Re: Comparison of 2 spectros using Delta e
Re: Comparison of 2 spectros using Delta e
- Subject: Re: Comparison of 2 spectros using Delta e
- From: david wollmann <email@hidden>
- Date: Mon, 17 Jan 2011 14:22:20 -0700
Thank you Terry for your input and insight. What you said made sense
to me. I had thought about the fact that I was measuring two different
targets and not the same patches. I will read one target again using
the DTP41 and spot mode with the i1 and see what the results are.
I also like the idea of treating it like its an offset press by
rotating the targets and placing them in different locations.
Certainly a good way to go about getting good data for averaging.
Thanks for the link to XRGA on Xrite's site and also Print Tools.
David
Re: Comparison of 2 spectros using Delta e (Terence Wyse)
On Jan 14, 2011, at 2:02 PM, david wollmann wrote:
Hello everybody,
I've been doing an exercise here comparing two different
instruments, an i1Pro and a DTP41. Both devices have the UV filter.
I made a target of the it8 basic chart since I can fit all the
patches in the area of an 8x10 inch print. I then printed both at
the same time on some 24 inch wide fine art paper using an Epson
9800.
The Delta E 94 values are
Total - 1.63
Best 90% - 1.50
Worst 10% - 2.81
The worst 10% are mainly in the cyans and a few light yellows and
one green patch.
I guess my question is what all can learn from this?
That inter-instrument agreement is an issue! It doesn't surprise me
that a (former) Gretagmacbeth instrument, the EyeOne Pro, and the
XRite DTP41 don't agree all that well. The "spectral head" and
optics on these instruments are quite different. You'll find better
agreement among Gretabmacbeth devices and among X-Rite
devices....but not so much to each other.
While you said you printed them at the same time, I would make sure
you read *the exact same chart*. Obviously, the DTP41 requires a
special layout to be read in strip mode...so I would print the DTP41
chart, measure it, and then with the EyeOne Pro in "spot" or "patch"
mode instead of strip mode, read the chart perhaps 3-5 times (in
different spots within each patch obviously) and average the results
and then compare that.
The other option would be to print your current charts several times
in differing orientations (4 prints at 90 degree intervals) to
compensate for subtle coverage/inking differences on your 9800. At
the very least, print as many as you can across the 24" width and
measure/average all of them. Believe it or not, patch layout makes a
difference AND differences will show up across the width of the
printer.
Since this is only a comparison between two devices with no
standard reference how would I determine which spectrophotometer is
giving me more accurate readings?
Not easy....maybe impossible. There was a Gretagmacbeth program
called NetProfiler that was an effort to resolve these disagreements
but it was only designed to work with GMB SpectroEyes.
You could use a standard reference such as the "Lab Reference Card"...
http://www.printtools.org/pt/color-reference-cards/lab-reference-card
...but that's designed to be used with hand-held spectros. You could
measure it using your EyeOne Pro but it's not formatted to work with
the DTP41. (they do offer a 36mm patch size version but without an
image or any further information, it's unclear whether it's
formatted to work with the DTP41 or not).
The only way to get some sense of which spectro is more accurate,
you'd need to introduce a 3rd or 4th spectro into the mix....even
then, you're only going to reach a *consensus* between the different
spectros, not which one is the most accurate in an absolute sense.
There's a new thing XRite came up with, XRGA, that attempts to
reconcile the difference between legacy XRite and Gretagmacbeth
instruments and their different calibration standards. You can find
out more here:
http://www.xrite.com/product_overview.aspx?ID=1336
I haven't looked into XRGA enough to know whether it resolves inter-
isntrument disagreements or not.
My subjective evaluation, passed on the screen representation of
the lighter tone patches, is that the i1 is "cleaner."
Not sure how you evaluated this (assigned profiles made with each
spectro to the image or what?) but if the raw measurements are
telling you that the EyeOne Pro is measuring lighter or whatever, a
profile made from this instrument would have the opposite affect in
the final print. IOW, if the spectro is measuring lighter and this
profile is used as a destination profile, the printed result would
be darker since the profile conversion would attempt to "correct"
for the lighter appearance of one spectro vs. another. Only way this
would NOT be true if both source and destination profiles were make
with the same instrument. Make sense?
Regards,
Terry
______________________________________
Terence Wyse, WyseConsul
Color Management Consulting
G7 Certified Expert
FIRST Level II Implementation Specialist
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