Photons per square CM

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I was just looking at a light source from a few miles away obscured by several trees and thought "wow, that's pretty bright for such a small light (an airplane's flash) to register from such a distance".

So, a small flash travels, say two miles, hits a small opening, say two or three inches, and travels about another quarter mile to my eye and STILL registers pretty bright. It must take quite a large amount of photons to pass between the gap in the leaves of a tree and still make a bright "impact" on my iris.

What kinds of photon density tests have been done and if so, how many would it take to register a reaction in our vision?

Cheers!
 

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