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I heard you should wait some time until your eyes adapt to darkness and are able to see smallest luminosity differences, otherwise you might overlook faint objects in the ocular. Are there any rules of thumb here?

What about looking for 5 minutes on your bright laptop, do you have to wait again the same time or wear some sunglasses? ;)

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up vote 22 down vote accepted

Actually, the Army and Air Force (and I assume the Navy) teach their life support troops about this (as well as the aviators), since so many missions are conducted at night. It is actually some very interesting physiology involved in this. This Army Instructional manual should be of help (PDF File).

On an average it takes 30 to 45 minutes for your rods to be fully dark adapted to night vision.

It also covers things such as Mesopic vision and Scotopic vision, eye health, and how to actually dark adapt. I suggest it as a good read. It's less than 14 pages.

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The US military, I presume? –  Peter Mortensen Jul 25 '11 at 1:52
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Yep. :) I think the uniform gave it away, right? –  Larian LeQuella Jul 25 '11 at 2:00
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It should be mentioned that dark adaptation occurs in two phases. Your pupils dilate in a few seconds, then your rods adjust over many minutes. Try this demo!: Take off your glasses/contacts or borrow someone else's in a brightly lit room at night. View a row of LEDs. I use a cable modem in a home office for this. Stand where you can just barely focus on the LEDs, then flip off all the lights. You start out with tiny pupils that make a lens with a wide depth of focus, so you can see the LEDs. When your pupils contract, the lens enlarges, the DOF shrinks, and the LEDs go blurry. –  Andrew Jul 25 '11 at 12:57
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An old camping trick, for after someone's shone a flashlight in your face -- put your hands over your eyes, and stare into the palms of your hands for 5 to 10 seconds. Your pupils will fully dialate, so you'll restore your basic night vision in seconds, not the 3-4 minutes they mention. (but not something you'd want to do when flying an aircraft, I'm guessing). You still have to wait to get back any color sensitivity, though. –  Joe Jul 27 '11 at 20:19
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