Re: a link to some interesting illusions

[Larry Woolf <Larry.Woolf@GAT.COM>]

At 10:06 AM -0700 2/4/2003, SSHS KPHOX wrote:
> Are our cones really in red, green and blue or are they all the same
> responding to the energy of the photons letting the brain do the rest?
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A nice presentation of color vision concepts is shown at:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colviscon.html#c1

See also references at:
http://www.sci-ed-ga.org/modules/materialscience/color/sites.html

Color scientists prefer to use the terms long-wavelength sensitive cones
instead of red cones, mid-wavelength sensitive cones instead of green
cones, and short wavelength sensitive cones instead of blue cones.

The sensitivity to light of a given wavelength for each cone is shown at:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html#c1

The  color that results from the stimulation of a cone at a given
wavelength (also depending on the spectrum of the illuminant) is shown at:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/cieprim.html#c2

I like to think of color vision as hardware + software.  The hardware are
the cones - that respond to different wavelengths of light.  The software
takes the output of each cone and converts it to color.  Here the model
used is the so-called opponent theory of color vision.
-----------------------------

> Is magenta really purple? Why do folks use violet and purple almost
> interchangeably?

Because they appear to us to be similar in color and we do not define
colors well in common usage.
So this question is difficult to answer precisely because it depends on how
you want to define these colors.
But in general:
Magenta light results from nearly equal amounts of red and blue light.
Both the "red" and "blue" cones are stimulated.
Purple light results from some red light and some blue light Both the "red"
and "blue" cones are stimulated.
Violet and purple are often considered the same color, but they are not.
The perception of violet can be achieved by a light of a single short (or
short range of) wavelength, near 400nm.  At this wavelength, only the
"blue" cones are stimulated. But somehow the perception of a reddish tinge
reappears at these short wavelengths - most likely due to some "software"
effects.  The reemergence of red at these short wavelengths is not well
understood at this time.





Larry Woolf,General Atomics,San Diego, CA 92121; Phone: 858-526-8575; FAX:
858-455-8568