visible light - Why isn't it allowed to use a flash when taking pictures in a certain place?

When I go to, for example, a museum I try to take some pictures.

Sometimes the museum staffs forbid me to use a flash. Do you know the reason? I don't think it is related to photo-electric effect, right?

Answer

From Amateur Photographers in Art Galleries: Assessing the harm done by flash photography. by Martin H. Evans:

The flash built into a digital compact has, typically, a GN value of about 6 to 9 (though some manufacturers are rather coy about revealing the GN rating). If one extends the calculations used by Saunders and by Evans to these little units, then one finds that if one of these is fired at full power at about 2.5 metres (ca 8 feet), it exposes the object to about the same quantity of light as that falling on it every one-eighth of a second in a 200 lux (ca 18.6 footcandle) gallery, or every half second in a dark 50 lux (ca 4.6 footcandle) gallery. Is it worth getting steamed up about such a tiny extra quantity of light, as far as pigment fading is concerned? Several photographers have already suggested that any trifling damage done by a few hundred of these little flashes in a day could be fully offset by closing the gallery and turning off the lights a few minutes early. A ban would be justified in rare cases, where large numbers of photographers might be taking many flash photographs very close to something that could reasonably be considered photosensitive.

So it appears to me the main reason for the ban is not related to the photoelectric effect.