Most, if not all, consumer-grade camera flashes emit a level and spectrum of light that is relatively insignificant in terms of harming library, museum, or archives materials.
Here is a link to Stephan Michalski’s oft-cited article that makes this assertion:
Though the cumulative effects of several flash exposures over many (many, many, many) years might indeed accelerate degradation of certain pigments, the same can be said for the repeated physical handling of any particular object – including exposure to normal room lighting conditions.
I think banning flash for preservation reasons is an overreaction.
Thomas Blake (archives-L)