Spectro question
Spectro question
- Subject: Spectro question
- From: "James B. Reswick, Jr." <email@hidden>
- Date: Mon, 2 Sep 2002 18:53:04 -0700
- Organization: Reswick Consulting
Hi All:
I was wondering if some folks could help me bridge another knowledge
gap...please let me state what I presume, and then my question...
I am using the Gretag EyeOne spectro with Gretag ProfileMaker 4.0 to create
output profiles.
To create a CMYK profile with the Gretag software, a CMYK target provided as a
TIFF file to print, and a text file of corresponding CMYK values for each
patch represented in percentages, are required. For simplicity, let's limit
discussion to one patch on that target.
For discussion, our representative patch has a corresponding set of values in
its associated text file of CMYK_C CMYK_M CMYK_Y CMYK_K equal to 0.0 100.0
100.0 0.0 . One might assume that this represents some color of red, but what
color? All that the text file (and its identical values in the TIFF file)
tells the printer is to (ignoring total ink limits and other non-profile
modifications) print 100% magenta and 100% yellow.
After the print dries, we measure our patch with the spectro, and a Lab value
for that patch is obtained. If my spectro were perfect, this Lab value would
represent the coordinates exactly describing the printed color.
So, now the profiling software wants to use this measured Lab value and the
value originally obtained from the target's associated text file for this
patch, to make an appropriate correction in the CMYK output profile to be
generated, to compensate for the device-specific variations in this particular
printer-ink-paper combination.
My question is (finally): How can the profiling software know what
corrections to make when comparing only the CMYK text values associated with
the target's patch and the measured Lab values for that patch, when the
information contained in the target's CMYK text values are relative
percentages? Wouldn't two device-independent coordinate descriptions for that
patch (one provided and one measured) have to be available for there to be a
comparison? Wouldn't an additional text file with Lab coordinates have to be
provided with the target?
The Measure Tool accepts Lab text files to reference, so that a Lab file can
be printed through an output profile and then compared against its respective
Lab data, but the Profile Maker will not accept Lab data as a reference.
As always, thanks again for your help...
Jim
email@hidden
(PS - Why does it always seem with color management that the more I learn, the
less I understand?)
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