Some time ago I read an article on a forward operating base (FOB) of the U.S. Marines somewhere in Afghanistan. Because of the threat of rocket attacks, the Marines on that FOB practice strict “light discipline” at night: there are shutters on the windows, no white lights are allowed, and people move around using small red-lensed flashlights.
This reminded me of when I was a kid and watched those Vietnam War movies in which U.S. infantry soldiers used flashlights that looked like this:
The intriguing thing is that the soldiers used red filters on their flashlights. “Why red?”, I wondered back then. Well, 15 or 16 years later, I still don’t know the reason.
Now that I am a bit more educated, I can think of two possible reasons why red flashlights may be a good idea:
- physics: it may happen that photons in the “red” part of the visible spectrum undergo stronger attenuation than “green” or “blue” photons. Or then, perhaps “red” photons are scattered differently than visible photons of other colors. If that’s the case, red flashlights allow their bearers to see what’s around them, but are harder to notice from far away.
- physiology: the human eye is more sensitive to some visible wavelenghts than others. At night, white light causes the pupil to shrink, thus impairing night vision. It seems that red light does not cause such pupil-shrinkage, so night vision won’t be decreased. Supposedly, this is why red lights are used to illuminate aircraft cockpits at night.
Therefore, I suppose that red flashlights are a good idea for both physical and physiological reasons. I searched for some material on this on the internet, and I found some stuff (like this discussion), but nothing very scientific.
Can anyone come up with a reasonable scientific explanation why the military uses flashlights with red filters? I have been waiting for a good answer for almost two decades, and your input will be appreciated.
Tags: Questions kids ask
July 29, 2008 at 00:10 |
Back when I used to do amateur astronomy, we always used dim red flashlights to find our way around — apparently, the human eye stays dark-adapted much better when exposed to red light than to white light. This has less to do with pupil shrinkage — pupils adjust quickly — than to the properties of the cells of the retina, which take half an hour to become fully sensitive when kept away from bright light, but lose their sensitivity very quickly when exposed to white light.
I’m not sure whether this is the reason for the light discipline, but the answers from the military guy on yahoo answers suggest that it is…
July 29, 2008 at 00:31 |
Thanks for your feedback!
Back in the days I went camping, I experienced the phenomenon you described: the eye loses sensitivity quickly when exposed to white light, and it takes a while to regain full night vision. You are also right that the pupils adjust very quickly to variations in lighting conditions.
The photoreceptor cells in the retina are quite amazing sensors. Now, I have to find an ophthalmologist to explain me the details.
So, it seems that there are, indeed, physiological reasons for one to use red flashlights. It remains to be “discovered” why red light is not so noticeable from afar.
July 29, 2008 at 04:31 |
On Sgt. Maj. Brett A. Stoneberger’s Combat Leader’s Field Guide (13th edition), on chapter 11 (Camouflage, Concealment, and Decoys), page 219, one reads:
Interesting. It seems that red light is good at protecting night vision, while blue/green light are good for concealment. Like in any engineering problem, there’s a trade-off. One can’t have it all.
July 29, 2008 at 18:45 |
I took a psyc class on sensation and perception, and it’s actually very interesting. Your eyes work like a chemical cascade amplifier. Your cones [blue-yellow response] have a very strong reaction to light. When they dump their neuro(transmitters? don’t remember the right word), they overwhelm your rods, which are themselves very sensitive [which is why you want them for use at night]. Basically your cones white-out your rods.
A consequence of all this is that it takes a good 45 minutes to an hour for your eyes to get to full night vision, which sucks if you have something important going on. So, you use the red filter to preserve your night vision.
On a different note, in addition to being red and grayscale sensitive, your rods are wired to your brain so they especially good at perceiving motion. When I let my dogs out at night, and can’t see them. I avert my direct gaze from where I think they are, so the image will form in my peripheral vision [hard to do when you are consciously thinking about it; cones are concentrated in the center of your retina, rods in the periphery, so your 'peripheral vision' really is a different vision mechanism]. You can’t get a perfect picture [it's all shadows in the dark end of the grayscale], but you can get a good bearing and a feel for the relative range.
