Visible light, IR and UV radiation can cause extreme and irreversible damage to many items commonly found in collections. In general, organic materials are the most sensitive to this damage.
Visible light and UV radiation can cause chemical reactions to take place within certain materials. Continued exposure can result in the breakdown of the material. This can be seen by colours fading and changing; fabrics, papers and photographs yellowing, losing strength and becoming brittle; woods bleaching, yellowing or darkening; and lacquers and varnishes turning yellow or brown.
In extreme cases disintegration of the material can occur. Damage caused by visible light or UV radiation is known as photochemical deterioration. UV radiation has a much greater potential for causing this than visible light, as it comprises higher energy.
It is important to realise that photochemical deterioration is cumulative and irreversible, and that you might not notice a change until severe damage has already occurred. Take note also that the degradation processes are not uniform and can therefore be difficult to predict, especially for an object comprised of more than one type of material. Therefore, when lighting any area where important or valuable items are housed, it is essential to take steps to keep the potential for damage to a minimum.
IR radiation differs from visible light and UV radiation in that it causes objects to heat. Although this does not cause photochemical deterioration, it can accelerate it. Heat also causes objects to expand, causing damage. This will be discussed in further detail in the Temperature and Relative Humidity section of this topic.