A white light source is often characterised in terms of its colour temperature, which is the temperature (units of Kelvin) of a Planckian black body source having the same chromaticity co-ordinates.

The sun is the natural standard for how we perceive colour and it is also the most obvious example of a Planckian black body source. The chromaticity co-ordinates of a black body source fall on the Planckian locus as plotted on the CIE 1931 Chromaticity Diagram. The Planckian locus is intersected by iso-lines, which are lines with a single colour temperature along their locus.

If the chromaticity co-ordinates of a white light source do not fall on the Planckian locus, it is characterised in terms of its correlated colour temperature (CCT), which is the closest colour to the colour of the white light source. This CCT value is applied to white LEDs since the colour bins extend above and below the Planckian locus.

Measurement of colour temperature for a white light source goes against convention in that a high colour temperature (e.g. 6000K to 10000K) is described as cool white, whereas a low colour temperature (e.g. 2500K to 3000K) is described as warm white.


See colour metrics for further detail.