My question is about a fairly common phenomenon related to light and vision.

When you are inside a room with lamps inside that also has windows, your capacity to see well can depend on both the light from the lamps as well as the light coming from outside through the windows.

Assume we are considering a room in which

  • a bright sunny day would provide enough light to see well inside the room,
  • the lamps provide enough light to see well in the room when it is dark at night.

One might expect that this means there is always enough light to see well, independent of how light it is outside. However it is my experience that sometimes when the light from outside is neither completly dark nor very bright, it can still be hard to see well. (This is usually around dusk or dawn.)

Explanation so far:

It seems to me this is most likely caused by some mechanism related to our vision. That the light from outside configures our vision for brighter surroundings than are actually present inside, despite the presence of the lamps.

So my questions are:

  1. Is the "explanation so far" true (to some extent)? (Any references are welcome)

and if not

  1. what would be a better way to explain this phenomenon or what important factors have been left out?

But if the above explanation is to some extent reasonable:

  1. Can we then expect to find an "optimal" outside brightness such that it is most difficult to see inside (given a certain configuration of windows, lamps etc.)? Or conversely do we somehow know that such an optimal brightness would be heavily dependent on differences in individuals sensitivity to light?
  • 1
    $\begingroup$ Most of this relates to the biological basis of vision (rods, cones, response of pupils to light levels) and not physics per se. Vision is complicated... $\endgroup$
    – Jon Custer
    Nov 14, 2019 at 14:29

1 Answer 1


I think the "explanation so far" does not exactly hit the point (Q1), and I try (Q2) another formulation:

I understand that the setting is a room only lit by the light from the windows ("when the light from outside is neither completly dark nor very bright"). If you feel that it is hard to see well the light intensity will be so low that your eyes are no longer sensitive enough, even though fully adapted. This will hold especially for the color vision. Take into account that with windows of average size the light intensity inside is much lower than outside, so even if you are happy with your vision outside it may be too dark inside.

When you switch on the light the vision will increase significantly. Switching on the light on a sunny day however will not significantly increase the vision – I think it goes logarithmically with the intensity.

Given this explanation I am unsure how to answer Q3.


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