With special thanks to George Sexton, IALD, one of the world's leading authorities and accomplished designers in the field of museum lighting, and Rogier van der Heide, IALD, of Arup Lighting and designer of current state-of-the-art museum projects.
Important reference material: IESNA RP-30-96, Recommended Practice for Museum and Art Gallery Lighting.
Light is a common cause of damage to paintings, drawings, books, artifacts, and virtually all types of historic and archival collections. It can cause paper to bleach, yellow, or darken, and it can weaken and embrittle the cellulose fibers that make up paper. It can also cause base materials and dyes used in documents, photographs, and art works to fade or change color. Paper, bindings, and media (inks, photographic emulsions, dyes, and pigments, and many other materials used to create words and images) are particularly sensitive to light, as are fibers, furs, and textiles. Most of us recognize fading as a form of light damage, but this is only a superficial indication of deterioration that extends to the physical and chemical structure of collections. Light damage is cumulative and irreversible.
Light provides energy, which causes the chemical reactions that produce deterioration. In photochemical deterioration, or photodegradation, light energy is absorbed by molecules within an object, which can then start many possible sequences of chemical reactions. Each molecule requires a unique minimum amount of energy to begin a chemical reaction with other molecules, called its activation energy. A common reaction is oxidation. Other reactions include photodissociation, in which molecules are broken into smaller pieces by photons, and the change of molecular shape, such as the denaturing of proteins. These reactions can be even more damaging if other environmental factors contribute; for example, for every 14-degree-Fahrenheit rise in temperature, the deterioration rate of paper (and likely other organic materials) doubles. Major contributing factors include humidity, surrounding atmosphere, environmental pollutants, and the age, condition, and current state of the object entering conservation.
Most designers know that ultraviolet light (UV) is destructive. Indeed, UV is the most damaging of all forms of light, and deserves principal attention. However, it is important to remember that all light causes damage, and UV, visible light, and infrared energy all need to be addressed as part of the equation.
Basic Lighting Principles
Aside from the design issues of visibility, appearance, artistic interpretation, and the like, there are two overriding principles of lighting design to address and manage the affects of light.
1. Control the Spectrum of Light
Think of each of the three main types of light--visible, UV, and infrared (IR)--as being sources of energy that can cause photodegradation. Only one of the three, visible light, is actually used for art display and curation. The other two--UV and IR--are not necessary for either task and can be gladly discarded. This is easier said than done, of course, and minimizing both is the key to successful design.UV gets our top attention, as it is the most destructive light energy. An easy way to assess the amount of UV present is to determine the percentage of UV to visible light. (See Table A.)
The higher the percentage of UV, the more filtering is required. The standard UV limit for preservation is 75 W/lumen. As a general rule, every light source should have a UV filter, and a perfect filter removes all radiation with waves shorter than 400 nanometers. For incandescent and HID lamps, glass filters are necessary to handle the lamp's heat; for daylight and fluorescent lamps, plastic sleeves and lenses can be used. Keep in mind though, any filter changes the color rendering (CRI) of the system, and the greater the filtering, the lower the CRI. For example, halogen light (CRI 100) through an Optivex dichroic filter will reduce the CRI to 95. And daylight, filtered through Low-E window or skylight glass, can have altered color, with CRI as low as 75-90 with some modern windows.
Don't forget IR radiation, either. There is precious little information about IR effects at this time, but practical experience suggests that the drying and baking impacts can be significant. Displays of especially sensitive materials will demand IR filtering. To minimize IR you can employ "cold mirror" lenses, "cool beam" PAR lamps, or dichroic MR16 lamps, all of which substantially reduce IR in display-lighting applications. In fact, the natural filtering effects of the IR coating on IR/HIR lamps reduces both IR and UV light, making these lamps ideal candidates for sensitive display applications.