Hi,
There is a lot of confusion about terms illuminant and white point. The
ICC spec itself, was very unclear on versions earlier than 4.0, lets see
if I could clarify a little bit:
For printer profiles:
- Illuminant is the light source being used to illuminate the
paper. ICC states clearly that it have to be based on D50. If you want to
use any illuminant other than D50, you have to chromatically adapt the
colors to D50 and then store the chromatic adaptation matrix in the CHAD
tag, which is optional.
- Media white: is the chromaticity of the unprinted paper as
illuminated by a D50 light.
For monitor profiles: Since monitors are emitting light, ICC states a
virtual reflective media is modeled by monitor. This media has D50
chromaticity, so:
- Illuminant: Is the white point of the monitor
- Media white: is D50 always.
That implies most monitor profiles have CHAD tag .
Also, what ICC calls ICC-absolute colorimetric, for CIE is relative
colorimetric. In practice, what ICC-absolute colorimetric means, is the
observer being fully adapted to destination illuminant. In LittleCMS you
have a function cmsSetAdaptationState to control this behavior.
Ø For relative colorimetric, XYZ values are normalized by the media white.
The media white is the measured XYZ values of the device white (i.e. the XYZ
values of paper illuminated by D50, or of a white patch displayed in the
monitor).
That is right for printers, but not for monitors, where a white patch is
always represented as D50 in the PCS
Ø Also, I learned that absolute colorimetric values are the relative
colorimetric values adapted to the profile illuminant which in most cases is
D50.
Thats right for monitors, but not necessarily for printers. In absolute
colorimetric, for example, going from SWOP to ECI printer, unprinted paper
comes yellowish, with relative colorimetric unprinted paper remains white
(no ink is dropped). Printer profiles with non-D50 illuminant are pretty
rare.
All the best
Marti Maria
The littleCMS project
www.littlecms.com
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Hector J.
Santos-Villalobos
Sent: Friday, May 16, 2008 7:04 PM
To: lcms-user@lists.sourceforge.net
Subject: [Lcms-user] Difference between media white point and illuminant
Dear all,
What is the difference between media white point and the illuminant inside
an ICC profile? I know the Relative colorimetric intent is media-relative;
its values are adjusted to the media white. Does this mean the measured XYZ
values are normalized by the measurement of the device white, or the XYZ
values are normalized by the illuminant (i.e. D50 XYZ values)?
==This is what I assume is correct ==
For relative colorimetric, XYZ values are normalized by the media white. The
media white is the measured XYZ values of the device white (i.e. the XYZ
values of paper illuminated by D50, or of a white patch displayed in the
monitor). Also, I learned that absolute colorimetric values are the relative
colorimetric values adapted to the profile illuminant which in most cases is
D50.
Is all this correct?
Thanks for your help (again),
Hector
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