Your eyes are frikkin’ amazing.
July 29, 2008 at 22:28 |
Wow, that’s a lot of really cool info. Thanks! I had no idea that the human eye’s structure was so complex, and that peripheral vision worked like that. Quite interesting. Also interesting is to compare photoreceptor cells in the human eye, with man-made photodetectors (such as CCD sensors in digital cameras).
July 29, 2008 at 20:55 |
I thought it might have to do with the reason the British wore red. The eye’s resolution and depth perception are worst in red.
July 29, 2008 at 22:32 |
According to Wikipedia’s entry on night vision:
So David and Rolland were right on target.
July 29, 2008 at 20:18 |
I thought it was a chemical reaction. Wait, here’s the wiki page. David, you were right on the money.
Night vision scopes seem to be very sensitive to the extreme low end of light, almost and sometimes into the Infared, depending on generation of the image tube and design intent.
So the explanations on the rods function is why red is great for night vision, ask any astronomer. Red lights everywhere. I myself have 2 red led flashlights, and replace the dome light in my truck with a red bulb on star party nights. Not worried about detection.
That is also why pirates wore an eye patch going in to battle so when they went into the dark below decks, they could swap the patch to the other eye and see immediately! Actually tested and proven on that most accurate of scientific shows, Mythbusters.
Averted visIon , looking just to the side of your point of interest, is taught to soldiers and astronomers. Takes some practice but really works! I’ve found I have excellent peripheral vision and excellent night vision; I don’t turn on lights late at night to get around. Starlight (on cloud free nights) is also sufficient. Freaks people out, mostly girls.
July 29, 2008 at 22:38 |
I thought that pirates were patches because they had lost an eye during some bloody piratical activity in high seas. Now, if they were patches in order to preserve night vision, then I must tip my hat to them. That’s a very good idea. And it sounds so simple! If that possibility has been mythbusters-tested, then it’s credible.
I had never heard of averted vision until you and David brought it up. Now that’s interesting stuff. I am almost embarassed that I had never heard of it before.
July 30, 2008 at 09:44 |
Here is a documentary made by the BBC on color vision in 1985. The documentary illustrates the human color vision with a remarkable set of experiments. I watched this in the early 1990s when I was a undergrad. I was literally blown away when I saw it the first time.
Dr Edwin Land, the inventor of the Polaroid camera, carries out experiments in order to demonstrate his theories on how we see colors.
http://xhgc18.blogspot.com/search?q=BBC+Horizon+-+Colourful+Notions
Use rapidshare links to download all the parts of the movie
Use HJSplit (Google ‘HJSplit’) to join the pieces together.
Unzip and watch…
Its strictly illegal to share copyrighted material. But this remarkable documentary was not made available to the public by the BBC after it was broadcast in the late 1980s.
If you feel that I have violated the terms and conditions of your blog by posting links to copyrighted material, please delete my post.
June 22, 2009 at 11:50 |
Hi Rod,
I’m a former US Army Infantry Officer, and stumbled across your question about why we use red light instead of white. There are a couple of reasons, all of which are pretty simple.
1. Red isn’t as detectable by the enemy. Shining a white light in a combat zone is a quick way to get yourself killed.
2. Red is used for map reading. Our military maps are designed to be read under red light.
3. Red works well with our night vision goggles, under extremely pitch black conditions. Otherwise, extra light sources are not necessary at all.
4. Red doesn’t destroy your night vision like white light does.
5. Standardization. If a US Army soldier sees a green, blue, yellow, or other color light in a combat zone, they won’t hesitate to shoot. A red light, on the other hand, probably means it’s a friendly.
Hope that helps you.