Re: Photoshop 6/7 problems with OptiCAL Monitor profiles
Re: Photoshop 6/7 problems with OptiCAL Monitor profiles
- Subject: Re: Photoshop 6/7 problems with OptiCAL Monitor profiles
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Thu, 13 Feb 2003 20:47:59 -0600
Bob Chase wrote:
>
When you look at the values of OptiCAL produced profiles using the Photoshop
>
Color Settings "Custom RGB..." Working Space Panel to display the OptiCAL
>
profile information as Photoshop sees it, the phosphor primaries numbers
>
bear only distant similarities to the OptiCAL Information palette numbers.
C. David Tobie wrote:
>
Little x and little y values as reported for Gamma 1.8 Whitepoint Native
>
(about 6500) or 6500 profiles with the EyeOne, the Spyder and OptiCAL and
>
Optix with the new Sequel calibrator compared to the colorant tags in the
>
profiles reported x, y values; all for the same monitor. All on a Mac, OS
>
9.2, Photoshop 7.
We seemingly have three different ways of viewing the chromaticities of the
monitor, all from a single measuring session:
a) OptiCAL Information palette numbers.
b) Photoshop "Color Settings" numbers.
c) Colorant tags from the ICC profile.
First of all, chromaticity values for a monitor colorimeter use *no*
reference white. They are absolute measurements involving only the color on
the monitor and the CIE standard observer functions.
When an ICC profile is made, the measured primaries must be adapted from the
measured chromaticity of the monitor white (whatever it may be) to the
chromaticity of D50. This means that simply extracting the red, green and
blue colorant tags from the ICC profile gives the *adapted* chromaticities,
not the original measurements. If you want to retrieve the original phosphor
chromaticities, you must adapt them back from D50 to the original monitor
white (found in the media white point tag). In order to do the "back
adaptation" correctly, the *same* adaptation algorithm must be used as was
used by whomever created the profile (OptiCAL). In ICC v4 profiles, there
will be a tag that says what adaptation algorithm was used. In pre-v4
profiles (like those Bob probably has), you will have to guess. If you guess
wrong, you will not get the correct original measurements.
In Photoshop's Color Settings dialog, the chromaticities reported (using the
method Bob described) will have been "back adapted" using the Bradford
transform. I don't know what method is used by OptiCAL, but it might not be
Bradford.
I suspect the discrepancies are related to adaptation, especially since the
problem seems to become more pronounced as the monitor white point is
farther away from D50. Bob, if you e-mail me an OptiCAL profile, along with
the numbers you see in OptiCAL's Information palette, I may be able to
figure out what is going on.
--
Bruce J. Lindbloom
email@hidden
http://www.brucelindbloom.com
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