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Colour Temperature is a way of describing a colour-quality of light. It is a measure of how much light of different colours is present. High colour temperatures have a lot of blue light and low temperatures have more red light. Unfortunately, in common language, light with a lot of red has become known as warm lighting - the colour of fire, whereas blueish lighting is described as cold - the colour of icy water. This is the reverse of colour temperature; the longer, less energetic - and so "cooler" - wavelengths of visible light are at the red end of the spectrum, whereas shorter, higher temperature wavelengths are at the blue end.
In film photography, not matching the colour temperature of the film & process to the lighting of the subject can result in pictures looking, for example, overly red or yellow (daylight film used under indoor lighting), or too blue (indoor film used outside). Similar problems result in digital cameras when the white balance and other colour balance settings are preset for the "wrong" type of lighting - although most digital cameras avoid this problem by having some automatic adjustment of colour.
Typical values for colour temperature are approximately
|Blue sky||up to 18000K|
|Sun plus light from a blue sky||6500K|
|Electronic flash or blue flashbulbs||6000K|
|"Daylight" fluorescent lamps||4500K|
|"warm" fluorescent lighting|
(including some low-energy bulbs)
|Tungsten filament domestic lighting||2800K|
It is well-known that heating a metal object will cause it to glow, first red, then orange, through to bright bluish-white. Technically, the colour temperature is that temperature, measured on the absolute scale in degrees Kelvin, to which a black object would have to be heated to produce light of similar colour - or Spectral Power Density (or Spectral Power Distribution) - SPD.