I used to be in the R&D department for a large company that included a major movie studio. They wanted to develop a system that would find people filming the sc

I used to be in the R&D department for a large company that included a major movie studio. They wanted to develop a system that would find people filming the screen in theaters. The system worked by exploiting the fact that all cameras at that time, including phone cameras, had a IR filter over the sensor that was a retro-reflector at IR wavelengths. Behind the screen at three separated places were three IR sources and detector cameras. If the detector cameras at the same points as the sources saw a bright dot from any two of the three locations that was coming back from the illuminator (the light was modulated so we could determine if it was ours) we knew we had a camera! The system worked, but deployment in theaters never happened in a big way, other than a few theaters used for screenings in LA.