Since people so often mention the military in regards to this topic…
Like another poster, I too was in the military and they actually gave us torches with red filters.
After many complaints about not being able to see really important stuff, like certain lines on maps, they gave us torches with red and green filters… After further complaints, they included blue filters too, but that garnered hatred from medics who couldn’t see blood when treating casualties.
In the end we all just gave up and bought standard Maglite AAs with our own money. They fit the pocket better, anyway.
This is based on old CPF threads where it was discussed at length. The way I heard it, human color vision is high resolution and most sensitive in the blue-green part of the spectrum, and quite insensitive at wavelengths longer than about 640nm. It is the vision from cone cells in the eye. Night vision or scotopic vision, the vision from rod cells, is much more sensitive than color vision, but it is monochrome and low resolution. Its purpose is to see where you are going instead of stumbling into things, not to see fine details.
It is not really possible to read using scotopic vision because of the low resolution. If you want to read you are supposedly best off with blue-green (cyan or so-called NV green) light, around 505nm, set to just barely bright enough.
On the other hand, to best preserve your night vision, you want wavelengths above 640nm, thus the 660-670nm deep red leds that we are finally able to get easily now. But you’d use that for finding your way around, not for reading.
In aviation cockpits were set up so that the person reading the map would have a dim green light and others would have red lights. Night vision gear was also set up to not pick up the blue-green light.
The Rigel Skylite astronomy lights (very dim, with adjustable brightness) have some models where there are both red and green leds to select.
Some species such as cats and owls have a structure in the eye called the tapetum lucidum, which is like a reflector behine the retina that focuses faint incoming light back into their photo receptors. That is what makes their eyes especially shiny. It means their night vision is even more sensitive than that of humans, but also of even lower resolution.
And UV light is horrible for the eyes. You can seriously damage them. I hate to think of all the guys who thought UV bulbs and UV sensitive posters were “the rage” and used UV light often… what damage might’ve happened to their eyesight.
I actually have a real-world use for red lights….well, sometimes green light too. I like to hang out outside in the backyard at night. Along with insect and creature shows, I also like looking for different stars and constellations. So if I need to see my gadgets, then I use red light and when I go back into the darkness, my eyes are already adjusted to the dark.
