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.
This is why astronomers, hunters, and military at night use dim red lights: they have least effect on night vision.
Most headlamps have such a light. Please use that dim red light, not the bright white light.
If you don’t have such a headlamp, just point the face of your phone down towards the ground. If you’re dark adapted, that should provide plenty of light to see where you’re going.
If you must use the flashlight feature of your phone, also point that down. Never point it at anyone’s eyes.
This has been a public service announcement. We return you to your irregularly scheduled outings.
For WWALS outings, see:
* Dark Adaptation, University of Minnesota
CC LICENSED CONTENT, SHARED PREVIOUSLY
Cheryl Olman PSY 3031 Detailed Outline
Provided by: University of Minnesota
Download for free at http://vision.psych.umn.edu/users/caolman/courses/PSY3031/
License of original source: CC Attribution 4.0
Adapted by: Savannah WhisenhuntWebvision: The Organization of the Retina and Visual System, Light and Dark Adaption by Michael Kalloniatis and Charles Luuz
License: CC BY-NC
Adapted by: Savannah Whisenhunt
-jsq, John S. Quarterman, Suwannee RIVERKEEPER®