Path: utzoo!attcan!lsuc!becker!bdb
From: bdb@becker.UUCP (Bruce D. Becker)
Newsgroups: comp.graphics
Subject: Re: CIE (1931) XYZ vs xyY
Keywords: CIE color Macbeth
Message-ID: <46268@becker.UUCP>
Date: 7 Oct 90 07:17:17 GMT
References: <1990Oct4.025925.22471@qualcomm.com>
Organization: G. T. S., Toronto, Ontario, Canada
Lines: 36

In article <1990Oct4.025925.22471@qualcomm.com> fpa@qualcomm.com (Franklin Antonio) writes:
|Recently purcased the book "Illumination and Color in Computer Generated
|Imagery" by Roy Hall.  Great book.  In this book, he gives the color
|coordinates of each square of the Macbeth ColorChecker(r) chart using
|CIE 1931 XYZ coordinates.  I ran out & bought one of these charts at
|my local camera store, and started writing a program to calibrate my
|computer's display.  The chart came with a leaflet which lists the
|color coordinates of each square on the chart, but it contains different
|numbers!  It also claims to be using the CIE 1931 system, but the three
|columns are labelled " x y Y " instead of " X Y Z ".  Anybody know what's
|going on here?

	The "x y Y" system is a normalized form of the
	"X Y Z" color coordinate space.

			X
		x = ---------
		    X + Y + Z

			Y
		y = ---------
		    X + Y + Z

	Note that  x + y <= 1.0 in this system, and that the
	locus x = 1/3, y = 1/3 exhibits equal energy contribution
	from each part of the visible spectrum. The Y component
	in both systems represents the luminosity function, which
	is a measure of perceived brightness independent of color.
	Because the "x y Y" system is normalized, Y can be scaled
	to any arbitrary reference value.

-- 
  ,u,	 Bruce Becker	Toronto, Ontario
a /i/	 Internet: bdb@becker.UUCP, bruce@gpu.utcs.toronto.edu
 `\o\-e	 UUCP: ...!uunet!mnetor!becker!bdb
 _< /_	 "I still have my phil-os-o-phy" - Meredith Monk