[argyllcms] Re: Calibration of my colormunki

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 28 Apr 2010 19:12:22 +1000

Richard Hughes wrote:
When argyll does a calibration of the sensor for my ColorMunki, where
is the calibration stored? On the device itself or somewhere on the

It stores it in a file named ".mk_XXXXXX.cal" in the current directory,
where XXXXXX is the instrument serial numbers.

I ask because I'm currently using -N for spotread if the user has
already calibrated once since starting gnome-color-manager. Is this a
safe thing to do?

It is if you are running consecutive invocations of tools
with the same current directory.

If it's stored on the device, I could fallback to requiring
calibration once a week or so, rather than every single time you start
the program, which is what I have now.

I wouldn't really recommend that. For these sorts of instruments the
calibrations should be more frequent than that, since things like
black level are very temperature dependent, and LED output levels
are also temperature dependent (The ColorMunki actually measures
its LED temperature by measuring it's forward voltage drop, and
compensates for the relative warm up effects when it is turned
on for each strip reading. There is no absolute temperature
compensation though, it depends on the white reference for that.)

In fact temperature effects is one of the known problems with measuring
deep display blacks, since often displays are warmer than ambient
temperature (and high brightness can also directly warm the instrument
sensor), so the black level tends to rise from the point the instrument
is attached to the display.
So calibrating on just every use is a bit of a compromise - those after
high repeatability will actually want to calibrate more often than that.
(The Spectroscan automatically recalibrates every 50 readings for instance.
 I suspect the Isis does something similar.)

Laboratory grade instruments do things like having a mechanical
calibration shutter, so that they can calibrate before every reading.

Graeme Gill.

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