Light

All wavelengths of light – visible, infrared, and ultraviolet (UV) – promote the chemical decomposition of organic materials through oxidation. Higher-energy ultraviolet is the most harmful.

However, light in all its forms, especially in the presence of atmospheric pollutants, leads to a weakening and embrittlement of cellulose, adhesives, cloth, and skin materials.

Light can cause some papers to bleach and others to yellow or darken; it can also cause media and dyes to fade or change colour, altering the legibility and appearance of documents, photographs, art works, and bindings.

The following factors concerning light should be known by all those responsible for preserving library material:

• Chemical reactions initiated by exposure to light continue even after the light source is removed and materials are put into dark storage.
• Light damage is irreversible.
• The effect of light is cumulative. The same amount of damage will result from exposure to a strong light for a short time as to a weak light for a long time. 100 lux (the unit of measure of illuminance) on a picture for 5 hours gives it an exposure of 500 lux-hours, equivalent to 50 lux for 10 hours.
• Visible and infrared light sources, such as the sun and incandescent light bulbs, generate heat. An increase in temperature accelerates chemical reactions and affects relative humidity.
• Daylight has the highest proportion of UV radiation and therefore must be filtered.