The chief light sensitive ingredient in film is salts of sliver (silver + iodine or chlorine or bromine). These compounds form tiny crystals that are off-white in color. When exposed to light, these crystals reduce to metallic silver; the other component is a member of the halogen family, Swedish for salt maker.
Violet and blue light rays are most energetic. Early films were only sensitive to these colors. Old black & white films thus image with weird results. Rosy cheeks and lips image too dark, often reproducing a black void of detail. Green foliage images too light also void of detail.
Additionally, all early films suffered from a defect called “halation”. Bright highlights often imaged blurred seemingly showing an encircling halo. The halation resulted with bright light traverses the film emulsion, playing on the film emulsion to base junction or pressure plate, it was prone to be reflected back into the film, exposing it from the rear.
Professor Hermann Vogel of Berlin Technical set out to solve the halation problem. In 1873 he hit upon the idea of dying the film emulsion yellow. He reasoned, the yellow dye would act as a blue blocking filter thus preventing the halation. His idea worked – to his surprise, the yellow dyed film gained sensitivity to green light. Vogel had also invented what we call orthochromatic film, sensitive to violet, blue and green. This film and its predecessor can be handled under red safelight. Red light is void of violet, blue and green.
Vogel and his graduate students went on to dye film emulsions using various colored dye. These dyes altered the way the silver salt crystals responded to different colors. Eventually they produced panchromatic films. These are sensitive to red, green and blue light. Pan films are least sensitive to green. We can used a super dim green safelight placed some distance away from the film and used for only a few seconds. Such green safelights help prevent darkroom workers from tripping and they can be used to inspect the film as it develops. In modern times, military night scopes, infrared type, can be used and are greatly appreciated, these work!