All emulsions are sensitive to the radiation emitted by lightning discharges such as static electricity. The density of the image formed is proportional to the speed of the emulsion.

All you need is a dry lab and you begin to get such discharges. You can even generate them with scotch tape. Put some tape onto a piece of plastic, sit in the dark to adapt your eyes and then peel the tape off of the plastic. You will see bright flashes.