[argyllcms] Re: Question regarding White Point and spotread

  • From: <robert@xxxxxxxxxxxxxxxxxx>
  • To: <argyllcms@xxxxxxxxxxxxx>
  • Date: Sun, 2 Nov 2014 13:40:54 -0000

I've repeated the spotread using the scanning rig and the results are pretty
much the same.

 

If I do repeated tests after setting a reference, the dE94 starts off at
around .005 and creeps up to around .02 over 15 or so readings.  Moving the
instrument changes the reading up to .25 max (over a 3cm square.  Changing
to the reflective read adaptor (without recalibrating) doesn't seem to
change things much (still around 0.3 max). 

 

Going back to my original question: could the error I'm seeing have to do
with the white point? If I measure the paper white using spotread I get a
value of 97.74, 0.2, -0.59, whereas xicclu gives 100.000000, -0.000062,
0.000060 for RGB of 1,1,1 (or a dELab of about 2.3).

 

Robert

 

  _____  

From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Brad Funkhouser
Sent: 02 November 2014 12:54
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Question regarding White Point and spotread

 

Have you spotread the patch 10 to 15 times in succession, to see your
measurement variability as the lamp heats up?  Also move the read point
around on the patch to mitigate small differences in direct reflections
caused by texture?  And even a tiny difference in instrument height above
the patch will change luminosity reading.  Is your spotread setup truly
identical to strip reading of original target?  Are you pressing down more,
or less for different readings, etc.





When I experimented with all these factors with i1pro, I was (wrongly)
expecting near perfection, and was surprised by the degree of variability.





- Brad






On Nov 2, 2014, at 3:59 AM, <robert@xxxxxxxxxxxxxxxxxx> wrote:

Hi Brad,

 

Between print and profiling I left the print overnight (minimum 16 hours).

 

After the spot test I left a couple of hours.  I've just redone the
measurement (so about 16 hours again) and the values are now:

88.01, -13.56, 87.23 (so a bit worse).

 

BTW . this may (or may not) be relevant: the profile was made using
i1Profiler (with 2584 patches) and not Argyll.  

 

Robert

 


  _____  


From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Brad Funkhouser
Sent: 02 November 2014 00:59
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Question regarding White Point and spotread

 






Curious... how long did the inks dry between print and measurement of the
profiling target?  And between print and measurement of the spot color test?






Thanks.






- Brad







On Nov 1, 2014, at 4:31 PM, <robert@xxxxxxxxxxxxxxxxxx> wrote:

I wonder if you would be kind enough to clarify something for me?

 

I'm trying to do a spot color test from a document through to print, and
this is what I'm doing (using Photoshop and Argyll):

 

1.      The spot color has a Lab value of 90, -14, 87 in Photoshop.
2.      I convert the document to the print destination space (relative
colorimetric with BPC).
3.      After conversion, the spot color is RGB 240, 247, 52 (or 0.941176,
0.968627, 0.203922).  The Lab value from Photoshop is still 90, -14, 87, as
expected.
4.       xicclu (rel. col.), with the RGB colors above through the profile
(forwards), gives Lab 90.33650, -14.520704, 87.115660. I assume that
Photoshop is effectively doing the same as xicclu but is rounding the
values.
5.      I also tried fakeread (rel.col) which gives me exactly the same Lab
values as xicclu.
6.      I print the image with no color management.
7.      spotread gives me Lab values of 88.987, -13.637, 87.268. This is a
dE-Lab of about 1.6 compared to the xicclu reading.

 

The dE-Lab error seems too high as I have only just calibrated the printer
(iPF6400) and profiled the paper (Canson Baryta, so good paper).

 

colverify has an option to normalise each file's readings to white XYZ, but
xicclu, fakeread and spotread have no such adjustment.  

 

I would have thought that the paper white would need to be taken into
account in comparing the spotread value to the image Lab value.  Should the
paper white be measured and the spotread value normalised?  If so, how
should this be done?

 

I appreciate your help.

 

Robert

 

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