Juergen Lilien wrote:
In a first try I attached the ColorMunki to the screen and let it take more than 1200 white point measurements (hires adaptive mode) in a row, this took roughly an hour. At this time the monitor was several hours in on state, so it should be pretty stable.
It's not unexpected that such an instrument will drift due to temperature effects over a time period like an hour, if it at a different temperature when the measurements first started. It takes a more expensive instrument (ie. with a heater/cooler on the sensor, or a temperature sensor and compensation tables, or some sort of internal calibration tile that it can automatically calibrate to) to be able to counteract such effects. This is a basic characteristic of using silicon for the sensor.
The color (X/Y) measurements are even more interesting, they are also decreasing but the variance is not constant, there are knots in the point graphs with less variance and the period between the knots is increasing over time - the pattern is more stretched. At the end the measurements seem to have reached a semi constant state with nearly no further decreasing tendency.
The "knots" could be quantization artefacts or something similar. I'm not sure that I'd say that the Y isn't stabilizing, as it does have a convex shape. 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.)
My first thought was to wait an hour with the ColorMunki on the screen, until the color measurements reach the stable bottom, before starting calibration and profiling. The problem is, that the first (highest) readings seem a lot more reasonable compared to my pre-calibrated monitor presets, and even after a self- calibration of the ColorMunki they never get back to the start level. Yes, this effect (temperature drift?) seems to be at least partly immune against self-calibration of the instrument!
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.
To prove this I started a second series of measurements with a self-calibration after each single measurement and I still see decreasing luminance and X/Y color values.
That's interesting, but without some sort of independent reference, it's hard to know exactly what is changing.
Should I reinstall the X-Rite software and check if I see the same decreasing measurement values?
I'd certainly be interested in any evidence that there is different behaviour with the X-Rite drivers.
And what is the practical relevance? The DE of the first measurement was 2,6 and it dropped to 0,7 at the end, a 1,9 DE white point difference should be visible(?).
You can't judge the visibility of a difference in degrees Kelvin or xy coordinate. You need to convert to DE to make this assessment.
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. [Note you can use these corrections independently.] Graeme Gill.