The measure of the colour of a light source by relating it to a theoretically perfect black body source of radiant energy, measured in degrees Kelvin (°K).
For instance a tungsten lamp rated at 3200°K means it emits light just like a ‘black body’ heated to this temperature. (Degrees K =°Celcius + 273).
The proportion of yellowish to bluish light in a light source: a bluer balance corresponds to a higher colour temperature ie: Daylight is around 5,500°K. 1
Unfortunately, when colour temperature is expressed on a Kelvin scale the relative blue content of light does not increase evenly with an increase in colour temperature. The Kelvin scale of colour temperature therefore has a non-linear relationship to both visual and photographic effect. This can be seen in a diagram as a crowding of the Kelvin scale on the left.
To overcome this difficulty a system of expressing the colour temperature on a Mired (pronounced ‘my-red’) scale is also used in photography and particularly in colour photography where corrections in colour quality are often needed.
1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston