I have seen sprays and covers that do a few things:
1. Spray the plate so a flash will be so bright so as to ruin the photo, like shooting in a mirror.
2. Polarize the plate (cover are cheaper) with a polarizing filter so that the plate appears to be blank when viewed through a camera lens that does not use a circular polarizer to "unpolarize" the light.
Both illegal in my state.
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