Light exposure of colorants with light of different spectral characteristics was investigated. Typical spectra of light sources used in accelerated light fading, filtered xenon arc and cool white fluorescence were area-normalized to the same lux level or the same total energy. The light absorbed by each colour channel was compared for the two cases. Using cool white fluorescene illumination at same lux level is about 50-65% as efficent as using indoor filtered xenon arc. The efficiency depends on the colour channel and is not very dependent on the exact dye spectrum and not very sensitive to the exact spectral composition of the light source. Comparisons with experimental data confirmed that the degradation of the colorant was dependent on the spectral power distribution of the light source and the absorption characteristics of colorants. For the cases where the experimental fading light source varies from the assumed display condition, the authors propose an approximation, that converts the exposure from one light source to another per colour channel. Two light sources are roughly equivalent as long as the area normalised spectra per colour channel are the same.