We need light to be able to see the objects on display, but the energy contained in its electromagnetic spectrum, both visible and invisible, is powerful enough to damage many materials. Too much light falling on a painting will cause pigments to fade, as it will on other dyed material such as textiles. The invisible Ultra Violet (UV) short wavelengths in light contain enough energy to penetrate deep into materials and can break chemical bonds. This will start breaking down the cellulose of which paper is made, as well as that of textiles, lacquer and wood. At the other end of the spectrum the long invisible wavelengths called Infrared (IR) produce heat when hitting a surface. Overheating speeds up reactions which subsequently add to the rapidity of decay.
As explained in the section on deterioration, light damages objects. The light from the sun is especially powerful in all wavelengths so it needs to be stopped from shining directly on to objects. While all wavelengths of sunlight and artificial light cause deterioration, UV is the most damaging, so it has to be carefully controlled.
The Museum has to make a compromise between exhibiting the objects so that they can be viewed comfortably by visitors yet ensuring that the minimum of harm is done. There are internationally accepted light levels for different museum materials.
Specialist light bulbs and fluorescent tubes have been designed to emit lower levels of UV light and these are fitted in most of the galleries. Standard fluorescent tubes are covered with UV filter sleeves. We have now adopted LED lighting which provides low heat emission, low UV emission and low power consumption.
Daylight can be controlled by blinds and curtains and by applying filtering films to windows and roof lights. Some roof light blinds are operated automatically in response to sunlight.
Glass is a natural barrier for the reduction of UV light. The new showcases are laminated which doubles the effect and removes most UV light.
Particularly sensitive objects are provided with individual curtains or covers that can be opened briefly by the visitor for viewing (see image above). Where possible, galleries are kept in complete darkness when the museum is closed to the public.