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1 Colour in photography > Colour temperature

Colour temperature

As described previously, incandescent light sources have continuous spectra. The relative proportion of short and long wavelengths produced by these sources varies widely. This variation depends on the temperature of the source. As an example we can take two 100 W household lamps, one run at correct voltage and the other underrun at a fraction of correct voltage. The underrun lamp is not only less bright but gives more orangey illumination. Similarly, if you compare correctly run 500 and 100 W lamps, the weaker source gives out a higher proportion of red wavelengths and a lower proportion of blue ones. It looks more orange-red.

The spectral power distribution curve gives information on the colour of the source, as we saw earlier. In photography, however, the colour of a light source is also expressed by a unit of measurement, the colour temperature. This is based on the fact that continuous spectra, emitted by incandescent light sources of equal colour temperature, are the same, so they give the same results on film. Colour temperature is defned based on the black-body radiator or Planckian radiator, as follows: the colour temperature of a white light source is the temperature in kelvin (K) of a


  

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