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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?

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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

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    \$\begingroup\$ Another remedy to the unbalanced spectral response was to have people wear white or gray make-up, and paint sets using shades of gray. \$\endgroup\$
    – supercat
    Sep 17, 2017 at 18:23
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    \$\begingroup\$ @ supercat 31 -- Makeup artist like Westmoreland & Max Factor win fame making up movie stars in Hollywood using shades that countered the weird effects of orthochromatic film. \$\endgroup\$
    – Alan Marcus
    Sep 17, 2017 at 19:00
  • \$\begingroup\$ How do they develop it in total darkness? (Might be another question but if the answer is short I'd like to know :) \$\endgroup\$
    – John Hamilton
    Sep 18, 2017 at 12:21
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    \$\begingroup\$ @ Hamilton -- Men and women with home darkrooms practice, with lights out, to spool film onto a spiral reel. The loaded reel is then placed in a light-proof tank with a special lid. Liquids can be poured in and out but light cannot enter. Now the developing fluids of the process are mixed to a specific strength and temperature. Once the tank is loaded, the lights are turned on. Fluids of the process are poured in and the tank agitated. In other words the reactions are done by time, temperature and chemical strength. Commercial labs have automated machines that perform these actions. \$\endgroup\$
    – Alan Marcus
    Sep 18, 2017 at 13:58
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    \$\begingroup\$ @ Hamilton – Over my 55 years in this business I designed and managed mega photofinishing labs. These were regional labs that processed and printed films for a 2000 drug store chain. In those days, people (amateurs) took pictures with their Brownie cameras, it was natural to bring them to the drug store for developing and printing. My 7 labs each had the capacity to develop and print 20,000 rolls per day. The lights in many areas were never on except for maintenance. We offered steady work for the blind. The blind can’t know that the lights are on. We installed special safeguards to alert. \$\endgroup\$
    – Alan Marcus
    Sep 18, 2017 at 15:41
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Quick simple answer - film aims to be sensitive to ALL visible colours, even if it will only record them as shades of grey. Mono paper needn't be.

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