http://www.getty.edu/conservation/our_projects/science/lighting/related_mats.html

Light is, of course, necessary in museums and everywhere else we work and display precious items. It does, however, damage collections by causing colours to fade and other material to degrade. When caring for collections we need to take measures, such as monitoring and controlling light levels, to minimise this damage, so that future generations of visitors can enjoy and appreciate them.

Generally speaking, all organic materials, that is, all things that originate from plants and animals are at greater risk of light damage than are inorganic materials such as stone, metal and ceramic objects.