Re: LAB as working/interchange space
Re: LAB as working/interchange space
- Subject: Re: LAB as working/interchange space
- From: Dan Reid <email@hidden>
- Date: Sat, 02 Jun 2001 19:19:51 -0600
On Thu, 31 May 2001 15:02:41 +0100, ross lists <email@hidden>
wrote:
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Message: 10
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Date: Thu, 31 May 2001 15:02:41 +0100
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Subject: Re: LAB as working/interchange space
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From: ross lists <email@hidden>
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To: <email@hidden>
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Hi,
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If I open a file that has CIE Lab values in Photoshop there is no dialog box
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asking about conversion. Presumably this means that the CIE Lab file has
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been opened into ICC Lab. So are the values correct?
The crux of the problem with v5 Linocolor software. Lincolor Elite used a
D65 white point for their flavor or LAB. Essentially this IS CIE Lab.
Linocolor provided a ICC profile that helped in converting the TIFF using
the Colorsync Photoshop filters to properly match Lab files from Linocolor
to Photoshop's D50 Lab.
Photoshop v5 installed Kodak created .pf (icc/icm) profiles that are
installed in v6 of Photoshop. Two Kodak .pf ICC profiles *pslabpcs.pf* and
*pslabint.pf* are the profiles used in conversion. The only difference I
can see is the A2B0 transform of the *pslabpcs.pf* has a 32 point CLUT while
the *pslabint.pf* uses a 16 point grid. Both are 16bit precision profiles.
Oh, and BTW, Apple's unity LAB profile (Generic LAB) has a 2 point CLUT.
Most US devices have standardized on the D50 Lab while European companies
favor CIE D65 Lab. Check out www.profilecentral.com to see if you can
download the CIE LAB ICC profile for conversion. Otherwise I can email it to
you off list.
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I am trying to use the values from a Macbeth Color checker to get an idea of
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color shift in a workflow including digital cameras. I have been using a
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file from the internet which has the checker colors in Lab not sure which,
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how do I find out.
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If ColorBlind is measuring in CIE Lab do the measurements relate to Lab
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values in Photoshop?
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I thought I had a reasonable handle on this, but apparently not. Are there
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any books relating to the subject which any of you could recommend.
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Thanks
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Ross
If you have a reflective color measurement device (i.e.. spectrophotometer
or colorimeter) you can update the file you have by typing in the LAB
values in Photoshop's color picker for each swatch measured and filling in
each swatch with the measured color. Make sure you are using D50 2degrees
LAB to ensure the measured numbers are correct for Photoshop.
The updated file will be useful to test out profiles (especially camera
profiles) in comparing to your reference, the ColorChecker. You'll get
better digital camera profiles with the DC Colorchecker though the original
is still good for checking profiles against your measured reference.
--
Dan B. Reid
RENAISSANCE PHOTOGRAPHIC IMAGING
Color Imaging Solutions Provider
http://www.rpimaging.com | email@hidden
Toll Free: (866) RGB-CMYK [ 866-742-2695 ]
Local: (505) 471-4126