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I'm no expert in darkroom photography, but it seems a little odd that there is a type of light that doesn't affect film or developing paper etc. The only way that I could think to explain it is:
either way, I'm curious to know how this handy tool is, as per title, safe.
Photo films and papers are made from salts of silver that naturally only darken when exposed to violet or blue light. In the early days of photography, this was all that was available. Therefore these films and papers are able to be handled under any light source that does not radiate blue light. By the way, the violet and blue frequencies of light are the shortest, and are the most energetic when it comes to inducing a chemical change. These early films and papers could all be be handled safely under red light as well as yellow light. These lamps do not emit violet or blue.
These blue-sensitive-only films did an OK job, with some exceptions. Women’s faces with cosmetics, like lipstick and rouge on the cheeks, came out weird. Warm tones reproduced super dark, and most times lips and cheeks turned black, void of detail on the finished picture. The bottom line is, many colors in nature reproduced incorrectly with this early blue-sensitive-only film.
The cure was accidental. Professor Hermann Vogel at Berlin Technical was trying to solve the problem of halation. This results when taking a picture of bright objects, like light sources or gemstones and the like. These objects play on the film with lots of light energy. This energy often goes completely through the film and hits something behind the film. The light is then reflected back into the film. The result is a halo around bright objects. The professor had one of his students dye the silver salts yellow, thinking the yellow dye would filter out the annoying reflected blue from the rear. He tried this dyed film and it did the trick, plus the film gained sensitivity to green light. He named this blue/green sensitive film orthochromatic (Latin for correct color). The year was 1857, and the quality of film reproducing the colors of nature moved forward by a big leap.
A few years later, one of his graduate students, experimenting with different dyes, discovered how to make films sensitive to blue, green and red light. This film was called panchromatic (the pan prefix in Greek means "all"). Thus panchromatic film reproduces all colors found in nature with high accuracy. The bad news was, the darkroom folks were forced to give up the red and yellow safelight. A super dim green safelight could be used for a short period of time during developing.
Photo papers remained insensitive to red for the most part - no need, as they work OK with just blue and green sensitivity. Modern variable contrast photo papers have two sensitive coats, one for blue light and one for green light. We can use a safelight on these papers; it is amber with reduced brilliance.
Films and papers that make color pictures are panchromatic, and most safelights are not safe. We can use infrared lamps with a specialized night vision infrared scope to view and handle most panchromatic films and papers, because these materials have low sensitivity to infrared.
it seems a little odd that there is a type of light that doesn't affect film or developing paper etc.
There are types of light that don't affect the rods and cones in your retina, too. You can't see infrared or ultraviolet light even though some other animals can. It's the same idea with photographic materials: they vary in their sensitivity to different colors.
Most film that you'd be likely to use is panchromatic, meaning that it's sensitive to all visible wavelengths. That includes black and white films like Kodak Tri-X and T-Max films. You can't use a safe light with panchromatic films, so you have to load them onto developing reels in complete darkness and put them in a light-safe developing tank before you turn on the lights. Black and white photographic papers, on the other hand, are mostly sensitive to light at the blue end of the spectrum, so light from a red safe light doesn't expose them.
I can't speak from experience about color paper, but I'd expect that it needs to be handled in complete darkness.
Update: I overstated things a bit above. Safe light filters come in colors besides red for other purposes. There's a dark green one that can be used sparingly during development of panchromatic film. That is, after the film has been in the developer for some portion of the expected time, you can use the green safe light to check progress. There's also an amber filter that can apparently be used with color paper. At any rate, the idea is the same: a safe light is designed to have minimal impact on the material. The intensity of the light is kept low, and the wavelength is chosen to be in the part of the spectrum where the material is least sensitive.
Basic silver halides used in darkroom photography are sensitive to blue light. Special chemicals are added to the emulsion to extend the sensitivity range to other visible colors. Some materials are sensitive to all colors (reversal and negative color films, reversal and negative panchromatic b&w films), some have minor gaps in sensitivity (color papers for making prints from color negatives), some have greater gaps (b&w papers, orthochromatic b&w films). If you know where the gaps are, you can use appropriate light when processing these materials. You will notice that some materials call for dark green light, some for olive green, some for red, some for 590nm yellow etc.
A "safelight" is only safe for black & white processing from a distance and not for extended periods of time. The materials are less sensitive to the red or brown light wavelengths used in these lights.
