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Environmental Control
Temperature and Relative Humidity | Guidelines for Temperature and RH | Pollutants | Guidelines for Pollutant Levels | Light | Guidelines for Light Levels
Light
Light energy, in the form of waves, is absorbed by molecules within an object. This absorption of energy activates a variety of chemical reactions that have the potential to damage paper-based collections. This is termed photochemical deterioration.
Different types of light (visible, ultraviolet, infrared) fall at different points on the electromagnetic spectrum.
Photo courtesy Jet Propulsion Laboratory / NASA
(Obtained from NASA website: http:// deepimpact.jpl.nasa.gov/ science/ spectroscopy.html)
Visible light falls between 400 and 760 nanometers on the electromagnetic spectrum (violet to red). Wavelengths below 400 nanometers (ultraviolet, or UV, light) have a greater frequency than visible light, and bombard an object with more energy in a shorter time, accelerating photochemical deterioration. At the other end of the spectrum, infrared light (IR, above 760 nanometers) causes heating, thermal expansion, and spot desiccation. Fundamentally, although some types of light may be worse than others, all light is damaging, including visible light.
Sources of Light
Collections may be exposed to both natural and artificial light. Natural light is high in both UV and IR light, while artificial lighting varies.
There are two types of artificial light sources: incandescent bulbs and discharge bulbs. Traditional tungsten incandescent bulbs are high in IR but do not emit much UV. However, tungsten halogen incandescent bulbs (also called quartz iodine) may have high levels of UV. Discharge bulbs include fluorescent tubes (which are the type of lighting most frequently seen in collections storage) and metal halide bulbs. Metal halide bulbs emit high levels of UV, as do many fluorescent bulbs.
In fiber optic lighting, optical fibers carry the light from a single light source to various locations (this is often used for exhibit lighting). Fiber optic lighting may emit some IR light, but any UV in the original light source is removed as the light passes through the fibers.
Remember, the damage caused by light is cumulative and irreversible. Light follows the rule of reciprocity; thus, shorter exposure at a higher light level is equivalent to longer exposure at a lower light level.
