Back in '79 it was well advertised to use a roll of B&W film. Pull the film out, let it expose in the sun; cut in 8 or 9 inch lengths, 24 exposure is enough to put across several people's face. Something about the silver coating on BW film is what does the filtering. Google can elaborate.

"How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?

Sherlock Holmes