# Seeing colors

1. May 28, 2006

### fargoth

im reading some of feynman's lecture notes, and i stumbled upon a statement which i do not understand...

he says that we can use ANY three different colors to produce any other color.
lets say we want to see the color X, and we use yellow blue and red (Y,B,R)
then X = yY + bB + rR.
where y,b and are are the color coefficients.
inorder to produce some colors (in my example lets say we want to get green) we would need to use a negative color coefficient.
i.e. the r should be negative while y and b have some positive value to produce green.

mathematically i can see it is so, but i couldn't understand how you practically make it... i'll give you the original quote:
now, what does he mean when he says "to add them to the other side"?
does he mean that if i use three flashlights with red, yellow and blue colors, we have to use the red flashlight from one side of a sheet and the other two from the other side?
doesn't sound right to me...
can someone explain what he means?
how can i produce green in the lab using red, yellow and blue flashlights?

2. May 28, 2006

### Mk

I don't think you can. Those are the primary colors of art, not physics. RGB (televisions and monitors) and CMYK (printing) are better to try this.

3. May 29, 2006

### fargoth

i think i understand what he means.

i think he is referring to the test which was carried out to build the chromaticity diagram - in which one eye would see a sample color and the other one a color created from RGB, but not all colors could be created from RGB, and when such a sample was presented to people they were asked to add some RGB colors to the sample untill they found the right match - and these colors you add to the sample are "negative amounts of color".

found a source that may have said it better:
anyway, if we'd use another shade of green at ~515nm and change the R to ~640 B to ~440 and G to ~560 we'd get much better color accuracy...
but i guess most people are sattisfied with the variaty we get from only these three colors... (and monitors produce much more shades then printers, so graphic artists have no use for these accurate monitors).
and another problem is that we're limited by what colors of phosphor (or LED's, or gas-discharge) are available.

although i can't see any difficulty in making LCD monitors use the better wavelength's and even add a fourth one...
The typical CRT display can cover less than half of the visible domain of colors.

http://www.cs.berkeley.edu/~sequin/CS184/IMGS/CromaticityDiag.JPG

Last edited: May 29, 2006