Light and display time

The energy in light fades colourants and makes materials yellow and brittle, and this damage is cumulative and irreversible. Museums therefore have to balance letting people see against preservation. For light-sensitive materials such as works on paper and textiles, light levels are generally kept as low as possible, for example 50 lux; for more light-resistant materials such as oil paintings and bone, 200 lux is a common guideline; stone, metal, and glass are barely affected by visible light. Ultraviolet is not needed for viewing, so galleries shut out daylight or add UV-filtering materials. The other approach is to limit time: rotating displays, or using timer switches and motion sensors to cut the hours of lighting. At the National Museum of History, painting and calligraphy on display are limited to eight hours of light a day and ninety days in total; in gallery 302 of the permanent display, Zhang Daqian's Lotus under the Moon and Sanyu's Lotus sit behind covers that open for only half an hour each morning and afternoon, so they can stay on view for the long term with less of the handling risk that frequent changes bring.

Notice how much dimmer the painting and calligraphy galleries are than those for objects; check the opening times first and be in gallery 302 for the half hour when the covers open.

Why is this gallery so dim? If a work could be seen for only half an hour a day, how would you look at it?