Lights

Light levels in storage areas should be low. Light accelerates deterioration of historical materials. It weakens paper fibers and makes them brittle. It can cause the paper or ink to bleach, yellow, fade, or darken. Any exposure to light, even for a brief time, is damaging, and the damage is cumulative and irreversible. Ultraviolet (UV) light is particularly damaging and should be avoided whenever possible. 

Monitoring Light Levels

There are specific devices that can be used to monitor light levels. Light meters measure visible light levels; and UV meters measure ultraviolet light levels – UV is particularly damaging to collections.  

Basic light meters can be purchased for just over $100.  UV meters are significantly more expensive, costing over $1000.  Some meters measure both visible light and UV; these are, of course, more expensive. 

Because light meters, particularly for UV levels, are expensive, your best choice is to take some precautions to keep light levels at a minimum. Since damage caused by light is a result of both the intensity and duration of exposure, your goal is to reduce both whenever possible. 

• Lights should be turned off when no one is in the room.
   
• If the room has windows, the incoming light should be blocked using heavy curtains, blinds, or shades.
   
• Historical materials should be exposed to light only when they are being used (or processed).
   
• When they are not in use, materials should be stored in a “light tight” container or in a room where light exposure is minimal.
   
• In areas where materials are used, incandescent bulbs should be used because fluorescent lights emit ultraviolet (UV) rays that can damage materials.
   
• If you must use fluorescent lights or expose your records to natural light, you can purchase UV filters for light fixtures and windows from archival suppliers. 

Visible light levels are measured in lux (lumens per square meter) or footcandles. One footcandle equals about 11 lux. Ideally, visible light levels should be maintained at 55 lux (5 footcandles). 

Incandescent bulbs are the best choice to avoid damaging materials.

The law of reciprocity.
This says that limited exposure to a high-intensity light will produce the same amount of damage as long exposure to a low-intensity light. For example, exposure to 100 lux for 5 hours would cause the same amount of damage as exposure to 50 lux for 10 hours.