[argyllcms] Re: Fluorescence

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 26 Aug 2005 22:41:45 +0200

Roger Breton schrieb:

Hi folks,

I finally got hold of Grame's "A practical Approach to Measuring and Modelling Paper 
Fluorescence for Imprimved Colorimetric Characterization of Printing Process" in 
IS&T Eleventh Color Imaging Conference.

But there is one thing that puzzles me about UV filtration.

Suppose I measure withtout UV filtration and my instrument uses a tungsten light source, 
as many have. Then, is it conceptually correct to say that, provided I observe the final 
proof under an tungsten light source that I will have effectively reproduce the 
"viewing" conditions built into the instrument? And that I should observe a 
color match? Because the tungsten lamp used to illuminate the proof emits the same UV 
every as the lamp in the instrument?

Roger,

yes, in this case the measured spectral radiance factors should be the same as you'll observe them in your actual viewing environment.

Regarding your statement "And that I should observe a color match?", it's not clear to me, what colors you want to compare, and how.

Independent from the fluorescence issue, if you intend to view the print under illuminant A, then you also should create the profile for an illuminant A spectrum (i.e. "profile -iA -o1931_2 ...") and not for a D50 spectrum, which implies that media white is the color of the unprinted paper under illuminant A.

Btw, keep in mind that Argyll records the mediaWhitePointTag in the "traditional way", i.e. *not* adapted from the viewing illuminant to D50. Thus, if you use a printer profile generated with "profile -iA ..." for proofing with ICC-absolute intent, you may not get, what you expect, if your expectation is an *illuminant relative* reproduction, as the term ICC-absolute is interpreted by the ICC. In order to get the ICC desired behaviour, you need to patch the mediaWhitePointTag and mediaBlackPointTag accordingly in the Argyll generated profile. On the other hand, you should not use the patched profile in conjunction with the gamut mapping modes of icclink, since the conversion to/from CIECAM02 requires the "true" absolute XYZ numbers, which can no longer be reconstructed from the patched profile. If necessary, keep two profiles (unpatched and patched) and use the appropriate one for each particular purpose.

(To avoid a misunderstanding, this ambiguity only applies, if the desired viewing illuminant is different from D50, and if the profile is thus created for the desired viewing illuminant. Once V4 profiles (or V2 profiles with CHAD tag) are supported by Argyll, this should also no longer be an issue.)

Regards,
Gerhard



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