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From: billd@fps.com (Bill Davidson)
Newsgroups: comp.graphics
Subject: Separate Gamma Values? (was: Re: standard RGB wavelengths?)
Message-ID: <10273@celit.fps.com>
Date: 25 Jul 90 23:07:35 GMT
References: <7833@ccncsu.ColoState.EDU> <101880022@hpcvlx.cv.hp.com> <139554@sun.Eng.Sun.COM>
Organization: FPS Computing Inc., San Diego, CA
Lines: 33

In article <139554@sun.Eng.Sun.COM> poynton@sun.com (Charles A. Poynton) writes:
>The final row is the chromaticity of white, which represents the relative
>contributions ("colour balance") among red, green and blue.  Unfortunately 
>most computer displays are WAY too blue.  Daylight has the same colour as a
>chunk of platinum heated to 6500 kelvin, hence the CIE defined standard
>illuminant D65 to have this "colour temperature".  Modern blue phosphors are
>about twice as efficient as red and green phosphors, with respect to the
>sensitivity of human vision and driving all three electron guns in a colour
>CRT with the same amount of beam current produces a picture that is about
>twice as blue as daylight -- very noticeably blue.  This is done to achieve
>the maximum possible brightness, at the expense of colour reproduction.  The
>additional brightness over a more sensible choice of 6500 K is only about
>5%, due to the eye's insensitivity to blue, but in a market that has
>historically had little interest in accurate colour reproduction, a 5%
>brightness increase was worth the penalty. 

This makes me worry about possible extra problems for gamma
correction.  My understanding of gamma problems is that linear changes
in voltage do no give linear changes in brightness.  A gamma curve
plots the changes of voltage vs. brightness.  If one color of phosphor
is more efficient than another color, might this also change the gamma
curve?  Should we really have three separate gamma values (one each for
red, green and blue)?  This might help bring the color balance back a
bit closer to the proper value.

I'm sure that the differences are small enough that for most
applications it doesn't matter but still, if there's an error in my
calculations, even if it's acceptable, I like to know about it.  I
don't recall Roy Hall's book mentioning this.  He does mention
correcting for a given color monitor but nothing about separate gamma
curves.  Am I just out in left field?

--Bill Davidson