Why / how is it possible that the red light used in darkrooms during key developing stages does not ruin the image on the film?
You didn’t expose the film to the safelight. You load the reels and cans in the dark or in a changing bag. The safelights were safe for the paper used for printing.
Very general answer, and I’m sure there were some film exceptions (as further posts will show up), but the red or amber light was safe for many B&W papers.
My office used to be a darkroom, probably one of the last few built in residential construction (around 2003). I remodeled it but still have a big-ass red safelight globe in a drawer.
This question has been asked before:
One answer by Mangetout:
"Red lights can only be used in darkrooms when you’re working with monochrome film and paper; the film is not very sensitive to red light (this is evidenced by the fact that red objects in a BW photo often come out very dark on the print).
The real red lights in darkrooms (as opposed to the ones you see on Columbo) are very dim.
There is (IIRC) a similar light for use with colour photography, but as it needs to avoid emitting wavelengths to which the colour film is sensitive, this doesn’t leave much to play with - the light is a very dim murky greenish colur I think - many people who process colour (manually in a darkroom as opposed to one of those photo processing monster machines) work in total darkness by touch."
The answer is that the photographic material in question is not sensitive to red light wavelengths.
It was a long time ago when I used to do my own (black and white) developing and printing, but my memory agrees with what NoClueBoy says: undeveloped film could not be exposed even to red light. I had a special tank that you transferred the film into without exposing it to any light, then you poured in a succession of chemical solutions to develop and then fix the image. After that, you could then make prints from the film using photographic paper that could safely be exposed to red light light, because it was made not to be sensitive to those wavelengths.
This Kodak pdf document lists filter numbers (Kodak’s), their colours and their uses.
Red photons have less energy than blue, and if a photon is below a critical energy it can’t cause the photochemical reaction - so won’t affect the film. Early film was actually pretty insensitive to red light, and it took some time to get it to the point where is was sensitive across the range of human sight. These films were known as panchromatic films - even though the actual result was a simple monochrome (ie black and white) picture. These films could be handled under a safelight. However the panchromatic films could not, and given the better aesthetic results with a panchromatic film, there was scant use for these older types.
Printing paper for monochrome prints has no such constraints, and typically has little sensitivity in the red. So most can be handled under a safelight. One interesting exception was Kodak’s Panalure, which was designed to allow black and white prints to be made from a colour negative. Printing onto ordinary B/W paper from a colour neg always resulted in very poor results. Panalure is sensitive across the whole spectrum, and has a spectral sensitivity balanced to provide a good result. It can’t be used under a safelight.
Colour papers can’t be used under a safelight. You get good at messing about in the pitch dark.
As hinted at, it is the frequency of the light MUCH more than the intensity which matters.
Different emulsions respond to different lights.
The only significant frequency/emulsion combination is the monochrome (black and white) emulsion and the red-orange (OA) light - there is/was t he Thomas light whose filters were precise enough to allow book-reading light intensity w/o spoiling b/w emulsion.
Jobo made a led-based lights for color, but, as noted, were limited to reading package labels at a few inches.
Great answer. Although I think you wanted an “orthochromatic” (which is not sensitive to red) there where you first had “panchromatic.”
I did intend panchromatic, but the sentence structure isn’t exactly good, and the way you read it more valid than the intent I had. Panchromatic is of course what the later films were, orthochromatic the earlier films.
Used to hand process 500 ft. rolls of aerial film. ( Yes, 9.5 inch X 500 feet. ) B/W only. Color of that size was seldom done in house by anyone in the business.
Done in the dark by feel, 2+ hours at a time depending… Temps, developer type, film type all had to be accounted for.
Had a very small weak green light we could use when we thought it was developed properly developed but you had to be quick. And never at the the start.
Ok, so let’s clarify it for everyone, just in case:
Orthochromatic films, which were the earlier films, were not sensitive to the red end of the spectrum. It’s sensitive to blue and green.
There were even earlier emulsions that were only sensitive to blue.
Panchromatic, as the pan- prefix suggests, is sensitive to all colors of the spectrum. Almost all your modern black and white films will be of this type.
There are still some orthochromatic films out there. Looks like Ilford still sells their Ortho Plus Copy product in sheet film form. Adox Ortho appears to be discontinued, and there is a product called Maco Ortho 25 that still seems to be available in limited quantities.
Since modern film is panchromatic, you cannot use a light when rolling and developing it. Only after it hits the fixer should you use any light. There may be some exceptions for special types of film, but we never used any kind of light whatsoever anywhere in the film development process. Now, in printing, yes, we used a weak red light. Black and white paper is not sensitive to the red end of the spectrum. It doesn’t need to be. When you’re printing a black and white image, the only information you have is the amount of light that goes through the negative, so the paper only needs to be sensitive to the amount of light, not the color of it, so you can use an emulsion that is insensitive to one part of the spectrum, if you want.
My life is a darkroom. One big, dark room.
With an Xbox at one end and a Lazyboy/minifridge at the other
I did not realize this as I only did black and white film development in high school and never made it to color film processing. Although I did realize that the Kodachrome it’s gives us the nice bright colors, it gives us the greens of summer, makes you you think all the worlds a sunny day oh yeah. I didn’t realize it was processed in total darkness by most do it your selfers.
That would probably have made her more gloomy! 
Old black and white photos of the Swedish flag look quite strange as orthochromatic emulsions reverse the gray scale: http://upload.wikimedia.org/wikipedia/commons/5/54/Swedish_flag_on_orthochromatic_film.jpg
Perhaps a nitpick, but I don’t think DIYers were processing Kodachromes themselves. K-2 development (which is what Kodachrome used) was only done by specialty labs back in the day.
And I don’t understand–black and white film was also processed in total darkness. Or are you referring to printing? If so, Kodachromes typically weren’t printed by the average shooter, being slide film.
An old physics anecdote that will appeal to anyone who’s spent time in a darkroom:
Rutherford and his assistant were doing scintillation counts, which required them to allow their eyes to adapt to total darkness for an hour before they could make accurate observations. One session, after fifteen or so minutes of waiting, Rutherford asked, “How many cups of tea did I have this afternoon?” “Three, sir,” the assistant replied. “I thought so,” grumbled Rutherford. “Where’s the sink?”
Yeah, I don’t think you could even buy K2 chems unless you had one of the machines purchased (or leased) from Kodak. My rep days for Kodak were way back in the early 1980s, so I could be mis-remembering.
E6 (and before that, E4) was readily available on the consumer/hobbyist market. I pushed lots of E6 kits.