[argyllcms] Re: How do I force dispcal to always map R0 B0 G0 -> R0 B0 G0 on the calibrated video LUT?

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 28 Dec 2009 10:12:37 +1100

Alexander wrote:
Why don't you try the adaptive mode and see what you

       spotread -e

vs. display mode:

       spotread -d

spotread -e appears to fluctuate less than half as much as spotread -d over
multiple readings, which is excellent. I'm definitely looking forward to
trying out dispcal -V when RC3 is released.

The measurements between the two modes differ a bit though. Should I be
assuming that the spotread -e measurements are more correct than the
spotread -d measurements?

Below are multiple measurements of the same gray patch on my CRT (set to
2048x1536 @85Hz / ~85cd/m2 white / ~0.01cd/m2 black) with an uncalibrated
video lut:

Graeme, do you have any insight about this?

Does the difference I'm seeing between the spotread -e and spotread -d modes
appear normal to you? For reference, I did use -N when switching between -e
and -d in order to eliminate the internal sensor calibration as an
additional variable.

> Avg 11.448286 11.986959 12.567771 41.195356 -0.781389 -8.206391
> Avg 11.129467 11.682361 12.249876 40.706755 -0.977966 -8.140483

That's a DE of 0.44 or so, which is not very great.

The fact that the non-linearity correction is reversed between
low and high gain may be a factor in the discrepancy that
will improve with the next release, but simply from an Engineering
point of view I wouldn't expect exact agreement between a mode which
changes integration time and gain vs. one with a fixed integration time.

The resulting color temperatures between the two modes seem close enough,
but the color coordinates and resulting Delta E values differ somewhat

Generally < 1 DE is regarded as insignificant.

They can't both be correct, so which should I trust as being more accurate?

Another thing that I found a bit curious was how much my measurements of a
completely black screen varied every time I did an internal calibration of
the sensor. Below are only -e results, but the -d measurements were also
showing similar changes between internal sensor calibrations when measuring
my near 0.00cd/m2 black-point.

That's the nature of a silicon sensor for you. The background noise is
proportional to temperature, so it can change noticeably for
very dark readings if the instrument changes temperature.

I also noticed what might be a degree of sensitivity to how well the
instrument is sitting on the white tile during calibration. You need to
make sure that it is sitting tightly, and isn't in bright light at the time.

The first internal sensor calibration gave me color temperature results like
the following:
CCT = 2500K
Closest Planckian temperature = 2500K
Closest Daylight temperature  = 2500K

The second internal sensor calibration gave:
CCT = 2500K
Closest Planckian temperature = 999999K
Closest Daylight temperature  = 34999K

The third internal sensor calibration gave:
CCT = 9000K
Closest Planckian temperature = 7000K
Closest Daylight temperature  = 7000K

See full results here: http://pastebin.com/f3cec1649

You're way down in the noise, so it's not that much of a surprise
that calculations that depend on the ratio's of the sensors values
bounce around wildly.

I don't expect the Eye-One Pro to be all that accurate so close to 0.00
cd/m2, but the above seems a bit odd. The first result seems sensible with
CCT, Planckian, and Daylight all having similar temperatures. On the second
the CCT stayed similar to the first, but the Planckian and Daylight
temperatures were being reported as out-of-range values. The third which was
taken about an hour later then the first also seems sensible, but it is
significantly different then the others. What do you think?

I suspect that is just the nature of it. It so happens that with
the CCT's skewed sense of "closest" it lands somewhere near a normal
value, but with the other readings truer sense of "closest" it reveals
that the point measured is almost equally distant from a couple of
points on the locus. In other words, color temperature isn't
a very meaningful measure for colors that aren't close to the daylight
or planckian locus.

Graeme Gill.

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