There are several reasons why people are often asked not to take flash photographs in museums. The primary concern is preservation of the art, as flash photography can cause significant damage to works of art, especially when it occurs in high volume. Some cynics have also suggested that prohibitions on taking flash photographs may have something to do with a desire to preserve gift shop revenue, but this is not in fact the case.
Flash photography can contribute significantly to the degradation process of a piece of art. Flashes produce both light and heat, which can trigger a variety of chemical reactions. Exposure to light and heat, for example, causes the cellulose in paper to break down, and damages many pigments, as anyone who has left a photograph in a sunny window for a few months may have noted. While a single flash is not a large issue, numerous flashes over the course of years of exhibition will cause a work of art to deteriorate more quickly. This leads to the formulation of policies forbidding flash photographs in museums, so that future visitors can enjoy the art too.
Concerns about light and heat also explain the environmental conditions in museums. Most reputable museums are designed in such a way that sunlight never touches the art, with specialized low-level lighting which allows people to see the art without causing damage. The air is often also kept cool and at a steady temperature, to ensure that the art is not damaged by heat or temperature fluctuations.
There are some other reasons why taking flash photographs in museums is frowned upon. For one thing, flash photography can be very disruptive to other patrons, especially people with medical conditions which cause increased sensitivity to light. In sites of cultural and artistic value, such as cathedrals, flash photography may also be viewed as disrespectful. Taking flash photographs in museums during events or ceremonies is also generally viewed as disruptive, as flashes can be extremely distracting.
That said, many museums have recognized the desire to take photographs of their collections. In response, many permit photography, as long as a flash is not used. The use of a tripod is highly recommended to compensate for the low lighting conditions, and photographers should try to be respectful of other patrons while photographing their favorite works of art.
Some museums disallow all photography due to copyright concerns, or by special request of an artist or the owner of a loaner collection. In these cases, taking photographs may result in a polite request to vacate the premises.