What effect does exposure to light have on a museum’s collections?
Color fading and deterioration of materials
Light acts cumulatively—it is the total exposure over time that matters! Light is a source of damage to artifacts and its effects are irreversible. Fortunately, modern lighting technologies offer a variety of solutions to help control many of the damaging effects from light.
Light is a form of energy. Light generates heat. Artifact deterioration is a result of chemical reactions that occur when an energy source changes the chemical structure of the object’s surface. The amount of energy given off by a light source can be illustrated in the light spectrum. The electromagnetic spectrum is divided into wavelengths of energy, which range from low (radio waves) to high (gamma rays). The range of wavelengths from light sources (daylight and artificial light) can be divided into three regions: ultraviolet radiation or UV (300-400nm), visible radiation (400-760nm) and infrared radiation (over 760nm). In essence, the shorter the wavelength of the light source, the more damaging to the surface of an object.
Of special concern to museum collections is the high proportion of UV energy in normal daylight (natural sunlight shining through glass). This is why the amount of light from windows is minimized in museum galleries. Artificial light sources used in the galleries and storage areas, such as incandescent, halogen and fluorescent are generally filtered or diffused.
How can light be measured?Intensity of light is measured with a light meter using units of measurement known as “lux” or footcandles. One footcandle is equal to 10 lux units. There are also special light meters that measure levels of UV radiation. In general, light exposure levels recommended for museum objects are as indicated below. Extremely light sensitive organic materials such as paper or textiles are exhibited at very low light levels for only limited periods of time.
|200 lux ( 20 footcandles)||most ceramics, glass, and metals|
|150-200 lux (15-20 footcandles)||oil and tempera paintings, undyed leather, lacquer, wood, horn, bone, ivory, stone|
|50 lux (5 footcandles) or less||watercolor paintings, dyes, manuscripts, prints and drawings, vulnerable textiles, photographs|
What is done in the Museum to reduce or control exposure to damaging light?
- Timer switches in galleries or inside exhibition cases
- Computer controlled systems to turn lights off when the Museum is closed
- Curtains or light-diffusing materials placed on windows
- UV-filtering plastic sleeves or tubes placed over artificial light sources