[argyllcms] Re: "Wrong" ambient light reading with colormunki
- From: Roger Breton <graxx@xxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 10 May 2010 21:33:11 -0400
Hi Marco,
Don't trust a reading of 0.09 cd/m2 with any instruments except from very
expensive colorimeters and spectroradiometers. Very few instruments have the
needed sensitivity to measure that few photons. You could aim for 0.5 to
1.0.
Best / Roger
> -----Original Message-----
> From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-
> bounce@xxxxxxxxxxxxx] On Behalf Of Marco Presi
> Sent: 10 mai 2010 21:01
> To: argyllcms@xxxxxxxxxxxxx
> Subject: [argyllcms] "Wrong" ambient light reading with colormunki
>
> 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.
>
>
>
>
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