Path: utzoo!utgpu!water!watmath!clyde!bellcore!decvax!ucbvax!agate!aurora!amelia!ames!elroy!mahendo!jplgodo!wlbr!scgvaxd!trwrb!aero!venera.isi.edu!raveling
From: raveling@vaxa.isi.edu (Paul Raveling)
Newsgroups: comp.graphics
Subject: Re: RGB to printer CMYK conversion
Message-ID: <4738@venera.isi.edu>
Date: 9 Feb 88 18:35:45 GMT
References: <10258@sgi.SGI.COM> <4180010@hpgrla.HP.COM>
Sender: daemon@venera.isi.edu
Reply-To: raveling@vaxa.isi.edu.UUCP (Paul Raveling)
Organization: Information Sciences Institute
Lines: 40

In article <4180010@hpgrla.HP.COM> douglasg@hpgrla.HP.COM (@Douglas Genetten) writes:
>Mr. Hue writes...
>
>>...Then use
>>a transmission-reflection densitometer and measure the densities of your
>>17^3 RGB triplets and 17^4 CMYK quads... 
>
>A common missunderstanding hides within this remark.  Densitomiters
>DO NOT MEASURE NTSC RGB.  They typically measure using very narrow-
>band filters which are designed to maximize sensitivity to variations
>in process ink sets.  These filters block out most of the visible
>spectrum---something you can't do if you want to measure color regardless
>of the color space you use. These filters would have to have broad 
>overlaping "standard observer" curves to make them useful for RGB 
>measurement.
>
>Using press room densities as estimates of NTSC RGB densities can cause
>large objectionable color errors, especially when measuring strange 
>pigments found in some color electronic printers.
>
	It sounds as if there's a specialized densitometer for
	printers.  About 10 years ago I used a transission/reflection
	color densitometer in Werner Frei's medical imaging group
	to get good RGB/CMYK measurements for color darkroom work.
	I don't recall the exact tolerances from its filter
	calibrations, but something under 1% in visible light
	seems familiar.

	Does anyone know if there are reasonably priced color
	densitometers available now?  Ability to do decent
	color measurements would help some aspects of our project
	(the ones dealing with digitized color maps), but
	the requirement isn't strong enough to warrant the
	$2,000+ price tag, in ~1978 dollars, of the instrument
	I mentioned above.


---------------------
Paul Raveling
Raveling@vaxa.isi.edu