Colour temperature is strictly applicable to light sources which may be precisely matched by a full radiator.
The concept is extended to include sources which give light that can be closely - but not exactly - matched by a full radiator. The expression Correlated Colour Temperature (CCT) is used to describe the light from such sources. This is the temperature at which a full radiator produces a light that most nearly matches the light from the given source. CCT is calculated by determining the isotemperature line on which the colour of the light source is positioned. Isotemperature lines are straight lines for which all colours on the line appear visually equal. δuv is used to specify the deviation from the blackbody locus. The maximum deviation for δuv is set at ±0.02.
CCT is not suitable for measuring light sources which have narrow-band spectral emittance curves that do not approximate any blackbody curve (for example, LED).