Night Vision

The ability to see in the dark is a matter of survival. Not only for nocturnal animals hunting for their next meal, or perhaps trying to avoid becoming somebody else's next meal, but also for humans. Evidence of that can be seen on every battlefield and in every military cockpit, both airborne and marine. If you can't see what's out there in the dark, you're vulnerable.

For the human eye, it takes about 30 to 45 minutes to fully adapt to darkness to the point that we have our maximum natural night vision. After that adaptation happens, all it takes is a brief burst of light from the white, yellow, green or blue spectrum to "bleach out" the rod cell photoreceptors in our eyes. The result is instant night blindness. 

Illumination sources that don't emit white, yellow, blue or green light don't cause night blindness. That's why a red spectrum of light is used onboard ships and aircraft at night when a light source is needed. Red light has a longer wavelength that doesn't attack the rod photoreceptors in or eyes and disrupt our night vision.

Once night blindness occurs, you have to start over with the adaptation process. But one trick to help retain at least some night vision is to close or cover one eye when an offending light source is approaching.  For example, if you are aware that car headlights are coming your way, close or cover one eye to preserve the night vision in that eye. The eye that remains open will be affected, but you'll still have some night vision left after the car passes. Of course, it's better to tightly shut both eyes in the presence of light if you are able,  but there are times when you must keep at least one eye open — while you're driving, for example. Be aware, however, that closing one eye will diminish your depth perception and can be dangerous while driving. 

But if you're hunting, or being hunted, and white lights (flashlights, helicopters, flares, etc.) are being used in the area, keep that trick in mind. It might save your night vision…or even your life.