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My new head torch has a mode that produces a red light. I've seen this several times and I have no idea when our why you would use it?

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Red light preserves your night vision.

Red lights are used in photography dark rooms, military bunkers, submarines, and anywhere you might find yourself working in the dark. It gives you light to see by, while at the same time keeping your eyes adjusted for the dark.

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I use my red light mostly when I'm when I'm around a camp fire, in the middle of the night when I'm in my tent, or when I have to go out to do my business in the pitch black. It's also excellent to use when there's a bright moon.

Using your red light in the tent is great because you won't blast your eyes out, or disturb anyone else in the tent, your eyes don't need to adjust to it like they need to adjust to white light. This is exactly why digital alarm clocks have red numbers. @Olin explains some of the science in his answer, and @radpin's comment to the OP links to this question on biology.SE.

Using your red light outdoors in the dark gives you much more peripheral vision. When you're using your white light; your eyes adjust to the light so you can only see what's being illuminated by your headlamp. When your eyes are adjusted to the dark, you can use the red light to illuminate the trail, but you will also be able to see everything that's being illuminated by the moon in the distance.

Try it out next time you're in the dark, give your eyes enough time to adjust to the dark, then turn on your red light. You'll be surprised how much you can actually see.

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    I think the primary reason for red lights being used in darkrooms is that it doesn't affect black & white film.
    – jamesqf
    Apr 5 '15 at 3:55
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    No, the primary reason for using red lights in dark rooms is so you can see what you're doing. B&W film is very much affected by red light, if it wasn't then the colour red would appear as black in every picture. B&W film is processed in pitch black, purely by feel. It's the photosensitive paper that isn't affected by certain intensities of red light.
    – ShemSeger
    Apr 5 '15 at 17:34
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    Red does appear black in B&W.
    – jamesqf
    Apr 5 '15 at 18:23
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    It's not for the film; it's the printing paper that's insensitive to the red end of the spectrum. When you handle film you have to do it in total darkness; when you make prints you can use red light so that you can see what you're doing. Sep 30 '15 at 11:41
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    @PeteBecker That's pretty much what I said in my first comment.
    – ShemSeger
    Apr 22 '16 at 18:05
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The automatic f-stop mechanism (iris) in your eye is less sensitive to red light than the shorter wavelengths. Therefore, red light provides some illumination, but doesn't trigger your eye to stop down and make you temporarily blind when you shut the light off. There are also other ways your eyes adapt to ambient brightness that are also less sensitive to red.

In contrast, if you were to read a map using a bright white light, your eyes would adjust to the brightness and you'd have trouble seeing for a little while after the light was shut off. Using red light instead of white decreases the eye's response to the brightness significantly.

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    The gist is right (red light does not disrupt night vision), but I think the mechanism analogy is not f-stop (iris diameter), but ISO ( film speed). See en.wikipedia.org/wiki/Purkinje_effect Apr 3 '15 at 14:19
  • I always thought the weak red light mode is for reading or working in the tent/hut.
    – Wills
    Apr 3 '15 at 18:10
  • @Wills: It's for any time you want to preserve your night vision. Apr 3 '15 at 18:18
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    The mechanical or camera analog would be aperture. The more it's open, the more light the eye can collect, but with less definition to the image.
    – Escoce
    Apr 3 '15 at 20:22
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In addition to the answers already mentioned, I prefer red light while outdoors in the summer because it seems to attract fewer insects. I always seem to have more insects flying around my face if I use white, and fewer with red.

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Night vision and, if you're camping with a partner, a nice way to not blind them around camp.

It is also a way to see in the dark without disturbing animals you're hunting, or the fish if you're night fishing.

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The focus is preserved by using red light. Human eyes adjust faster to red light than to white light when shifting from darkness to light.

Emergency personnel and first responders also use red lights. They aid in the preservation of night vision and the reduction of overall light signature in low-light environments. Red light does not induce the human eye pupil to shrink to the same extent as more bluish/white light.

It also reduces light pollution.

Less blinding light When wearing a headlamp on a camping trip or other nighttime activity, white light can be dazzling and uncomfortable for those directly exposed to it. Red lights, on the other hand, are less intrusive and distracting.

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