Outdoor Lights in Twilight

Don’t seem to work as well as they do at night. It’s not just that the lights themselves aren’t as noticeable, but the areas that they illuminate aren’t as easy to see. Why is this?

I can think of two possibilities. Either because at night the darkness elsewhere makes it easier for the brain to focus on the areas being looked at. Or because the pupils of the eye become more expanded (due again to the darkness elsewhere) at night, allowing in more light.

It is often more difficult to see properly at twilight than it is either at night with the same illumination as at twilight, or in the daytime with sunlight alone.

My WAG on this is that although the sun has gone down far enough that the ground is no longer directly illuminated, the sky is still lit by the sun’s rays from over the horizon, the eye reacts to the (comparatively)bright expanse of sky by shrinking the pupils, but this results in inadequate perception of objects on the ground.

I sincerely believe this to be correct, but it is a guess.

Good WAG, and I wouldn’t be surprised if that plays a part at times, but the effect can be observed even on overcast days.

There are two types of sensors in our eyes - rods and cones. Cones respond to different colors(color vision), whereas rods only respond to basic changes in light (thus black/white vision). Cones function better at daylight levels of light, and rods function better at night.

At twilight, you have a transition period as the cones stop working and the rods kick in. While the rods are trying to adapt to the lowered light levels, detail is a lot harder to perceive. (Think about what your vision is like if you stare at a bright light)

Here is a link that describes the effect in a little more technical detail.

And just because this is one of those things that has stuck in my mind for years, the mnemonic to remember the function differences between rods and cones is that ice cream cones are colorful and magicians’ rods are black.