| Excerpt |
Light is an agent of deterioration that can cause damage to museum objects. Light causes fading, darkening, yellowing, embrittlement, stiffening, and a host of other chemical and physical changes. Light is a form of energy that stimulates our sense of vision. This energy has both electrical and magnetic properties, so it known as electromagnetic radiation. The energy in light reacts with the molecules in objects causing physical and chemical changes. Because humans only need the visible portion of the spectrum to see, you can limit the amount of energy that contacts objects by excluding UV and IR radiation that reaches objects from light sources. All types of lighting in museums (daylight, fluorescent lamps, incandescent (tungsten), and tungsten-halogen lamps) emit varying degrees of UV radiation. This radiation (which has the most energy) is the most damaging to museum objects. Equipment, materials, and techniques now exist to block all UV. No UV should be allowed in museum exhibit and storage spaces. Control of ultraviolet light is relatively straightforward. The standard limit for UV for preservation is 75 µW/ l (international system). Any light source with a higher UV emission must be filtered. Control of visible light is more problematic. It is essential to understand that light damage is cumulative, and that lower levels of illumination will mean less damage over the long term. Another important concept in controlling visible light is the law of reciprocity. This says that limited exposure to a high-intensity light will produce the same amount of damage as long exposure to a low-intensity light. For example, exposure to 100 lux for 5 hours would cause the same amount of damage as exposure to 50 lux for 10 hours. For many years, generally accepted recommendations in the preservation community have limited visible light levels for light-sensitive materials (including paper) to 55 lux (5 footcandles) or less and for less sensitive materials to 165 lux (15 footcandles) or less. In recent years, however, there has been some debate about these recommendations. Some have argued the importance of aesthetic concerns: older visitors need more light to see exhibited objects well, and any visitor will find that more fine detail is apparent and colors appear brighter as light levels increase. In addition, the assumption that all paper objects are equally sensitive to light has been challenged. Scientists at the Canadian Conservation Institute (CCI) and others have begun to gather data on rates of light fading for specific media and colors in an effort to begin developing more specific guidelines based on the International Standards Organization (ISO) Blue Wool light fading standards. It is important to remember that even with such guidelines, some fading will occur. The goal is to achieve a workable compromise between exhibition and preservation. |
