A set of 24 pigment samples and 17 dyed textiles containing colorants that are moderately or extremely sensitive to light were exposed to light from four different sources: daylight fluorescent lamps, tungsten halogen lamps, fluorescent lamps similar to those used in museums and a simulated daylight source. Colour measurements made before and during exposure to light were used to calculate the exposure necessary to produce a colour change of 5, 10 or 20 colour difference (E₀₀) units under each light source. The results indicate that a given colour change requires a lower exposure from the two ‘daylight’ sources, which emit appreciable ultraviolet radiation, than from the other two sources. There is little difference in the exposure required to produce a particular colour change between the tungsten halogen and ‘museum’ fluorescent lamps. The results emphasise the damaging effect of ultraviolet radiation on a range of colorants. They also suggest that, providing the light levels are controlled to a similar level, neither of the two common museum illuminants (tungsten halogen and fluorescent) is more damaging than the other, and that the choice of lamps can therefore be made on the grounds of aesthetic, economic and energy efficiency criteria without affecting the deterioration of the object.