If you go somewhere truly dark, like the Okefenokee Swamp, or even Banks Lake after dark, you, or at least other people, are trying to get dark adapted, so they can see the stars, planets, meteors, etc. That takes many minutes, and can be destroyed in seconds.
Most people these days live in cities with lights on all the time, even at night, so they are not used to dark adaptation. Here’s how it works.
It takes 7-8 minutes for the rods that enable dark vision to become sensitive enough for dark adaptation to become obvious.
It takes another 13-22 minutes (20-30 total minutes) for the rods to reach maximum sensitivity.
During all that dark-adaptation time, bright light can reverse the process rapidly.
“So if you’re trying to get dark-adapted, it’s crucial to avoid light—it can undo hours of dark adaptation in seconds. All the rhodopsin you have built up over the previous 30+ minutes disappears, and it will take time for your retina to replenish it.”*
So if you’ve come on a dark outing, please do not shine bright white lights.
Especially, do not shine such lights in people’s eyes: you will destroy their night vision, and it can take half an hour for it to recover.
If you are sitting around a campfire and need more wood, the firelight should be enough to see.
If people are in the woods looking at the stars, do not walk up on them with a bright light. That’s like running up on kayaks or canoes in a power boat. Sure, the wake will subside, but everybody knows not to do that. Getting back night vision takes even longer.
If you must use a light on such an outing, please use a dim red light, which has the least effect on night vision. Continue reading