Seeing (Level 5)

Dusk from Inside, Dusk from Outside
Here are two views of the same place at the same time. It was dusk and in the first view, I was inside looking out through the windows. My room inside was illuminated with yellowish incandescent lights, but it looked fairly neutral to me because of adaptation. Looking out through the window, outside appeared quite dark and bluish like in the left side of this image. However when I went outside, things looked fairly normal since my visual system could adapt to the low light level and bluish color of the sky light (the sun had set). In that case, when I looked inside the house, the illumination looked very bright and yellowish. When we are adapted to the bright and yellowish light inside, then outside looks dark and bluish. When we are outside and adapted to the dark and bluish light, then inside looks much brighter and yellowish. The only thing that changes is the way our visual system adapts to the scene (and I made my camera mimic that behavior!).

Why Can I See Well Outside When My Mom (Who's Inside) Thinks It's Too Dark to be Out?

Adaptation! It all comes down to how our visual systems adapt to their environment. We are normally interested in identifying objects in our environment and less interested in identifying the color and level of illumination. Imagine a white piece of paper. When we view it on a bright sunny day, it looks white. When we view it at dusk after the sun has set, it looks white. When we view it under the very yellowish illumination produced by incandescent light bulbs (or the even more yellow illumination of a candle), it still appears white. This is because our visual system has adapted to the prevailing levels and colors of illumination in order to be able to better judge the relative colors of objects in our world.

When your mother is inside at dusk, she has probably turned on some lights and gradually adapted to that level of illumination. As it gets darker outside, she does not adapt to that change and when she glances out she notices that it appears very dark outside and calls her children in. The children, on the other hand, have been outside the entire time and adapted to the gradual change in the color and amount of light. The world outside still looks completely normal to them and they can still see fine when their mother seems to arbitrarily decide that it is too dark to be out. This change in appearance due to adaptation is illustrated in the images above.

Adaptation is a very powerful property of our visual system (and other perceptual systems) that allows us to easily detect changes in the world around us. For example, if you eat some sugar you will adapt to sweetness and when you taste some plain water it might seem bitter or sour. When you are very hot, a cool glass of water might feel very cold, but if you just came inside on a cold winter's day, that same glass of water might feel warm. If you spend time in a room full of tobacco smoke, you will gradually adjust to the smell and it won't seem as bad is it would if you walked into the same room from the fresh air outside. If you are in a room full of loud noises, then you can't hear a quite voice while if you are in a perfectly silent room, you can hear almost anything. These are all examples of adaptation and our visual system is very capable of adapting to changes in color and light level as well.

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Ever wonder ... How is color perception different from hearing?


Updated: Apr. 19, 2011