Early photographic films were not very sensitive to red. (It showed up as black on the resulting photo.) That's called 'orthochromatic film'. And it's relativel

Early photographic films were not very sensitive to red. (It showed up as black on the resulting photo.) That's called 'orthochromatic film'. And it's relatively safe to use a dim red light (often called a 'safe light') with that. Photographic paper which is what is used to produce a 'positive' print is only reacting to black and white, so it can be made from 'orthochromatic chemicals' without problems, and it is more convenient because you can use a red safe-light while you are processing it. Later photographic film was about as sensitive to red as the other colours, so all colors showed as shades of grey, while black was black. That's called 'panchromatic film'. You must use total darkness when transferring/developing this film. I learned to do the the entire film-processing sequence solely by feel umpteen decades ago. An offshoot to the dim red effect, is the movies where you see submarine control rooms using dim red lighting. This is supposedly a way of preventing a loss of night-vision when looking though the periscope at night.