Light of a particular wavelength has a particular spectral colour. White light is made up of a large number of spectral colours of different intensity. The more lines "fill" the spectrum, the better the colour rendering property of the lamp.

Spectral radiation distribution is characteristic of a particular type of lamp. Thermal radiators have a continuous spectrum; in the case of high-pressure and low-pressure discharge lamps, the spectrum depends on the composition of the additives used – e.g. the halogen in a metal halide lamp or the phosphor blend of a fluorescent lamp. Daylight also has a particular spectral radiation distribution; it is dominated by short-wave blues.