[argyllcms] Re: ColorMunki measurement drift

  • From: Juergen Lilien <jp.lilien@xxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 28 Sep 2010 05:37:01 +0200

Graeme Gill wrote:

Hello Graeme!

> Without an independent instrument to confirm that
> the display is perfectly stable, there could also be display
> drift effects, in spite of it having been on for several hours
> (Consider: if the display is affected by it's temperature, then
>  anything that changes the rate at which heat is dissipated from
>  the display and/or instrument may affect the readings. So a change
>  in air temperature due to ambient temperature change, room heating/cooling
>  cycles, a door being opened or closed etc., could have subtle effects.)

You are absolutely right, there are many variables.
For example, one point that I don't have full control of is the
self-adjusting capability of the monitor. It has a temperature sensor
to stabilize itself, but I can not control/observe when it is actively
altering the screen. I can only hope the implementation is good and
actually does its job (stabilizing) and the intervention is minimal
after having the monitor on for several hours.
What I can do is to manually trigger the monitor to measure and
self-calibrate its brightness level. When I do this after the ColorMunki
measurement has drifted (shows a lower luminance than at the beginning)
the ColorMunki measurement reading does not change. My assumption
is, that there is no actual internal luminance drift in the monitor,
otherwise the self-calibration would have altered the backlight
brightness, which had to show up in the ColorMunki measurement.
So at least the luminance drift seems to happen in the ColorMunki
and not the monitor.

>> The problem is, that
>> the first (highest) readings seem a lot more reasonable compared
>> to my pre-calibrated monitor presets, ...
> I don't see why you are assuming that the pre-set is the reference.
> The point of using an instrument is that it is assumed that it
> will be more accurate that the state of the display.

I bought the ColorMunki in the hope that it is more accurate
than the monitor, otherwise I wouldn't have spend the money.
But I have a suspicious mind and need some evidence. ;-)

In general the better NEC monitors seem to be really precisely
factory calibrated, they also need to measure each monitor
individually to program the uniformity compensation. What remains
is the problem of backlight aging, but they claim to compensate
this also - time will tell. Actually I found a message on the
luminous-landscape forum where a user suggests to evaluate the
accuracy of probes with a new NEC PA display (preset 6500K). ;-)
Let me quote his measurements (with SpectraView II):

Spyder3 colorimeters - 7500-8000 K,
i1Display 2 colorimeters - 5700-5800 K,
i1Pro spectros - 6450-6550 K,
ColorMunki spectro - 6400-6600 K,
NEC MDSVSENSOR2 colorimeters - 6450-6500 K.

Because of this, I have more confidence in my first ColorMunki
6610K measurement reading than in the later drifted (but more
stable) 6700K.

>> As I understand the new -I option will only help to counteract
>> drifting in the brightness level measurements, but does not
>> help with the color/white point drift?
> It can help with instrument black point drift and display white
> point change. It won't make the instrument more accurate in
> an absolute sense, but minimizes the effect of discrepancies
> between the instrument and display on calibration and profiling.

I asked, because in the Dispcal doc you only mention "Display white
level drift compensation" with the -Iw option, but not white point
compensation. I have to try it...

Best regards, Juergen

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