[argyllcms] Re: Calibration of my colormunki

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 29 Apr 2010 13:02:54 +1000

Ben Goren wrote:
Does the calibration really need to be saved to a file at all, then? Why not 
just keep
it in memory and never write it to disk?

It's because of the nature of the Argyll toolset. Each tool
is a separate executable process that gets created and
destroyed. So to keep the cal. in memory would require some sort
of independent server, and that would be complete overkill. Saving
the calibration in a file is simply to allow the -N option to work,
so that a sequence of tools can be used with one calibration at the start
of the sequence.

...and, any chance of adding an option to have chartread (and maybe dispread) 
every N patches for a recalibration? Perhaps even a sliding scale with frequent
recalibrations early on, and longer intervals if the instrument appears to have

It would be nice to add something like this. I have added it as a manual
option to spotread, and a similar thing could be done with chartread,
but I'm not sure how it would be managed with dispread.

I'm also thinking it might be helpful to have some sort of an auto-test mode 
calibrates, runs the lamp for N seconds, calibrates, repeats, and reports when 
stabilized. And perhaps even an option to have the instrument warm up with the 
lamp on
for x seconds before starting to read the chart.

I'm not sure that would work. Certainly for the i1pro the lamp has
it's own thermal dynamics, and it's hard to model (I've tried). The
thermal dynamics gets more obvious as the lamp ages (which is why
they should/would replace it when sent in for recalibration). For
all their disadvantages (lumpy spectrum and lack of UV), LED's have
the advantage of not suffering so much from thermal dynamics and
aging, and the thermal compensation in the Munki works very well.

The munki readings vary imperceptibly from one to the next
on the same spot, irrespective of the timing, whereas excellent
consistency for the i1pro demands exactly the same timing,
or letting it cool down completely between readings.

In practice it's not a huge problem, since repositioning
errors/variation dominate ie. the paper location variance
is of the order of 0.3 delta E for most media, which is
larger than the lamp thermal dynamics errors. By letting the
lamp cool down between readings and carefully not repositioning
the instrument I can get reading consistency of about 0.005 - 0.020 delta E.
with the i1pro, similar to the Munki. When it comes to strip reading,
there are worse problems (strip edge detection, total integration
time, and lamp thermal changes), and once again the LED with thermal
compensation has an advantage with regard to the latter.

Graeme Gill.

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