Re: Monitor calibration software/hardware
Re: Monitor calibration software/hardware
- Subject: Re: Monitor calibration software/hardware
- From: Marco Ugolini <email@hidden>
- Date: Fri, 21 Sep 2007 22:04:54 -0700
- Thread-topic: Monitor calibration software/hardware
In a message dated 9/21/07 4:53 PM, Chris Murphy wrote:
> As for validation I think it's OK to use the same measurement device
> for calibration validation. This is essentially pass/fail, and uses a
> behind the scenes set of metrics to determine that the expected
> behavior is what's occurring.
It's good to see that we agree on that point, Chris.
I am of the opinion that the monitor profile validation procedure (in
general, independently of the specific software package it comes with) is a
test *of the internal consistency of the profile itself* -- nothing more. In
other words, it either confirms or denies that the L*a*b* values as measured
off the calibrated and profiled display for certain device RGB patches
correspond to what the profile predicts they should be when those same
device RGB numbers are (1) tagged with the profile itself and then (2)
converted from that to L*a*b*. If the predicted L*a*b* values match the
measured L*a*b* values within a defined DeltaE tolerance, it's a match. If
not it's a fail. That's all.
In that sense, validation tells the user whether the profile is internally
sound or not according to the colorimeter used to create it. And for that
purpose, it makes perfect sense that the measuring device be the same as the
one that generated the measurement for the profile. Validation closes the
circle, so to speak.
Whether the colorimeter itself is accurate or not, in an absolute sense
(e.g., when compared with measurements of the same color samples taken with
a reference-grade spectroradiometer) is a separate issue. Basically, either
we trust the colorimeter or we don't. Lacking access to a spectroradiometer,
such trust, in turn, is based on trust in the opinion of more experienced
colleagues and specialists, and also in our own satisfaction with the
results. It's not rocket science, but it beats calibrating and profiling "by
eye" by a very long shot. And I believe that there is wide consensus on
that. (In saying this, I have in mind the empty boasts of one particular
"expert" who brags of being able to obtain perfect results simply by eye,
and claims to have done so more or less since the times of the Huns. Some
people on this forum know whom I'm talking about.)
Also, please remember that the validation procedure can be performed
*independently of one's profiling software*, by using a higher-grade
instrument (for example an EyeOne Pro) with MeasureTool (which is part of
ProfileMaker). One can create custom RGB charts by sampling the RGB Cube in
whatever resolution one desires (4x4x4, 8x8x8, 10x10x10, etc.). I measured a
16x16x16 testchart that I created (4,096 patches), a procedure that took
about 2.5 hours to complete. I also measured an 8x8x8 testchart (512
patches), which took a little over 20 minutes.
As you can see, one is not restricted to the number of color patches that
standard validation procedures use. One can create and use custom RGB
combinations of increasing subtlety as the number of patches increases.
Whether or not one wishes to spend 2.5 hours to verify with a
spectrophotometer the profile built with a colorimeter is a matter left to
the individual user and their desire and willingness to undergo that level
of effort.
Marco Ugolini
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