Light causes permanent damage to library materials. This damage is both immediate and cumulative.
When we think of light damage, we most often think of fading. However, light damage can also include internal breakdown of library materials both physically and chemically. Light damage depends on 1) the type of light, 2) the intensity of the light, 3) the length of exposure time, and 4) how sensitive the materials are to light.
UV light is the most damaging to library collections, but visible light is also quite damaging.
UV levels are measured in milliwatts/lumen. UV levels above 75 should be avoided where at all possible. Collection areas should stay around 50 milliwatts/lumen or lower if possible.
The intensity of the light is based on the amount of radiation a light emits at one time. Intensity is usually measured in lux or foot-candles. Lux is the intensity of light over one square meter of space. 50 Lux/5 foot-candles is sufficient for visibility and a safe light level for the majority of items. Long-term light levels of about 10 foot-candles should be avoided.
Exposure time is dependent upon the intensity and types of light waves plus the sensitivity of the object itself. The higher the foot candles and lumens, the lower the exposure time for any library items. Different colorants in book cloth will be more sensitive to light than others. Remember that light damage is permanent and conservation cannot restore original book covers back to their original colors. Contact conservation if a particular library item is suffering extreme damage in regular lighting.
The four-flap boxes with mylar spines do not keep book spines safe from light damage. Most spines in our Special Area Studies collections have already suffered from light damage so this is not an issue and security of the item is considered more important. However, if a curator wishes to maintain the spine colorant of a book that has yet to experience light damage, please discuss other housing options with conservation.
Different object types, colorants, mediums, etc. have different sensitivities to light. Do not assume that one book bound in textile will have the same cumulative damage as another book bound in textile if they are different colors or different textile mediums. Different paper will also be affected by light differently. Brittle books, in particular, will suffer more under extreme light levels as they are already susceptible to chemical breakdown. White pages of books may darken with light damage, so they are also susceptible. Again, contact conservation if an item is undergoing more severe light damage than other items around it.
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