: heavily overcast sky

"Color temperature" is sometimes used loosely to mean "white balance" or "white point". Notice that color temperature has only one degree of freedom , whereas white balance has two (R-Y and B-Y).