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Re: [Phys-l] perception of variations in hue



The explanation in the web reference has at least part of the explanation.
Many optical illusions are due to the fact that we do not see color exactly
according to the classical color theory, but rather we see the differences
in color due to suppression by nearby cones. Essentially when a cone fires
its signal is suppressed by nearby cones firing in response to the same
illumination. But where the color is an abrupt change the cones are not
surrounded by cones similarly illuminated and a signal is sent. Essentially
we see only the boundaries which means our brain receives a line image.
When there are 2 colors involved, I would assume that the brain combines the
two colored lines and interprets it as white. So we can readily interpret
cartoon images or line drawings because that is what the brain actually
gets. This reduces the amount of information sent to the brain.

There is also another effect that you can in a darkened room. You can
reproduce a full color image with only 2 wavelengths of light projected
through appropriate b&w slides. So the brain can interpret 2 color signals
as white, even though they do not combine to that in the color wheel.

The signal sent to the brain is not exactly a derivative but really a
convolution of the given cone with negative contributions from the
surrounding cones. It does have many of the properties of a derivative or
gradient amplitude, but there is also messing around with the amplitude of
the signal. Your senses of hearing and sight are probably both
approximately logarithmic, and certainly not linear.

I do wish the textbooks in the "physics of color" would acknowledge the fact
that the 3 color theory has been know for many decades to fail in natural
lighting due to the constancy of color. The constancy of color, independent
of illumination, is according to the Hughes institute the most important
fact in color perception. This was demonstrated by Edwin Land at an APS
meeting during the last millennium.

John M. Clement
Houston, TX

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Wednesday, February 09, 2011 12:23 AM
To: Forum for Physics Educators
Subject: [Phys-l] perception of variations in hue

If you want to see something that is (a) pretty and (b) weird,
check out
http://www.av8n.com/imaging/disk-rays-no-rays.htm

It demonstrates that /perceived/ brightness is different from
photometric brightness ... different in a big way. One can
monkey with the hue so as to produce the perception of a
bright ray-like feature, even when there is no such feature
in the actual photometric brightness.

Making a hue-circle with constant perceived brightness is not
an easy task.

If anybody knows of any relevant references, and/or knows the
name of this illusion, please let me know.
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