At my museum and in other historical museums, the glass plates have been
mounted in boxes that have been wired with a push-button to illuminate
the plate from behind for only the length of time the button is
pressed.  The box is mounted in an area that is already somewhat in
shadow from natural daylight, or the overhead lights are redirected
elsewhere.

 The plate may be seen but the time it is exposed to bright light is
limited- I would also recommend you choose light bulbs that are museum
quality and will not damamge the plate from exposure to UV that are in
regular light bulbs.

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