Duke University Law Library
Recently, an optometrist responded to my frustration over reduced night vision by saying "Light is our friend." That is certainly true in most circumstances, but it is not true for library and archival materials. Light is not a friend to paper and book bindings.
As a matter of fact, light causes a chemical reaction in paper and bindings that is not only destructive, but irreversible as well. Over time, light causes cloth covers to fade and pages to change color. Fading, in itself, may seem to be only an aesthetic change, but is actually the visible result of a process that will weaken and embrittle a book.
Are all light sources equally damaging?
In general, there are three main sources of light in libraries: sunlight or natural light, fluorescent light, and incandescent light. If presented in order of their potential to destroy library materials, the list would read the same. Sunlight is the most destructive, as it is brighter and contains more ultraviolet (UV) light. UV light, although invisible, strikes papers and bindings with greater force and more rapidly than visible light, therefore accelerating deterioration. Sunlight, in all forms, is damaging, but direct sunlight is extremely bad and should be avoided entirely.
Flouorescent lighting, because it emits some UV light, falls in second place. Incandescent lighting, although more slow to destroy, will also discolor and embrittle papers and bindings that are regularly exposed.
How do you know whether your collections are overexposed?
If you have ever seen what appears to be the imprint of a bookend on a book's cover, you have seen evidence of light damage. What you actually witnessed was a book cover that had faded around the bookend. Even if you have not seen something like this, chances are very good that your collections are being damaged by light. To avoid damage, preservation sources generally suggest that visible light levels not exceed 55 lux for sensitive materials (art work, exposed pages, etc.) and 165 lux for less-sensitive materials.1 In contrast to this, the recommended light level for a reading area is at least 300 lux and for a study area at least 700 lux. Clearly, this is an issue where preservation concerns must be balanced against user needs. This balancing act is not a consideration where UV light is concerned. UV light is not visible light, so it can be reduced without affecting vision. Preservationists recommend keeping UV radiation at less than 75 microwatts per lumen.2 Determining whether the lighting in your library exceeds these limits can be fairly simple but potentially costly. Light meters for visible light range between $100-$200, but UV light meters are much more expensive, usually costing over $1000. Whether or not a library wishes to purchase light monitoring equipment to precisely measure exposure levels, there are many things that can be done to reduce light damage.
What can be done to lessen the damage?
First, and foremost, never allow collections to be exposed to direct sunlight. Even indirect sunlight contains some UV radiation, but is preferable to direct sunlight. If it is impossible to move books away from windows or skylights, consider installing blinds or window film with UV protection. If window film is used, check the lifespan and replace as needed.
There are several things that can be done to protect books from artificial light sources. One simple and inexpensive way to increase protection is to choose bulbs with a lower wattage when fixtures are relamped. If the light source is fluorescent, consider shielding the bulbs with a UV filter. There are two types of filters readily available. One type wraps around the fluorescent tube, the other consists of sheets placed inside the fixture cover. Whichever type is used, make sure the relampers are aware of their presence so the filters are not inadvertently discarded. Fluorescent bulbs are available that are pre-coated with UV protective materials, but are rather expensive for general collections, costing as much as $35 each.
If shelving books on top stacks exposes them to nearby lights, try to avoid using the top shelves, or install canopies or covers that will provide a protective barrier. Books that are in closer proximity to the light source are also affected by any heat produced, placing them at greater risk. Libraries should consider installing timers on stack lighting so that lights automatically shut off when not in use. Timers may also be combined with motion detectors that switch on lights when a person approaches. Timers and motion detectors may be especially useful for areas where sensitive and/or valuable materials are kept.
Light Exposure & Displays
Be mindful of light exposure to library materials placed in display cases or exhibit areas. These spaces are usually well lighted to highlight displays, are often enclosed, and materials may sit closer to light sources than normally. In addition to filtering UV light and taking other precautions already mentioned, care should be taken not to keep items on display for lengthy periods of time. If books are displayed with open pages, it is advisable to turn the pages occasionally so that the same two pages are not constantly exposed. Light damage slide rules can be used to measure how light intensity and length of exposure in a particular display area will affect exhibited materials. The slide rules, created by the Canadian Conservation Institute, are inexpensive and available from many preservation supply companies.
Any exposure to light is harmful to library collections and should be controlled as much as possible. The effects are gradual, but permanent.
1 1 lux is the amount of light intensity needed to illuminate one square meter of spherical space.
2 For a description of what is meant by the term "lumen", see K. J. Macleod, Museum Lighting (Ottawa: Canadian Conservation Institute, April 1975, reprinted May 1978), 7.