Photochemical deterioration occurs when energy from light interacts with the molecules of organic materials. This process contributes to chemical reactions that cause color change or fading as well as reduction in the tensile strength of materials. While we typically use the term “light” to refer to visible electromagnetic radiation, materials are also exposed to ultraviolet (UV) radiation through everyday lighting sources and daylight streaming through uncoated windows.
Fading is one of the most significant and visible effects of light damage on materials. UV radiation reacts chemically with colorants, such as pigments and dyes, to cause loss of color. This damage can be seen in textiles (such as cloth book covers), dyed leather, and in printed or painted illustrations. Some film-based materials such as photographic negatives and slides are also vulnerable to color fading.
In addition, light can serve as a catalyst for activating oxidants in the atmosphere, which can lead to oxidation. Photo-oxidation of lignin, which is found in high levels in early wood-pulp paper, causes darkening or yellowing of the paper, often seen in old newspapers.
Photo-oxidation due to light exposure has caused the darkening of these historic newspapers, while acid hydrolysis has led to brittleness.
All light can damage materials, but generally speaking, the shorter the wavelength, the more damage can be done. Short wavelengths equal more light energy hitting an object, thus increasing the amount of energy interacting with the object. This means that UV radiation, with its short wavelengths, causes more damage than visible light, which has longer wavelengths. On the other end of the spectrum, objects feel infrared (IR) radiation as heat, which (as we’ve already seen) speeds up chemical reactions.
When Pepperdine renovated Payson Library, all new light bulbs that were installed into the new facility were light-emitting diodes (LEDs). Most types of LED lights do not emit (or emit in very small amounts) UV or IR wavelengths, the two most damaging kinds of light wavelengths. However, visible light can also activate many organic molecules, causing chemical change. Thus, motion sensors were installed in the preservation environment so that lights turn off automatically when no individuals are in the storage spaces. In addition, we use protective housing for materials most vulnerable to light.
LET’S BE GREEN
LED light bulbs are not only better for collection preservation, they are also much more energy efficient. One LED light bulb might last 25,000 hours, while a fluorescent light bulb emitting the same light would last 8,000 hours, and an incandescent would last 1,000 hours. This equals energy and cost savings.