Effects of Light Exposure
Light is a small part of the electromagnetic spectrum to which the eye is sensitive. Its presence is essential for the most rewarding perception of almost all art. Yet exposure to light can result in cumulative and permanent damage to light-sensitive objects. Low light levels over a long period of time can cause equal or even greater degradation as intense light for a short period. Damage occurs because light is radiant energy. This energy causes irreversible change, either through radiant heating or photochemical action.
Radiant heat may create a rise in temperature that causes a reaction on the surface of an object, such as cracking, lifting, and change in color. Photochemical action is a chemical change at the molecular level and is the more profound transformation resulting from the exposure to light. The most potent sources of these destructive energies come from just beyond the limits of visible light—ultraviolet (UV) and infrared (IR) light (fig. 1). The longer wavelength, infrared, is a common source of radiant destruction. Ultraviolet rays, with their shorter wavelength, can be a significant basis for photochemical destruction. These wavelengths should be filtered and eliminated as much as possible to prevent damage. Visible light is essential for the perception and appreciation of art, but it, too, contributes to degradation and damage of objects. Irreversible damage caused by light can include color fading, yellowing, and embrittlement. Since all damage is cumulative and irreversible, the duration and intensity of light exposure should be carefully monitored and limited.
At the National Gallery, we use an assortment of tools, techniques, and materials to assist us in limiting and controlling light exposure to art. For monitoring, we use several types of light meters that measure both foot-candles (a measure of light intensity) and microwatts per lumen (a measure of visible light emitted). We also employ several types of films and filters that significantly eliminate the UV part of the spectrum. We pay close attention to the types of illumination used for the display of art and work closely with the Gallery’s lighting department to minimize light exposure without detracting from the visual enjoyment of the works.
For additional resources and further information, including tips about caring for your own collection, please refer to the sources below.
American Institute for Conservation of Historic and Artistic Work
Library of Congress
Smithsonian Museum Conservation Institute
Canadian Conservation Institute
Rea, Mark Stanley, ed. The IESNA Lighting Handbook. New York: Illuminating Engineering Society of North America, 2000.
National Committee to Save America’s Cultural Collections. Caring for Your Collections. New York: H. N. Abrams, 1992.
Long, James S. Caring for Your Family Treasures. New York: Heritage Preservation and H. N. Abrams, 2000
Temperature and Humidity