Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!watcgl!awpaeth
From: awpaeth@watcgl.UUCP
Newsgroups: sci.physics,comp.graphics,comp.sys.ibm.pc
Subject: Re: Color questions: color systems, "black bodies", EGB palette
Message-ID: <1142@watcgl.UUCP>
Date: Sat, 30-May-87 14:47:56 EDT
Article-I.D.: watcgl.1142
Posted: Sat May 30 14:47:56 1987
Date-Received: Mon, 1-Jun-87 02:36:16 EDT
References: <1937@druhi.ATT.COM> <725@bsu-cs.UUCP> <520@uhccux.UUCP> <1656@ames.UUCP>
Reply-To: awpaeth@watcgl.UUCP (Alan W. Paeth)
Distribution: comp
Organization: U. of Waterloo, Ontario
Lines: 62
Keywords: physics
Xref: utgpu sci.physics:1441 comp.graphics:660 comp.sys.ibm.pc:3913

The Boltzmann equations define the spectal energy of a perfect black-body
radiator. For use in color, these values can be converted into cie
chromaticity coordinates (they are well tabulated), which in turn can
be mapped into RGB coordinates through a change of basis, if you know the
chromaticity of your monitor. Enclosed are some sample values.


    /Alan Paeth

---------------
CIE chromaticity coordinates for a black-body at T in deg K.
 lambda(max) is the peak wavelength in nm of the Stefan-Bolzmann curve.

Dominant wavelength is not available for ~6500 degrees Kelvin, as the curve is
nearly flat through the visible portion of the spectrum here. Standard daylight
is approximated by illuminant D65, but does not lie along the locus of points
for blackbody radiators.

 Temp-K	x'	y'	lambda(max)

  100	.735	.265	695
  300	.734	.266	684
  500	.721	.279	641.5
 1000	.625	.345	606.7
 1500	.586	.393	594.8
 2000	.526	.413	588.9
 2500	.477	.414	585.2
 2854	.4476	.4074	583.5
 3000	.437	.404	582.9
 3500	.405	.391	580.9
 4000	.380	.377	578.9
 4500	.361	.363	577.2
 4800	.351	.356	575.1
 5000	.345	.352	572.6
 5500	.332	.341	0
 6000	.322	.332	0
 6500	.313	.324	485.7
 7000	.306	.371	483.7
 8000	.295	.305	481.5
10000	.281	.288	479.4
24000	.253	.253	477.0
99999	.240	.234	475.7

---------------------------------------

3x3 multiply to convert XYZ (cie chromaticity) into RGB

(1) set X=x', Y=y', Z=(1-x'-y') and then

(2) compute

   |R|       | 1.73 -.48 -.26 |    |X|
   | |       |                |    | |
   |G|   =   | -.81 1.65 -.02 |  * |Y|
   | |       |                |    | |
   |B|       | 0.08 -.17 1.28 |    |Z|

where these RGB are those defined the the NTSC which standardizes
broadcast colors for North America television transmission (this is the
inverse matrix of the definition, rounded to three significant places).

    /Alan Paeth