can also be utilized to predict the extent to which materials will be damaged during exhibits. UV radiation must be eliminated to the extent it is physically possible; it is recommended that light with a wavelength below 400 nm (ultraviolet radiation) be limited to no more than 75 microwatts per lumen at 10 to 100 lux. Furthermore, exposure to natural light is undesirable because of its intensity and high UV content. When such exposure is unavoidable, preventative measures must be taken to control UV radiation, including the use of blinds, shades, curtains, UV filtering films, and UV-filtering panels in windows or cases. Artificial light sources are safer options for exhibition. Among these sources, incandescent lamps are most suitable because they emit little or no UV radiation. Fluorescent lamps, common in most institutions, may be used only when they produce a low UV output and when covered with plastic sleeves before exhibition. Though tungsten-halogen lamps are currently a favorite artificial lighting source, they still give off significant amounts of UV radiation; use these only with special UV filters and dimmers. Lights should be lowered or turned off completely when visitors are not in the exhibition space. Relative humidity (RH) The exhibition space's relative humidity (RH) should be set to a value between 35% and 50%. The maximum acceptable variation should be 5% on either side of this range. Seasonal changes of 5% are also allowed. The control of relative humidity is especially critical for vellum and parchment materials, which are extremely sensitive to changes in relative humidity and may contract violently and unevenly if displayed in too dry an environment. Temperature For preservation purposes, cooler temperatures are always recommended. The temperature of the display space should not exceed 72 °F. A lower temperature of down to 50 °F can be considered safe for a majority of objects. The maximum acceptable variation in this range is 5 °F, meaning that the temperature should not go above 77 °F and below 45 °F. As temperature and relative humidity are interdependent, temperature should be reasonably constant so that relative humidity can be maintained as well. Controlling the environment with 24-hour air conditioning and dehumidification is the most effective way of protecting an exhibition from serious fluctuations. Length of the exhibition One factor that influences how well materials will fare in an exhibition is the length of the show. The longer an item is exposed