Re: X-Rite PhaserMatch Calibrations
Re: X-Rite PhaserMatch Calibrations
- Subject: Re: X-Rite PhaserMatch Calibrations
- From: Graeme Gill <email@hidden>
- Date: Thu, 26 Nov 2009 13:08:27 +1100
Joseph Yates wrote:
PhaserMatch has one measure a 140 Patch target with a spectro (we use a
EyeOne revD) and then sends the measured calibration data to the printer.
What is the nature of this chart ? Does it look like 4 sets of 35
colorant step wedges for instance ?
PhaserMatch also allows one to ³Export² the measurement data (as CIElab
values) as well as ³Average² several measurements before sending this
calibration data to the printer.
Does this Lab data get sent to the printer, or does PhaserMatch compute
the calibration and send the calibration information to the printer ?
I¹m trying to determine how well this calibration ³works² - by measuring a
2nd target after a calibration is sent to the printer and comparing the
CIElab values against the reference values...
So therein lies my problem trying to determine what these expected CIElab
values are.
You are making many assumptions here, the main one being that the
calibration process is attempting to match some set of Lab numbers.
It may not work like this at all. Most calibration systems are
setup to be white point relative for instance, so the target
changes with the color of the paper. Some are also colorant
maximum density relative, meaning that the output may get linearized,
but not made a consistent absolute level. Some may take the Lab
numbers and process them in some way before linearization (ie.
convert to XYZ and then apply a per colorant matrix to extract
a colorant level analog), etc.
PhaserMatch during the measurement process actually displays on screen the
actual Lab values but only a picture representation of the expect value.
Both Xerox and X-Rite have been not been of any assistance in helping me
find these reference values for the 140 patch target.
Even if the PhaserMatch software is generating the calibration, it's
targets may be in some other form than Lab values, may be in
some reference file or may be downloaded from the printer itself
(ie. the targets may be something that is set during testing
at the factory).
I¹ve search through the application support folder and could only find the
TIF CMYK target which as a last resort I suppose I could extract the Lab
values from but as Photoshop only indicates whole numbers of Lab values as
opposed to the 4 decimal points of the exported PhaserMatch Lab values I
was searching for something more exact.
How is Photoshop meant to know the corresponding Lab values for
the PhaserMatch CMYK ? It would need a model of how the printer
is meant to behave, ie. an ICC profile of target PhaserMatch
behavior. Were you supplied with such a profile ? Is there
any expectation that the printer will match this profile after
calibration, rather than the profile merely being a typical
characterization of the printer ?
[Converting from CMYK to Lab via a profile with floating
point is pretty easy - see icclu for instance <http://www.argyllcms.com/doc/icclu.html> ].
Second-
Once I have these reference values and the exported PhaserMatch values
I¹d like to compare the DeltaE differences between the two sets of values.
I¹ve searched around and found some sites that show what the formula is
but this is way beyond me and so am asking if some generous member of the
list had a pre-existing Excel sheet that I could plug these 2 sets of CIElab
values into. Please contact me off list.
<http://www.argyllcms.com/doc/verify.html> is one approach.
Graeme Gill.
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