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July 19, 2007

Not Always Believing

Sometimes Reality Is What You Make It

We're told that seeing is believing, but don't count on it. The visual system has some uncanny tricks for creating an impression of seeing what it "knows" it ought to, rather than what's actually out there. One example is the eye-brain system's ability to correct for ambient conditions, enabling us to perceive things in their correct colors even when illuminated by tinted light. Another is that we compensate for parts of a scene that are in shadow, seeing relatively light and dark areas in their correct relationship, even though a darker unshadowed zone might be reflecting more light than a lighter shadowed one. A striking illustration of the latter is known as the "Same Color Illusion," and was brought to my attention by my good friend, physicist Nick Herbert. It can be seen at http://antwrp.gsfc.nasa.gov/apod/ap070717.html

So it seems that our visual apparatus doesn't perform very well as a light-meter. But that doesn't really matter, because that isn't its function. We can make light-meters to measure absolute luminance. Its job is to make sense of the surroundings in the most meaningful way conducive to reliable decision and action.

 
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