Re: evaluating pantone colors with photoshop
Re: evaluating pantone colors with photoshop
- Subject: Re: evaluating pantone colors with photoshop
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Wed, 18 Sep 2002 09:54:02 -0500
I have spent most of my professional life dealing with situations rather
like yours: You are put to some test to see if you pass muster. The very
first thing you must do (*before* performing the test) is to validate the
conditions of the test. If instead, you go ahead and do the test and then
afterward complain that the test wasn't fair, it comes across like you are
trying to make excuses.
In your particular case, you have a tagged RGB test file. That means you
have everything you need to figure out what Lab values should result for
each RGB patch. This relationship is fully defined by the embedded sRGB
profile. It has absolutely nothing to do with Pantone colors at this point.
By conceding to the customer that this is a Pantone test, you are
unnecessarily placing the outcome of the test in your customer's ability to
have properly encoded Pantone colors into sRGB in the first place. I would
not assume they did that properly, if I were you.
This assumption about encoding Pantone into sRGB is complicated by the fact
that sRGB uses a D65 reference white and most Lab measurements of printers
(at least in our industry) are made relative to D50. So the embedded sRGB
profiles you are given by the customer were adapted from D65 to D50 using
some (probably unspecified) adaptation. (I can probably tell you the
adaptation method used if you send me the profile off-list.) As was recently
discussed on this list, there are many different flavors of the sRGB
"standard" profile floating around.
My recommendation would be for you to make a table of the colors. For each
color, list the following information (Excel would be a good tool):
a) The sRGB values found for the patch from the image file.
b) The Lab value this represents (use my CIE Color Calculator or someone
else's to find this).
c) The measured Lab value of the printed patch.
d) An indicator showing if this color was inside the gamut of your printer.
e) The delta E between (b) and (c) above.
BTW, your printer should use absolute colorimetric rendering intent for this
test!
Another option (one that I much prefer for absolute color matching tests)
would be to use the "Profile Evaluation Images" from the "Color" section of
my web site. A test based on these images circumvents all the ambiguities
introduced by having more than one profile involved (e.g. sRGB and a printer
profile).
Bottom line: Make sure both parties fully understand and agree to the test
conditions beforehand. I don't think you are at that point yet. Yours should
be an absolute color matching test, not a Pantone color matching test, IMO.
--
Bruce J. Lindbloom
email@hidden
http://www.brucelindbloom.com
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