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.