Exposing an object to light, whether it is a book or flat item, causes damage that is cumulative over the lifetime of the object. The damage done by light cannot be restored and the item is permanently altered. By keeping the light levels as low as possible while still allowing for adequate viewing of the item, the rate degradation is reduced. This includes color fading and the physical breaking down of the item. Minimizing the amount of time something is exposed to light, even if the levels are low, will also control damage. So we must control the quantity and quality of light exposure to minimize the cumulative damaging effects of light on objects.
Another point to remember is that not only visible light does damage, but also light outside the visible range, such as ultraviolet and infrared. All light will cause permanent chemical changes in the item, so it is important to monitor light, especially in an exhibit setting, and choose the most appropriate light level for each item.
Some objects are more light-sensitive than others, and require lower light levels. Within archival collections this may include photographic materials, textiles, and color media (printed color, watercolor, tempera, etc.). In an exhibit you may see that these types of materials have lower lights levels than perhaps oil paintings or metal objects.
Here we see a book and its protective box. The spine label is made of the same material as the book cover and was once the same color. The book retains its original color, but the spine label on the box reveals ambient light damage.
-Giselle Simón, Conservator