Most laptop computers are designed to a price-point and have cheap LCD displays. The native display response is generally awful and can be improved by calibration. This post describes calibration for an Acer Aspire 5742 laptop with WLED (White LED backlight) display. It complements a previous post about calibrating a NEC P221W display. I have modified the Microsoft Excel workbook for checking and reporting the results to Open Office format (download ArgyllView).
Fedora 20 has very limited support for the Intel HD graphics in the Aspire 5742. The display brightness hot-keys do not work (no hardware display adjustments) and there are no relevant Display settings (no software display adjustments). I did not attempt to install proprietary video drivers.
To disable any display profiles in Fedora 20, open Settings/Color, select the relevant display and switch colour management OFF.
Argyll CMS can reset the graphics card look-up tables with:
Display warm up
I measured the display response every 10 minutes or so until the white level and colour temperature stabilised:
dispcal -yl -r -X WLEDFamily_07Feb11.ccss
-X CCSS file selection.
-r report the display response (uncalibrated if dispwin -c was run first).
WLEDs emit less heat than CCFL (Cold Cathode Fluorescent Lamp) backlights and tend to warm up quickly. However, it took 30 minutes for colour temperature to stabilise.
Display uniformity (optional)
I measured the response at nine positions on the screen:
dispcal -X WLEDFamily_07Feb11.ccss -r -P0.05,0.05,1.0 (top left)
dispcal -X WLEDFamily_07Feb11.ccss -r -P0.05,0.5,1.0 (middle left)
dispcal -X WLEDFamily_07Feb11.ccss -r -P0.05,0.95,1.0 (bottom left)
dispcal -X WLEDFamily_07Feb11.ccss -r -P0.5,0.05,1.0 (top centre)
Moderate variation in white level and colour temperature measurements above could be due to age. When new, uniformity measurements for the Aspire 5742 were very good.
Native response (optional)
First, I checked the basic display response:
dispcal -X WLEDFamily_07Feb11.ccss -r
Black level = 0.53 cd/m^2
White level = 218.7 cd/m^2
Approx. Gamma = 2.57
Contrast ratio = 411:1
White Correlated Daylight Temperature = 7604 K, DE 2K to locus = 11
The black level above is moderately high, the white level is very high and the white point is far from the daylight locus (DE > 2).
Second, I measure a series of grey patches:
spotread -X WLEDFamily_07Feb11.ccss -T
-T report correlated colour temperatures
The neutral response was very poor, with a huge 8189 K colour temperature (Closest Daylight Temperature) standard deviation (excluding black and 80% black).
Finally, I used spotread again (without the -T option) to measure RGB patches and check the display colour gamut.
I calibrated for general use, with native white point, 2.2 gamma and moderate 160 cd/m2 white level. I attempted a D65 white point (e.g. sRGB), but the calibrated response appeared ‘muddy’.
For narrow gamut diplays, I prefer the native white point. Human colour vision can adapt to different white points.
dispcal -v -yl -X WLEDFamily_07Feb11.ccss -ql -g2.2 -b160 Aspire5742F150313
-v verbose mode.
-ql quality = low (fastest).
-g2.2 target gamma = 2.2.
-b160 target brightness = 160 cd/m2 (no hardware adjustment for the Aspire 5742 display under Fedora 20).
Aspire5742F150313 is the name of the .cal file output
Calibrated response (optional)
First, I loaded the calibration:
Then I measured the display response, the neutral response and the colour gamut.
dispcal -X WLEDFamily_07Feb11.ccss -r
Black level = 0.52 cd/m^2
White level = 159.6 cd/m^2
Approx. Gamma = 2.19
Contrast ratio = 305:1
White Correlated Daylight Temperature = 7511 K, DE 2K to locus = 11
Compared to the Native response, calibration reduced the white level and gamma. The contrast ratio also decreased because the black level remained high. Cheap displays can only achieve high contrast at high brightness.
Greyscale colour temperature after calibration averaged 8579 K and was fairly uniform (moderate standard deviation of 235 K, excluding black and 80% black).
Calibration hardly affected the colour gamut (shown in the gamut plot under Native response above).
The command targen is used to create test values, dispread measures these patches and colprof computes the profile:
targen -v -d3 -f400 Aspire5742400patches
-d3 colourant combination 3 = Video RGB.
-f400 number of patches = 400.
Aspire5742400patches is the name of the .ti1 file output.
dispread -v -yl -X WLEDFamily_07Feb11.ccss -k Aspire5742F150313.cal Aspire5742400patches
-k applies the calibration file Aspire5742F150313.cal and includes it in the output .ti3 file.
Aspire5742400patches is the name of the .ti1 file input and the .ti3 output.
colprof -v -D”Aspire 5742 150313 F20 native 160 cd/m2″ -qh -al Aspire5742400patches
-al creates a L*a*b* look-up-table profile (default).
Aspire5742400patches is the name of the .ti3 file input and the .icm output.
Loading the display profile (in Fedora 20)
I installed the new profile with the following procedure:
- Copy the new profile to /usr/share/color/icc/.
- Open Settings/Color and select the display.
- Switch colour management ON.
- Add the new profile.
- Select the new profile and Enable it.
I used spotread to measure a series of sRGB patches in Gnome Image Viewer and compared these measurements to reference data. Gnome Image Viewer is colour-managed. Last time I checked, Shotwell Viewer was not.
The Aspire 5742 shows large errors for sRGB patches (DE > 2). For narrow gamut displays and native white point calibrations, the Calibrated response checks above may be more informative than these colour differences.
The Aspire 5742 is not recommended for colour work. However, calibration has tamed the brightness and improved the greyscale response (for the native white point). I find the calibrated display is satisfactory for viewing sRGB images when I’m away from my NEC P221W display.