Virgina Postrel writes,
The anti-photo policies of museums don’t necessarily make sense, except as some kind of revenue enhancer. Prohibiting flash is one thing. And I don’t blame the Louvre for blocking photos in the often-crowded Italian painting gallery. But prohibiting all photos in an uncrowded museum filled with works in the public domain is unnecessary—unless you think it will generate sales in the museum store.
I can understand that a museum might prohibit flash photography because it’s annoying, but I think that gift-shop revenue is the real motivator. When it comes to old paintings, there’s no copyright anymore, so museums have to find other ways to keep you from making copies.
Apparently, some museums imply that flash photography causes paintings to degrade more quickly, perhaps causing the pigments to fade or something. On first consideration, this seems plausible because at close range the flash can be brighter than the sun, and the sun can certainly fade paint.
It turns out, however, that flash photography is essentially harmless.
The key intuition is that it always takes the same amount of light striking a piece of film to expose it properly, and for that amount of light to reach the film, it first has to bounce off the painting. So every photograph of a painting involves hitting it with the same amount of light. It could be a high-speed flash fired in a 1000th of a second, or a 1-second long exposure in a dimly-lit museum gallery.
In other words—and this is the whole point—a flash picture is equivalent to leaving the museum lights on for one extra second.
(My 1-second figure is just a reasonable guess based on some photographic sources and a little playing around with my camera. The actual exposure will depend on the film speed and aperture, the actual museum lighting, and maybe the tones present in the painting. But once those are chosen, it’s still the same amount of light for flash as for ambient light photography. The same principle also applies if you’re shooting digital.)
So, if 300 people take pictures every day, that’s equivalent to leaving the museum lights on for an extra 5 minutes each day.
Technically, a photographic flash usually emits more ultraviolet light than ordinary museum lighting, and paint pigments are known to suffer more damage from UV light. Also, certain types of chemical damage are disproportionately worse for high-intensity light. However, studies by professional conservators indicate that neither of these factors contributes to the aging that art works undergo while on display at a museum.
So, if you want a good picture of a painting but the museum won’t let you take a flash photo, you can always take one in ambient light. Of course, it’s hard to hold the camera steady for a full second in your hands, so you’d have to shoot with the camera mounted on a tripod.
And wouldn’t you know it, most museums prohibit the use of tripods.
I guess you’ll just have to buy a photo from the gift shop after all.
Further reading: Here’s an article on the subject. Or there’s this book: Effects of Light on Materials in Collections: Data on Photoflash and Related Sources, summarized briefly here.