The changes in colour in five identical sets of 25 light-sensitive pigments and other materials, exposed to light under selected neutral density filters, were measured. It was found that in the majority of cases the amount of damage, measured as colour change, caused by a given light exposure in lux-hours was independent of the level of illumination. These findings demonstrate that, for many light sensitive materials, the reciprocity principle is followed and that limiting the overall dose of light to which an object is exposed during a period on display can be an appropriate method of controlling deterioration. -- ICCROM