I read about these a while back in a science journal where they discuss all the latest scientific...umm, well, the scientific science.

Anyway, they ended up with this design going to market, but the early prototypes started with a torch filled with a couple D-cell Energiser batteries stuck up the ass to light one's way in midnight ablution
duties.

Apparently the testing phase wasn't pleasant at all, and the whole idea was somewhat flawed, and eventually they pivoted to this design, the one you have.