Light, Ultraviolet, and Infrared
Visible light is necessary for museum environments but is damaging to many collection materials. Standards developed in the preservation community recognize that light levels must be high enough to view objects for display, collection care, and research, but anything above that should be limited to prevent unnecessary damage. The most commonly seen type of light damage is fading of color, both natural pigmentation and dyes, and paints applied to items of material culture. But light damage also manifests in other visible forms such as yellowing, bleaching, and in some cases darkening. In addition, there are unseen chemical changes, such as cross-linking of coatings and the physical weakening and embrittlement of organic materials.
Light level or intensity is commonly measured in lux (lumens per square meter) or footcandles (fc). One footcandle is slightly more than 10 lux. Light damage, which is cumulative and irreversible, is a function of light intensity multiplied by the duration of exposure. Consequently, low lights left on 24 hours a day will cause the same damage as higher light levels over a shorter time. For example, items exhibited 24 hours a day with 50 lux of light will experience the same amount of light damage (50 x 24 = 1200 lux-hours) as artifacts exhibited for 6 hours a day at 200 lux (200 x 6 = 1200 lux-hours). Reducing damage can be accomplished by lowering light levels, or by reducing the amount of time that exhibits are lit.
Light and the Electromagnetic Spectrum
Visible light represents a very narrow portion of the spectrum of electromagnetic radiation, which also includes ultraviolet radiation (UV) at the short end, and infrared (IR) wavelengths at the long end. UV and IR do not directly impact our viewing experience, but like visible light, they can damage materials.
Ultraviolet (UV) radiation is measured in microwatts of ultraviolet radiation per lumen of visible light (µW/l) and can be measured using a handheld UV meter. The high energy of UV radiation is particularly damaging to collection materials. UV is not visible to the human eye and therefore removing it from museum lighting causes no change in appearance. Daylight is generally the strongest source of UV; fluorescent, metal halide and mercury vapor lights also emit UV radiation. Ideally, UV should be as close to zero µW/l as possible, and lamps emitting UV above 75 µW/l should be reduced.
When absorbed, infrared (IR) radiation causes a temperature rise. IR is also beyond the detection of the human eye. The effects of heat on collections are covered more specifically here, but it is important to recognize that light radiation acts as a catalyst in the oxidation of materials—particularly organic materials.
Controlling Light and UV Exposure
Different types, sources, and levels of light will be necessary for different parts of a museum environment. For example, storage environments require light levels high enough for research to be conducted, but there is no need for daylight, and lights should be off when not in use. If daylight can not be eliminated in some areas due to design priorities or other factors steps should be taken to minimize potential damage. For example, objects less susceptible to light damage could be chosen for exhibition.
Lighting within museum exhibition spaces may be divided into two general categories: ambient lighting of interior spaces overall, and targeted task lighting of objects, which may be inside or outside of display cases. Different types of light fixtures may be combined, or if unavoidable, a mixture of daylight and artificial light sources.
Methods for Reducing Total Light Exposure
- Window shades, films, and filters (for windows or light fixtures)
- Decreasing the number of light fixtures
- Decreasing the wattage of bulbs
- Using light dimmers, viewer-activated switches, or motion sensors
- Rotation of collections on display
Methods for Eliminating UV
- Excluding daylight
- Using UV-absorbing plastic on windows. This type of plastic can be purchased as thin films that can be adhered to the glass, or as thick sheets (e.g. Plexiglas) that can be used as secondary glazing on windows (or sometimes in place of existing glass). A large sheet that completely covers the entire glass can be hung and attached to the inside of the window frame.
- Using low UV output light fixtures
- Using UV filtering shields and sleeves (available as thin plastic sleeves or hard plastic tubes) over fluorescent and other lamps. Both should be properly sized to cover the entire light fixture and must be reaffixed when light bulbs are changed.
There is a dearth of research on exactly how long most UV-filtering plastics, films, and varnishes will maintain their efficacy, but information from suppliers suggests anywhere from 5–15 years. Research done by the Canadian Conservation Institute (CCI) suggests that 10 years should be considered a general lifespan for UV-filtering plastics and films. UV levels should be checked periodically to assess the efficacy of these materials.
Light and Collections
Organic Collections: Zoology, Botany, and Material Cultural
Vertebrate and invertebrate zoology collections, botanical collections, and collections of material culture constructed from plant and animal materials are highly susceptible to light damage. Fading, discoloration, loss of pigment, embrittlement, and chemical breakdown are real dangers with these organic materials. Controlling light levels should be a priority for collections in storage and on display.
Skin, fur and feathers in vertebrate collections should not be subjected to light levels above 50 lux (5 foot candles) for extended periods. Photooxidation in these materials is associated with fading of natural coloration, yellowing, embrittlement, loss of cohesive strength, and increased water sensitivity.
The pigmentation, sheen, and iridescence of entomology specimens are extremely light-sensitive. This is true even of specimens in amber, where UV in combination with other environmental factors plays a significant role in the deterioration. Excessive light exposure can lead to darkening, crazing, and fracturing making the examination of the insect inclusions difficult or impossible.
In fluid preserved specimens, radiation in the ultraviolet range enhances the degradation and discoloration of fluid and specimen by accelerating oxidation processes. Specimens should never be placed in direct sunlight and it should be recognized that glass (of either specimen jars or specimen cases) does not effectively filter UVA (315–400 nm), which is more damaging than visible light.
Many organic materials in collections of material culture are highly light sensitive including the animal materials listed above (fur, feather, and skin), organic dyes and pigments, some papers (especially those with high lignin content), textiles, natural resins, and some plastics. While other organic materials such as wood, basketry, ivory, bone, horn, and some painted surfaces are moderately sensitive. As many objects in collections are composed of multiple materials light levels must be set with the most sensitive components in mind.
Inorganic Collections: Paleontology, Geology, and Material Culture
While many mineral specimens are quite impervious to light damage, some can have interesting and complicated reactions to visible light, ultraviolet, and infrared. For example, when illuminated on display, realgar slowly converts to pararealgar due to light exposure and other non-ideal environmental conditions.
Most fossil specimens are not directly affected by visible light or ultraviolet (though specific mineral components can fade, change color, decompose, or change phase in response to high light levels), but light can affect adhesives and consolidants used in their preparation or preservation, as well as affecting housing materials. Sub-fossil materials, such as horn sheaths or complete mummified carcasses, are an exception and may remain quite sensitive to light.
Material culture constructed from inorganic materials such as stone, metal, ceramics, and glass are generally not light-sensitive. Some mineral-based pigments can be affected by light including thearsenical pigments, realgar, and orpiment. However, many inorganic materials may be part of a composite structure that includes light-sensitive organic materials, including surface coatings that may not always be immediately evident.
National Park Service Conserv-O-Gram: Choosing UV-filtering Window Films
Canadian Conservation Institute Notes offer practical advice about issues and questions related to the care, handling, and storage of cultural objects. Relevant Notes include: