Light levels need to be measured when the exhibition is prepared. UV light meters will check radiation levels in an exhibit space, and data event loggers help determine visible light levels over an extended period of time. Blue wool standards cards 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.