from Iconoduel weblog
Flash Bulbs and Artifact Preservation: Myth Debunked!
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The restricted use of flash photography in museums and art galleries can be said to be fairly commonplace. The reason behind such a restriction has at times been attributed to conservation concern in terms of excessive exposure for light-sensitive artefacts.

However, as the above weblog posting set out to show, there is not a shred of scientific basis for such a concern. In fact, the post quotes Stefan Michalski (from the Canadian Conservation Institute) from an earlier Conservation DistList post in which he argues that actual increase in light exposure from photography flashes (using actual measured figures) would be rather insignificant:

"Assuming the gallery lighting is the lowest most museums can tolerate, 50 lux (5 foot candles) then each flash adds the equivalent of one second of normal gallery exposure. So, 300 amateur flashes a day is equivalent to adding five minutes to the display day."

In fact, Michalski cited more defensible reasons for restricting flash photography as : "copyright, or as a disturbance to the act of contemplation (my personal vote) but there is no preservation reason."

The other posts in the original Conservation DistList discussion thread could be viewed here.