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You see in a lot of museums, art galleries and even aquariums about not using flash photography or just any photography so that it protects whatever is on display. Is there potential to use two-way mirrors with the mirror facing inwards as a protection barrier? Or is this likely to cause too much light reflection back onto the object inside.

Any information on existing methods to protect pieces would also be appreciated!

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  • \$\begingroup\$ Family member who was a ward at the Louvre: flash is prohibited because it is a nuisance to humans (visitors and wards). And since the visitors don't really care (or know how to disable the flash) the ward on duty in Mona Lisa's hall can be recognized by the red eyes, and the paintings don't fade away (ML is a copy behind a glass, but the other aren't...). \$\endgroup\$
    – xenoid
    Commented Apr 21 at 7:58
  • \$\begingroup\$ I highly doubt the above claim that the Mona Lisa, which you see on display at the Louvre, is a copy, the suggestion being that the original is squirrelled away somewhere for safekeeping, never to be seen by anyone. \$\endgroup\$
    – osullic
    Commented Apr 21 at 10:43
  • \$\begingroup\$ You can read about the current display/lighting environment of the Mona Lisa here, and in greater detail here. \$\endgroup\$
    – osullic
    Commented Apr 21 at 10:47

2 Answers 2

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Semi-transparent mirror works by both reflecting light on the same side and passing through light from the other side. For the effect to appear, one of the sides needs to be lit significantly brighter than the other. Thus on the darker side, light passing through will dominate, while on the brighter side, reflecting light will dominate.

So when protecting museum items with such mirror, they should be put on the brightly lit side of that mirror for them to still be visible. A flash would still make the other side brighter and thus the photo would capture only reflections.

The reason flash photography is discouraged is that light is a carrier of energy, and that energy affects the lit surface destructively. In this context, the aim is to keep displayed items in best possible shape for decades, or even centuries.

So such mirrors are not used in museums and galleries, because using them would require constant bright lighting, and that would be a counter-productive strategy for avoiding short light bursts.

Current strategies are the prohibition messages already mentioned in question, accompanied with generally dim lighting and obstruction of direct sunlight reaching the displayed items.

In aquariums and zoos, the concern is different - flashing light in nature usually denotes approaching adverse weather conditions and as such would cause unnecessary stress for the inhabitants. Thus no need to use dim lighting, or protect from direct sunlight (exceptions may occur depending on habits of the specific species).

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The wavelengths of concern are primarily the high energy UV spectrum which can cause photochemical destruction. Visible light is not the issue...

However, the majority of UV wavelengths are absorbed by plain glass (≤300nm); acrylic glass absorbs even more. And both can easily be treated to absorb remaining wavelengths of concern... like a UV filter for a lens.

 (below 300

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