2

On a bright summer day, I can look down a straight highway and easily make out signs and some details of typical roadway objects that are at least hundreds of meters away, perhaps a kilometer or more.

At night, I cannot easily read the black text on a road sign until I get within ~15 m, even though most of it, the yellow background, is retroreflecting my bicycle's headlight. Of course, a car's headlights are much more powerful and make it easy to read from much farther back.

It's obvious that more light means we can see better (up to a certain point). But I wonder, if all of our rod and cone cells are already working during the day, why does visibility (seem to) continue to increase with overall brightness? Would the daytime long-distance clarity be most enabled by:

  • minimization of shot noise associated with that point in the field of view, after the usual inverse-square-reduced contributions from a faraway object
  • something vaguely similar to the operation of a pinhole camera, in which the pupils' reduced size also minimizes off-axis light entering the eyes
  • some combination of the above

or something else?

  • 1
    Related, Pinhole camera: How "How to See Without Your Glasses" works?, as the entrance pupil of the eye gets larger in the dark. – Farcher Aug 24 '19 at 06:51
  • I've noticed the same sort of thing in that if I'm in a restaurant and I don't have my reading glasses, I can see the menu much more clearly if I shine a bright light on it. It works well but I was puzzled as to why it works. Of the two possibilities you listed, I'm thinking that a pinhole camera effect is more likely. Another possibility is that the explanation may lie somewhere deeper in the physiology of human vision, which is actually much more complicated than most people may think. There's a very interesting book titled "Sleights of Mind: The Neuroscience of Magic" about that. –  Aug 24 '19 at 19:23

0 Answers0