[argyllcms] "Wrong" ambient light reading with colormunki

  • From: "Marco Presi" <marco@xxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 11 May 2010 02:00:45 +0100 (BST)

Hi,

I am trying to calibrate an LCD display by using a colormunki.  As I am
doing these things for the first time, I am trying to compare results with
the x-rite software coming with the device and argyll.

I found that the ambient light measurement is constantly "lower" if made
with argyll. I have been very careful in keeping the same conditions while
during the measurements: as an example, when the x-rite software returns
80 lux, argyll returns approx. 50 lux. Doing the same measurement on a
debian gnu/linux box gives exactly the same result (please note that the
x-rite software is not available on linux). I tried the same measurements
at different night times and at different days: the values I obtain can be
reproduced with good accuracy (x-rite software returns 80, argyll approx.
50).

how could this be fixed? could this problem affect all the calibration
process? (I mean, if the sensor reads with an offset, this error is made
also during the patch examinations?)

In addition, when completing the calibration with argyll (I tried to read
the docs carefully), the black point looks too bright: I tried to
compensate it by using all the options I found (-B, -f,), but nothing
changes. I am attaching here the output of dispcal: at the beginning, it
declares that the target black level is the "native", but it then set the
target black to 0.9. However, as can be seen from the "dispcal -yl -R"
output, the monitor is able to reach black levels as low as 0.09 cd/m^2

So, why during calibration dispcal decides to shift up the black point
which results in a lower contrast?!? This behavior is also identical both
in windows and linux.


Thanks for your attention!


Ciao,

Marco


zsh 1009 % dispcal -yl -R
Set instrument sensor to calibration position,
 and then hit any key to continue,
 or hit Esc or Q to abort:
Calibration complete

Place instrument on test window.
Hit Esc or Q to give up, any other key to continue:
Uncalibrated response:
Black level = 0.09 cd/m^2
White level = 91.39 cd/m^2
Aprox. gamma = 2.17
Contrast ratio = 975:1
White chromaticity coordinates 0.3283, 0.3400
White    Correlated Color Temperature = 5688K, DE 2K to locus =  2.1
White Correlated Daylight Temperature = 5689K, DE 2K to locus =  2.9
White        Visual Color Temperature = 5634K, DE 2K to locus =  2.0
White     Visual Daylight Temperature = 5763K, DE 2K to locus =  2.7
Effective LUT entry depth seems to be 8 bits
The instrument can be removed from the screen.



---------
zsh 1008 % dispcal -yl -b90 -gs -t5500 -a40 -f -v5 sm206bw
Setting up the instrument
Instrument Type:   ColorMunki
Serial Number:     2021361
Firmware version:  288
Chip ID:           01-60d06812000029
Version string:    'colormunki FW V1.32  Build Nr. 1303'
Calibration Ver.:  6
Production No.:    35276
Set instrument sensor to calibration position,
 and then hit any key to continue,
 or hit Esc or Q to abort:
Calibration complete

Place instrument on test window.
Hit Esc or Q to give up, any other key to continue:
Display type is LCD
Target white = 5500.000000 degrees kelvin Daylight spectrum
Target white brightness = 90.000000 cd/m^2
Target black brightness = native brightness
Target gamma = sRGB curve

Display adjustment menu:
Press 1 .. 7
1) Black level (CRT: Offset/Brightness)
2) White point (Color temperature, R,G,B, Gain/Contrast)
3) White level (CRT: Gain/Contrast, LCD: Brightness/Backlight)
4) Black point (R,G,B, Offset/Brightness)
5) Check all
6) Measure and set ambient for viewing condition adjustment
7) Continue on to calibration
8) Exit
Doing check measurements
Black = XYZ   0.05   0.05   0.06
Grey  = XYZ  19.96  20.14  26.49
White = XYZ  87.09  90.23  88.64
1%    = XYZ   0.77   0.79   1.05

  Target Brightness = 90.00, Current = 90.23, error =  0.3%
  Target 50% Level  = 19.63, Current = 20.14, error =  0.6%
  Target Near Black =  0.90, Current =  1.07, error =  0.2%
  Target white = x 0.3325, y 0.3475, Current = x 0.3275, y 0.3393, error =
 4.38 DE
  Target black = x 0.3325, y 0.3475, Current = x 0.2969, y 0.3013, error =
16.88 DE

Press 1 .. 7
1) Black level (CRT: Offset/Brightness)
2) White point (Color temperature, R,G,B, Gain/Contrast)
3) White level (CRT: Gain/Contrast, LCD: Brightness/Backlight)
4) Black point (R,G,B, Offset/Brightness)
5) Check all
6) Measure and set ambient for viewing condition adjustment
7) Continue on to calibration
8) Exit
Exiting
The instrument can be removed from the screen.





Other related posts: