Lighting of Library Materials
Why should librarians, archivists, exhibit designers, and curators care about lighting?
While exhibit goers and researchers need good lighting to see items in exhibits and research rooms, light deteriorates collections. Appropriate lighting requires striking a balance between the needs of the visitor who wishes to see the collection item or exhibit and the needs of the collection item being displayed to be protected from premature fading, embrittlement, yellowing, color shift, or darkening.
Collections items damaged by light can not be repaired or fixed by conservators. Light damage is permanent, irreversible, and cumulative, meaning that each exposure to light poses some damage that eventually causes significant change in the item including weakening, embrittlement, yellowing, darkening, color shift, and other issues depending upon the nature of the item exposed to light.
Since conservators can not repair light damage, they must instead prevent the damage to collections by working with exhibit designers to plan and minimize all light exposures, particularly those in exhibits and research rooms. Conservators set lighting levels to ensure that collections items receive minimal damage from lighting, while researchers and exhibit goers are still able to see the items. Providing surrogates or facsimiles, either microfilm or digital, prevents light damage. However, surrogates don't work for all uses.
Therefore, conservators and exhibit designers ensure that lighting system are not allowed to heat up holdings, cause deterioration, or to expose items to excessive light. Items that are extremely sensitive to lighting are either exhibited only as facsimiles (e.g., digital copies) or are exhibited or viewed only for short periods under controlled conditions, so as to extend their life expectancy.
What specific problems does lighting cause to collections?
The most common problems from light exposure are:
- Infrared radiation heats materials, leading to their accelerated aging and embrittlement, as well as their yellowing.
- Ultraviolet radiation causes materials to disintegrate or become weak, while also causing pigments or dyes to change color, and lignins in paper, as well as resins, starches, and glues to yellow or darken.
- Visible light fades the colors of collections items, as well as causing darkening and yellowing of some collections items, as well as color shifts in dyes and pigments.
What materials are most vulnerable to light damage?
When reviewing materials, keep in mind the reciprocity between duration and intensity of light as low lights for long periods and high lights for short periods may cause equivalent damage.
Exceptionally Light Sensitive Items: Avoid exhibiting exceptionally light sensitive materials such as salted paper prints (photographs), as well as calotypes, Talbotypes, and variants. Instead use copies for exhibits and research if possible.
Highly Light Sensitive Items, include:
- architectural reproduction processes
- dyed organic materials
- drawings and documents using ball point or felt tip pens or iron gall ink or colored pencils
- most photographic processes, including negatives, prints, slides, and direct positives
To avoid damage for all of these materials, display them only for short periods, swap them out or replace the originals frequently, and monitor them carefully.
Moderately Light Sensitive Items, such as:
- Enameled items
- Oil paintings with pigments such as alizarin (madder) lake and carmine
- Tempura paintings
- Undyed organic materials
- Wooden materials (e.g., boxes, printing blocks)
The appropriate lighting level must be fitted to the specific item.
Not Light Sensitive Materials, such as:
- Ceramics, but not photographs on ceramics
- Glass, but not photographs on glass or lime glass
- Metals, but not photographs on metal
- Oil paintings made with azurite, carbon black, ochres, iron oxide, ultramarine, terre verte, and umbers
- Stone, such as signature seals
These materials may be relatively little impacted by light, allowing them to be viewed under higher light levels under appropriate environmental conditions than the previous categories.
How do repositories manage the risks of light damage?
Conservators prevent light damage by limiting light exposures to a bare minimum by controlling the key factors upon which light damage depends including:
- Duration: the length of the exposure to light,
- Intensity: the strength or light level of the exposure, usually measured in lux or footcandles with a light meter,
- Wavelength: the wavelength of light to which the materials are exposed, whether natural light, UV light, or infrared light (Note: shorter wavelengths of light cause greater damage), and
- Sensitivity: the type of materials being exposed.
Increasing any of the above factors (i.e., exposure duration, light intensity, shorter wavelengths or more sensitive materials) increases the level of collections damage.
Conservators ensure that repositories do not exhibit items that are at the end of their ability to handle light exposure or highly light sensitive items without suitable protections. Most repositories keep exhibit lighting levels as low as possible. One idea is to not exceed 5 footcandles in most exhibit lighting. When exceptionally sensitive materials are displayed, do not exceed the most conservative suggested light levels, often 3 footcandles, or instead use visitor activated lighting to limit lighting duration.
Human eyes can adapt to viewing materials in low light situations, given a little time for the eye to adjust. The older the human eye, the greater the need for contrast to assist with viewing. A minimum of 30 lux (3 foot candles) is necessary for human color perception, although older visitors may prefer somewhat higher light levels. The key factor is to ensure there is a good contrast ratio for enhanced viewing. Always, lower light exposure is better for the collection items.
When reviewing materials for exhibits, keep in mind the reciprocity between duration and intensity of light as low lights for long periods and high lights for short periods may cause equivalent damage. So, the rule is to swap out sensitive materials in exhibits with other items and to keep light levels low. During exhibit planning, acceptable light exposure limits are set based upon an item's history and conditions. Schedules for swapping out items are determined for the life of the exhibit.
Conservators use a variety of tools to check light levels. Blue wool standards may be included in exhibit cases containing sensitive materials. Conservators monitor and check visible light levels throughout the exhibition to ensure they remain constant.
What other lighting factors pose risks to collections?
Light damage can also be made worse by exposure to other environmental factors such as pollutants, oxygen, very high or low relative humidity, high temperature, and particulates. Lighted collections materials absorb heat from lights such as reading lamps, spot lights, fluorescent ballasts, or any lamps in enclosed and unventilated spaces. This heat leads to increased chemical reactions and collections ageing, thus shortening the life expectancy of the archival and special collections. Photochemical actions lead to darkening and changing of colors and pigments, embrittlement of paper and other support materials, fading of media, fraying of fabrics, weakening of paper fibers, and yellowing of paper.
One commonly used formula is that for every 9-10 degrees Fahrenheit increase in temperature, library and archival collections deterioration doubles in speed. When temperature and relative humidity cycle widely, and particulates are present, collections face a worst-case scenario.
Low and stable temperatures, appropriate relative humidity, and good particulate filtering preserve collections best when you implement these recommendations in concert with light exposure limitation, light intensity control, and light monitoring. Many repositories use fiber optic lights with built in acrylic filters within contemporary cases to limit light exposure and intensity, while maximizing visibility. External lights are kept a minimum distance of 24 inches between lighting and collections when using fluorescent lights and 36 inches when using incandescent lights.
Exhibit lighting may even pose legal issues, as it has recently become the subject of lawsuits as loaning repositories have sued exhibiting repositories when loaned documents visibly faded during exhibitions. This risk may be managed by exhibiting copies for light sensitive items, or using very low light levels. Over time, additional lawsuits may encourage even more stringent control and limitations on exhibition lighting for repositories nationwide.
What sort of exhibit lighting is recommended?
Many repositories use fiber optic lighting with built in acrylic filters for new exhibit cases and major case renovations. When this is not enough light, they may also use filtered overhead lights that are located far from the object being exhibited to avoid heat damage to collections items. If fluorescent or incandescent lighting must be used in exhibit areas or for lighting cases, conservators ensure they are kept outside of the case and that they filter all light frequencies < 400 mn. If external case lighting is not possible, the lighting is kept in a separate chamber of the case with full venting, heat filters, and a heat dissipating fan.
Historical case lighting tends to be incandescent, which is currently being phased out of production as of 2016. Historical cabinet lighting often is compact fluorescent, while under shelf lighting is often tungsten, halogen, or metal halide. Often this historical lighting is hardwired, small, customizable in length, and both the electrical elements and the heat source are held outside the case. If electrical or heat sources are inside the exhibit case, it is a good idea to use these cases to display only copies and facsimiles.
What other challenges does exhibit lighting pose?
The four most common archival and special collections exhibition lighting challenges repositories face are:
- Two Dimensional Items, such as posters or documents, that are mounted on vertical surfaces for which we may use UV filtered spotlights, optical projectors, or low intensity wall washing lights.
- Exhibit Case Reflections or Glare, which we may solve by facing the case towards a wall or angling the glass towards the viewer,
- Heat Build-Up in Cases, which we solve by using fiber optic lighting, or exterior to the case lighting sources that are focused from high above, or by having air vents and fans in the case to cool it down, and
- Shadows in exhibit cases, which is solved by the use of diffusing filters across the top of the case glass or alternatively the use of dichroic reflector lamps and heat filters.