Lighting can have a significant impact on exhibits. It can influence the environment of the exhibit space, highlight items of importance, accent areas of the space, lead viewers from one room to another, and, most importantly, it can have a significant impact on the life of an item.
How does light negatively impact archival materials?
The three largest contributors of degradation to archival materials are: light, heat, and oxygen. All three of these contributors play a part in causing displayed items to degrade.
Light and oxygen partner to cause photo-degradation. Photo-degradation is initiated by a photon being emitted from a light source, and being absorbed by molecules. The photon provides energy that is transferred as electrons. The electrons cause molecules to jump from a ground state (stable) to an excited state (unstable). The excited state of the molecules can cause them to spontaneously oxidize or hydrolyze. This process can be observed when photo-degradation causes archival items to yellow and become brittle, or when it causes organic dyes to fade. If lamps are set-up too closely to archival items, heat from the them can act as a catalyst for photo-degradation, causing it to occur more rapidly.
The temperature and relative humidity of collection storage spaces are often closely monitored. The lighting of exhibit areas should be monitored equally as closely to preserve the archival items on display. How can this be done? Information on light can be collected using a UV metre, an ELSEC Handheld Environmental Monitor, or a Blue Wool standard card to examine the lux and UV levels. Lux is the unit of illumination, used to measure the intensity of light in one square metre. UV or ultraviolet light is a particular wavelength that causes rapid fading of organic materials.
It is not recommended to use natural sunlight to illuminate an exhibit space because of the extensive damage it causes to archival items. Sunlight is too intense, emits too much heat, and has high UV levels. There should not be any windows in an exhibit space, though if there are, it is recommended to place UV filtering film on the windows and to cover the windows with blackout curtains. UV filtering film should be replaced every ten years. UV filtering light tube covers can also be purchased for fluorescent light tubes, which also emit low levels of UV.
It is recommended to display items for no more than three to four months. Archival items without coloured media can be illuminated up to 150 lux, though archival items with colour, as well as art on paper, should be illuminated up to no more than 75 lux.
Our next blog in the Exhibit Series will discuss archival materials that are particularly vulnerable to deterioration when on exhibit.
For more information on light and light damage, please visit the Northeast Document Conservation Centre.