Interesting, if a little left field.
Google is usually your friend, and curiosity led to this exchange of correspondence under Freedom of Information Act from 2011. Not very enlightening, though:
https://www.whatdotheyknow.com/reque...ion_photoflash
More widely, "Operation Photoflash" seems a rather literal name for an exercise allegedly involving the use of photoflash bombs: which to my admittedly fairly sceptical mind suggests that this probably falls the wrong side of the "myth/reality" line.