Damage
to cultural heritage caused by light
Long or regular exposure to artificial or natural light may cause irreversible damage to objects. This damage may manifest itself as discoloration or fading, or result in a change of a mechanical nature (brittleness…). It is accepted that damage increases with length of exposure and intensity of lighting.
Exposure to light
Rather than determining exposure length in weeks or months (which may not be correct in the case of irregular lighting conditions), it is advisable to monitor the luminous exposure, expressed in lux hours (lx.h), that is to say the quantity of light (called illuminance) multiplied by time of exposure (in hours). This can be achieved using a cumulative data logger, and in the case of natural light, whose characteristics continuously change according to time of day, external weather and location in the exhibition room, a data logger has to be adjacent to each object. This is very often impractical for budgetary, aesthetic or other reasons.
Risk to artefacts
Depending
on their chemical composition, physical structure, history or climatic
environment artefacts may exhibit very different reactions to light exposure.
It may happen that visually similar objects react differently from each
other when exposed under the same conditions. This makes it almost impossible
to predict, without performing specific experiments, how long an object
can be exposed with the minimum risk of deterioration. Nevertheless, objects
may be grouped roughly into four categories according to their supposed
vulnerability to light based on empirical experience. For instance, organic
materials, synthetic dyes, textile and graphic documents are more fragile
than easel paintings.
Accepted annual luminous exposure limits from published
data
| Category | Examples of artefacts * | After 100 years, a just noticeable fading will result from a maximum luminous exposure of # |
high sensitivity ISO 1,2,3 |
graphic documents, albumen prints, colour photographs, parchment, leather, textiles, tapestry, natural history specimens | 10 000 lux h per year |
moderate sensitivity ISO 4,5,6 |
oil paintings, tempera, wood, polychrome sculptures, bone, ivory |
100 000 lux h per year |
low sensitivity ISO 7,8, above |
stone, metals, ceramics, B/W photographs | 3 000 000 lux h per year |
* Luminous exposure values from Appendix
8a, p. 138 of: J. Tétreault, Airborne Pollutants in Museums, Galleries
and Archives: Risk Assessment, Control Strategies and Preservation Management,
Canadian Conservation Institute, Ottawa, 2003, 168 p.
# A more exhaustive list of artefacts and materials belonging to each
ISO category can be found in the manual cited above.