You probably know it from museums, exhibitions or even caves with a guided tour. "It's prohibited to use a camera as the flash would fade fabrics, wall paintings" and so on. On the other hand, some of these places use CMH spotlights, fluorescents or true daylight enters through windows.
I wonder if a camera Xenon flash is really such a big problem or rather a good excuse. The power of a built-in flash might be a sort of a cloudy/overcast outdoor light intensity, lasting for a few tens of milliseconds. And this is only within a 3 m (or 10 ft.) range. External flashes are several times stronger and some of them can be zoomed together with your lens.
Is it possible they would degrade material colours worse than those 12 hours/day running CMH spotlights? In the extreme case, it'd be a few thousands of flashes (tens or a few hundreds of seconds of the light) daily. Is there something wrong with their spectra or do the sudden light intensity changes a kind of "sledgehammer effect"?

Indeed, in sixth grade we took a field trip to an art museum, and we had to keep any camera flashes turned off. Based on what I've read, the UV light created from a xenon flash lamp can indeed discolor some materials, but it takes flashes of unrealistic number and intensity to make a perceptible difference. And camera flashes are covered with UV-filtering plastic, making the effect even more negligible. So in essence, no, it's practically not a problem.

There are theories that the real reason is to prevent people from taking high-quality photos for copyright reasons, or to keep museums with a large number of people in line moving quicker.

Also, the constant flashing of cameras distracts other visitors from the artwork. There are probably multiple reasons...

The "official" reason I've met was, the flashes are annoying the other visitors.

For the UV exposure: There are shown different pieces, each of them sensitive in a different manner.
The fixed lighting installation in the museum could be designed to take these specific limitations into account, so those items not sensitive at all are placed in areas lit even by a natural daylight, but with those most sensitive ones you won't find any fluorescent nor HID light source.
The problem with visitors is, you can not control them that much, so the only working way is to not allow the use of the flash at all.

By the way here in most museums or similar exhibitions any photography, nor filming is usually just not allowed at all, even without the flash.
The official reason is the security (it is explained as the photos "accidentally" taking the security items make the robbery plans easier). Well, I'm not sure how that really works, I have strong doubts...

And camera flashes are covered with UV-filtering plastic, making the effect even more negligible. So in essence, no, it's practically not a problem.

True. I've just checked that plastic on my oldest camera and there's absolutely no degradation (such as yellowing) present. And this plastic is just about 1 mm away from the flash tube. Some discharge lamps (especially MV/MBF but probably also MH ones) cause plastics yellowing due to UV /B(?) leaks through the outer bulb glass. So the UV is indeed an insignificant problem here.

For the UV exposure: There are shown different pieces, each of them sensitive in a different manner.
The fixed lighting installation in the museum could be designed to take these specific limitations into account, so those items not sensitive at all are placed in areas lit even by a natural daylight, but with those most sensitive ones you won't find any fluorescent nor HID light source.

That's a good explanation. Incandescent (halogen) spotlights might be a problem due to overheating the exhibit and (if not glass shielded) by to adding a tiny bit of UV, too. Today, I assume LED spotlights would be the best option here.

Why do they prohibit any photographing (even without the flash) might be also because so many people can't control their cameras well, use them in the point-and-shoot mode and have no control over the flash, letting the camera decide.

Btw. I've also visited a few places (e.g. the "Skanzen" in Stockholm) where they explicitly told us when visiting an indoor exhibition where spinning textiles using original equipment took place - taking pictures OK but please no flash! Obviously for the discolouring reasons listed here.

"UV exposure" is plausible. They may think of worst-case UV exposure adding up over years, as most works in "old art" presented in museums are intended to be preserved for centuries. Go calculate how much continuous exposure is that equivalent to

"visitor distraction" is plausible

"sledgehammer effect" i dont think exists in museums, but may exist in caves : The life of the cave may be related to some bacteria etc that lives on the walls, now that may be sensitive to the flashing. Dont really know anything about it just guessing

"security" thing is fake - Team Rocket can just go ahead and visit the museum, who needs pictures ?

"copyright" thing is plausible but only as an excuse. The museum often sells postcards etc with pics of its presented items. However, the copyright term for most items is usually long expired so IMHO there are no legal grounds to the "copyright" claim



Israel Museum in Jerusalem is lit exclusively with incandescents, all very heavily dimmed - i think down to 1500K or less. Some of the display cases have miniature lanterns inside them the size of a pen cap, and those are miniature mag-lite like incandescents too

I remember going there and photo'ing all day long (including the lighting). I did not use flash, i waited for people to move out of the frame while taking (without asking them to move), and everyone was ok with it

I think too the "security" is indeed just an excuse: I've seen that just in the former eastern bloc, to make extra money (as very frequently you may buy "photo permit").
And indeed elsewhere the filming/photos (without any artificiallight/flash) are OK and there are even way more valuable items are dislayed...