Light (sometimes also referred to in the professional literature as radiation) is best thought of as a spectrum consisting of ultraviolet light (UV) at the short end, visible light and infrared at the long end.
UV light is measured in microwatts of UV radiation per lumen of visible light (µW/l). The high energy of UV radiation particularly damaging to artifacts. UV light 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 light fluorescent, metal halide and mercury vapor lights also emit UV radiation. UV light can be measured using a UV meter. Ideally UV light should be as close to zero as possible and light sources emitting UV measurements above 75 µW/l should be reduced.
Visible Light is, of course, necessary in museum environments. The standards that have evolved in the preservation community recognize that levels of light must be high enough to adequately view artifacts on display but anything more than that causes unnecessary damage and should be limited. Visible light levels are measured in lux (lumens per square meter) or footcandles (FC). One footcandle is slightly more than 10 lux. Light levels can be measured using a light meter.
Recommended practice in the museum/historic home community for acceptable level of light required for viewing collections on exhibit, based on experience and a number of studies is given below. The underlying logic behind these numbers is that any level of light in excess of the minimum amount necessary to adequately view an object on exhibition causes unjustifiable damage.
|Levels of Susceptibility to Light Damage & Types of Materials||Recommended Levels of Illuminance|
| Category 1: Most Susceptible
e.g. textiles, cotton, wool, silk and other natural fibers, most paper-based materials, watercolors, fugitive photographic images, most organic-based natural history specimens, fugitive dyes, watercolors, some minerals.
|50 lux (5 foot-candles)|
| Category 2: Susceptible
e.g. high quality paper with light stable inks such as carbon black, modern black and white gelatin silver photographs, textiles with stable dyes.
|100 lux (10 foot-candles)|
| Category 3: Moderately Susceptible
e.g., oil and tempera paintings, bone, ivory, wood finishes, leather, some plastics.
|200 lux (20 foot-candles)|
| Category 4: Least Susceptible
e.g. metal, stone, glass, ceramic, most minerals and inorganic natural history specimens.
|Dependent upon exhibition situation|
Infrared (IR) radiation, when absorbed, causes a rise in temperature. IR light is also beyond the detection of the human eye. The effects of heat on collections are covered more specifically in the section on incorrect temperatures but it is important to recognize that light radiation acts as a catalyst in the oxidation of materials – particularly organic artifacts.
Light damage, which is cumulative and, once sustained, irreversible, is a function of light intensity (in lux or footcandles) times length of exposure. Lights that may be set at low levels but are on 24 hours a day will cause the same amount of damage as higher light levels do in a shorter period of time. For example, artifacts exhibited with 50 lux of light which is kept on for 24 hours will receive the same amount of light damage (50 x 24 = 1200) as artifacts exhibited at 200 lux where the light is on for only 6 hours when the exhibition is open to the public (200 x 6 = 1200). Reducing the effect of light damage can therefore be done by lowering overall lighting levels as well as reducing the amount of time that exhibits are lit.
The most commonly considered type of light damage is fading of dyes or pigments but light damage also manifests in other visible forms such as changes in colored pigments or bleaching of wood artifacts (and in some cases darkening of some types of varnished wood). In addition there are unseen chemical changes such as cross-linking of varnishes, and the physical breakdown or embrittlement of organic materials such as cellulose fibers.
Controlling Light and UV Exposure
Different types, sources and levels of light will be necessary in different parts of a museum or historic home environment. For example, storage environments require light levels high enough for curatorial work to be conducted, but there is no need for daylight and lights should be off when not in use. In other areas daylight might create a desired effect (e.g. historical accuracy in a historic room) but steps should be taken to minimize its damaging effects and, in those particular spaces, objects less susceptible to light damage should be chosen for exhibition.
Lighting may be divided into two general categories: ambient lighting of the overall space and task lighting of the artifacts. Again, different types of light fixtures or, if absolutely necessary, a mixture of daylight and artificial light may be combined.
- Methods for reducing total light exposure include:
- Window shades, films and filters
- Decreasing the number of light fixtures
- Decreasing the wattage of bulbs
- Using light dimmers, viewer activated switches or motion sensors
- Rotation of artifacts on and off exhibit
- Methods for eliminating UV light include:
- Eliminating daylight.
- Using UV absorbing plastic on windows. This type of plastic can be purchased as thin films (acetate) that can be cut to shape and adhered to the glass or as thick sheets (e.g. Plexiglass) that can be used as a 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.
- Applying UV absorbent varnishes to window glass. This should only be done by an experienced contractor as the varnishes when applied poorly are ineffective and aesthetically undesirable.
- Using low UV output light fixtures.
- Using UV filtering shields and sleeves (available as thin plastic sleeves or hard plastic tubes) for fluorescent fixtures. Both should be properly sized to cover the entire light fixture and must be reaffixed when light bulbs are changed.
- White paint containing titanium dioxide can be applied to window surfaces. This method is not as effective as others but can be cost effective and simple in areas like storage where the aesthetics are less important concerns.
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 suggest anywhere from 5-15 years. Research done by the Canadian Conservation Institute (CCI) suggests that 10-15 years should be considered a general lifespan for UV filtering plastics and films.
- Canadian Conservation Institute (CCI) Notes 2/1 Ultraviolet Filters
- Colleen Boyle, Frank Preusser, and Terry Schaeffer, 2011. Aging Properties of Select UV-Blocking Window Films. WAAC Newsletter, 33:1