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Re: [Phys-l] sun's true color

Notice I did say the eye-brain system. The apparent whiteness independent
of illumination is predicted by the retinex model. Actually the processing
appears to be fairly close to the eye. Also it works with perception of
black, gray, and white. One is able to distinguish them even when the black
paper may reflect more light than the white one. Land demonstrated this
with a Mondrian array of randomly placed papers, and spotlights aimed so the
black at one end replected the same amount of light as the white on the
other end. Land designed a simple electrical array which mimicked the eye's
response to color and brightness.

The CIE engineering approach to color management depends critically on other
factors which are not under control. It assumes a constancy of perception
which does not happen. Fortunately it works fairly well for the machines it
is designed for. The CIE approach would work better if all TVs had the same
surrounding illumination. At one time Sylvania tried this by surrounding
the screen with a bright frame.

Unfortunately the conventional teaching of color completely ignores the fact
the color constancy is maintained under varying lighting conditions. The
Howard Hughes institute in their online book on the senses states the "color
constancy" is one of the most important aspects of human vision, yet it is
generally ignored by the physics texts.

John M. Clement
Houston, TX



John,
You have just started to tap this subject because besides the
physiology of the eye, the brain can change our perception as well. Take a
white sheet of paper outside and you will see it as white. Same that same
sheet of paper inside under an incandescent lamp will also be white. This
even though the temperature of the sources of light vary by over a factor
of two. Your brain "knows it is white" so it makes appear so. In the
C.I.E.
color space chart which is used to model the eyes response the location of
white depends on the temperature of the source.
This by the way is not the same effect that people find when they
buy an item in a store with florescent lights and take it outside and see
it change. In this case it is because the light source is not a blackbody
source and the reflected colors are a quite different mix from those of a
blackbody, i.e. not a smooth distribution.
Visual perception is function of not only the light detector but
the minds interpretation of the image. The power of the brain to create an
image in our blind spot is another example of this.
Gary


At 02:01 PM 8/25/2008 -0500, you wrote:
Well, how do you define color? The problem is what we see is heavily
influenced by a variety of factors.
1. There are 2 different pigments for one of the cones (green as I
recall),
and which you get is genetically determined.
2. The distribution of cones varies from one person to the next.
3. The perception of color varies a lot from one person to the next so
that
one person sees grey and another beige.
4. The eye-brain system compensates for the color of the incident light
so
that a red square remains red under a variety of illuminations. It does
it
by comparing colors across edges. This is why the 3 color model does not
work well for realistic situations. This was demonstrated by Land in the
60s and accounted for by his retinex model. Incidentally you can get
fairly
good color reproduction of photos with only 2 colors in a totally
darkened
room. So the 3 color model is actually a good model for captured images,
not for the eye.
5. When drawing isolated lines, the number of distinguishable colors is
very small. I recall it is around 8 or so.

I suppose the only "scientific" way to define it is to characterize the
color by the peak in the spectrum. Of course by that measure some stars
would have ultraviolet color, and others infrared. And there should be
"green" stars if the peak falls there. However, the eye might not notice
that.

So it is no wonder we can't agree on the color. The colors of the double
stars might not actually change, but it may be an effect due to a change
in
the background which is influenced by atmospheric conditions.

John M. Clement
Houston, TX


If the color of a star is the color we observe with our eyes, then
there
are indeed green stars. I'm surprised many astronomers say green
stars
don't exist, because many observational astronomy buffs know that they
do. Of course the green color we observe is not the blackbody
radiation
color itself, but is the color we perceive after the light has passed
through other stuff, either in the space surrounding the star, or in
the
corona itself.

If you want to observe colored stars, it's a lot of fun to observe
double stars of contrasting color. Unfortunately these are often hard
to find by star hopping, but they are easily found if you know their
names or their celestial coordinates, and you have an 8-inch or larger
"goto telescope" that is accurately aligned.

There are two double stars in Hercules that are each classified as as
consisting of one red star and one green star. These are alpha-
Hercules
and 95-Hercules. I always try to find these for my astronomy class,
and
the students and I all agree that the "green" star truly appears
green,
although some will say bluish-green, and the color does seem to vary a
little from time to time.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l