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[Physltest] [Phys-L] Re: color



One interesting exercise is to go into some program and draw objects =
with specific RGB values and then get out the spectroscopes. (Person=
ally I like PPT because then you can fill the entire screen and get r=
id of the menubars by starting the slideshow.) Start with pure color=
s: RGB =3D 1,0,0; 0,1,0; 0,0,1. On the monitors we have, B & G are b=
ands in their respective parts of the spectrum, while R is a set of s=
harper lines in the red & orange part of the spectrum. I'm sure othe=
r brands of monitors (or LCD monitors which we didn't have handy) wil=
l have 1) different spectra for each color and 2) different observed =
colors. Then play around with combinations like 1,1,0; 1,0,1; 1,1,1.

It would be fun to have a bank of different monitors and see all of t=
hem simultaneously.

Obviously, the choice of which phospors the screen uses will affect t=
he observed colors. Three primary colors can produce a wide range of=
color sensation, but, as John highlighted, you certainly can't get E=
VERY color.

Tim F




-----Original Message-----
=46rom: Forum for Physics Educators on behalf of John Denker
Sent: Sat 1/29/2005 1:26 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: overloaded terms (was: color)
=20
Frohne, Vickie wrote:
... "Blue" is not, and as far as I know, never has been
a technical term in physics,

Those with wider experience might disagree. This includes
-- Joint Photographic Experts Group (the authors
of the widely-used JPEG standard)
-- Commission Internationale de l'Eclairage (CIE)
-- Society of Motion Picture and Television Engineers (SMPTE)
-- International Color Consortium (ICC)
-- Pantone (the almost de-facto standard for what
inks and process colors are supposed to look like)
-- the "xcolors" utility on my computer

For details look at the xcolors output. There is a definite
color called blue and it does not even remotely resemble the
color called cyan.

For additional details, see e.g.
http://www.color.org/wpaper1.html
and references therein, and/or the easier-to-interpret
diagram at
http://www.av8n.com/imaging/color-management.htm#fig-gamuts

You will see that although different standards have not-quite-
identical blue points, they are not wildly different. For
sure none of these experts have a notion of "blue" that
overlaps with their notion of "cyan".

One cannot measure "blueness" and assign a number to the
property of "blue."

Oh yes I can. Here's one example:

The blue point of the sRGB system has CIE chromaticity
(x,y) =3D (0.1500, 0.0600).

There's a high probability that the display tube on
your computer uses sRGB or something similar.

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Folkerts, Timothy J wrote:

... That's why three colors (not four or five or two) are
"primary". By choosing three inks or three lights that each
stimulate primarily one receptor, then a rather accurate
recreation of any color perception can be produced.

That's a charming theory ... too bad it doesn't agree with
the observed facts.

One problem (among others) is that the human receptors have
a nontrivial degree of overlap. There is no way to choose
three primaries such that each primary stimulates only one
receptor. As a consequence, for mixing lights with any
three primaries, there will always be a substantial part
of the color space that is out-of-gamut. Look at the
gamut diagrams at e.g.
http://www.av8n.com/imaging/color-management.htm#fig-gamuts

The situation is even worse than the diagram makes it
look, because (obviously) I cannot use your computer
monitor to exhibit the colors that are out-of-gamut on
your monitor. You can see where the out-of-gamut colors
sit on the diagram, but you cannot directly see what
they are supposed to look like.



The situation is even worse, much worse, if you're mixing
inks rather than lights. Artists who earn their living
doing oil-painting are keenly aware that two paints that
look virtually identical will behave very differently if
you try to mix them with other colors. They have a
little bit of theory (this and that 'should' make green)
and a lot of ad-hoc rote knowledge (this and that actually
makes mud, but something else actually makes green) ...
and in practice if they want a nice green they go to the
store and buy a tube of green.
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