[argyllcms] Re: Display calibration

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 01 Dec 2007 22:32:34 +1100

Frédéric Mantegazza wrote:
I received the i1 Display 2 colorimeter for the photoclub, and made a first profile. Results are strange... First, when I load the profil in iccexamin and lprof, they both say that White Point is near 12000K! But during measurements, argyll said 6700K.

Which version of Argyll are you using ?

The profile white point certainly look strange.
I suspect that iccexamin isn't computing the white point
temperature the same way as Argyll though, or the extreme white point
is showing up the difference between Visual Daylight Temperature
(which is where Argyll uses a modern measure of closest to
the daylight locus - CIEDE2000), vs. the traditional CIE 1960 UCS
space color difference formula used for Correlated Daylight Temperature,
or Correlated Color Temperature (black body locus).

Second, it seems that red color is over-saturated...

Neither the calibration nor channel response curves look unusual.

Here are the commands I used :

$ targen -v -d3 -f200 A901HT
$ dispcal -v -y c -d2,1 -m A901HT
Setting up the instrument
Place instrument on test window.
Hit Esc, ^C or Q to give up, any other key to continue:
Display type is CRT
Target white = native white point
Target brightness = native brightness
Target gamma = 2.200000

Press 1 .. 7
1) Black level (CRT: Brightness)
2) White point (Color temperature, R,G,B, Gain)
3) White level (CRT: Contrast, LCD: Brightness)
4) Black point (R,G,B, Offset)
5) Check all
6) Continue on to calibration
7) Exit
Doing some initial measurements

This is strange. You specified -m, yet dispcal hasn't skipped
the monitor adjustment step.

Adjust R,G & B gain to desired white point. Press space when done.
  Initial B 110.80, x 0.1985, y 0.3899, VDT 6762K DE 30.2
/ Current B 110.72, x 0.1984, y 0.3907  VDT 6753K DE 30.2  R+  G-- B+

Note the white point is correlated to 6753K with a delta E of 30 ! This means
that the white point is far from actually being at 6753K, merely that 6753K is
the closest daylight temperature to the actual white point.

If you want to actually have a white point lying on the daylight temperature 
locus,
you need to adjust the RGB gain controls so as to reduce the delta E to
near zero. Notice that the interface is suggesting you reduce green to
move the white point in the right direction, or possibly increase Red and Blue.

Adjust CRT Contrast or LCD Brightness to desired level. Press space when done.
  Initial 110.72
/ Current 80.77

Press 1 .. 7
1) Black level (CRT: Brightness)
2) White point (Color temperature, R,G,B, Gain)
3) White level (CRT: Contrast, LCD: Brightness)
4) Black point (R,G,B, Offset)
5) Check all
6) Continue on to calibration
7) Exit
Doing some initial measurements

Adjust R,G & B offsets to get target x,y. Press space when done.
   Target B 0.81, x 0.1978, y 0.3919
/ Current B 0.86, x 0.1812, y 0.4670  DE 24.4  R+  G-  B++

The black aim point is affected by the choice of white point.

Press 1 .. 7
1) Black level (CRT: Brightness)
2) White point (Color temperature, R,G,B, Gain)
3) White level (CRT: Contrast, LCD: Brightness)
4) Black point (R,G,B, Offset)
5) Check all
6) Continue on to calibration
7) Exit

Commencing device calibration
patch 6 of 6
Black = XYZ   0.05   0.11   0.12
Red   = XYZ  16.38  16.37   1.88
Green = XYZ  12.68  58.07  10.56
Blue  = XYZ  11.90   6.92  72.16
White = XYZ  40.74  80.85  84.70
patch 64 of 64
Initial native brightness target = 80.845374 cd/m^2
Had to scale brightness from 80.845374 to 80.818901 to fit within gamut
Target white value is XYZ 40.724326 80.818901 84.670188

Notice the strange X value - this is because you've let dispcal calibrate
to the native white point, and you've set your native white point to be
extremely green.

Adjusted target black XYZ 0.05 0.11 0.11, Lab 1.20 0.00 0.00
Target black after min adjust: XYZ 0.05 0.11 0.11, Lab 1.22 0.00 -0.00
Current gamma curve offset = 0.065136, Gamma curve power = 2.420341
Total Iteration 3, Final Samples = 64 Final Repeat threshold = 0.800000
Creating initial calibration curves...
Doing iteration 1 with 16 sample points and repeat threshold of 3.200000 DE
patch 16 of 16
Brightness error = 0.000000 cd/m^2
White point error = 0.000000 deltaE
Maximum neutral error (@ 0.277258) = 2.569797 deltaE
Average neutral error = 1.061537 deltaE
Computing update to calibration curves...
Doing iteration 2 with 32 sample points and repeat threshold of 1.600000 DE
patch 32 of 32
Brightness error = 0.000000 cd/m^2
White point error = 0.000000 deltaE
Maximum neutral error (@ 1.000000) = 1.000000 deltaE
Average neutral error = 0.527490 deltaE
Computing update to calibration curves...
Doing iteration 3 with 64 sample points and repeat threshold of 0.800000 DE
patch 64 of 64
Brightness error = 0.000000 cd/m^2
White point error = 0.000000 deltaE
Maximum neutral error (@ 1.000000) = 1.000000 deltaE
Average neutral error = 0.450462 deltaE
Computing update to calibration curves...
The instrument can be removed from the screen.

$ dispread -v -y c -d 2,1 -k A901HT.cal A901HT
$ profile -v -A "901HT" -q m -a s A901HT

Graeme Gill.


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