Why must unexposed and undeveloped film be handled in total darkness? Why can analogue, unexposed and undeveloped monochromatic photographic paper be handled in red/ yellow safelights without getting fogged?
The light sensitive goodies that comprise film are the metal, silver, and one or more members of the halogen family of elements (Swedish for salt maker these are -- chlorine, bromine, and iodine). Once a halogen is combined with silver, a light sensitive crystal results. These are coated on film or paper to make photographic film and photographic print paper.
In their natural state, these "haloid" crystals only respond to blue and violet light. Such films and papers can be safely handled in a darkroom illumined by a lamp that is void of blue and violet. This would be red, yellow, or amber safelights.
The fact that photo paper is only sensitive to blue and violet light is OK when it comes to making prints from black & white negatives. This means we can do the printing and developing of papers under safelight conditions (red or yellow or amber). Now early photographic film was also sensitive only to blue and violet. The problem was, some subjects imaged quite weird. Women who use lots of red makeup look wonderful when we gaze at them, but when photographed using these early films, the red makeup goes black, thus rouged cheeks, lips and natural ruddy completions image too dark.
The remedies were to add what is called sensitizing dyes to the film making recipe. The first attempts forced the film to become sensitive to green light, plus it retained its sensitivity to blue & violet. This film was called “orthochromatic” from the Greek, meaning suitable for all colors. Now this film is insensitive to red and thus it’s the type of film you want. Sorry to report that though still made, it is not readily available as a roll film. You can handle and develop orthochromatic under red safelight.
The next breakthrough in film making was to add more and different sensitizing dyes forcing the film to become sensitive to the three primary light colors which are red, green & blue. Such film is call “panchromatic”. The prefix pan is Greek for all and chromatic is Greek for color scale. Panchromatic is developed in total darkness. However, a feeble green light can be used for a few seconds if needed