How do I calibrate two displays to the same color? (LCD, LED backlight and CCFL backlight)

Question

I've got two displays. One is approximately six years old: Fujitsu P24-5 ECO (LCD/IPS, CCFL backlight), the other one is a DELL U2412M (LCD/IPS, LED backlight).

I've tried to color calibrate both displays (LinuxMint, dispcalGUI, 6500K, 140cdm²) with three different colorimeters that I could access (an old-ish Pantone Huey Pro, a brand-new Colormunki Smile and my very own brand-new Colormunki Display).

With all three colorimeters (and a handfull of different settings concerning the profile type etc, including color correction matrices for the Dell Display with the Colormunki Display - couldn't find correction matrices for the Fujitsu) the results were essentially the same: Checking with the eye the white point seemed to be very close for both displays, as were brightness etc. But on closer Inspection the Fujitsu does have a slight red-ish tint (compared to the DELL, that doesn't yet imply which one is closer to the "real" colour). This is almost not noticable (i. e. only with side-by-side comparison) for sunsets or brown leafs, but it's a big difference for lighter skin tones (giving someone a sunburn only on the Fujitsu Display).

On yet closer inspection the TRC curves of the display profiles (that I can view from the colour panel in my system settings) look a bit different:

The Dell TRC curve has all three colors of RGB neatly on top of each other.

The Fujitsu TRC curve seems to indicate that red is somehow intensified.

That said, I don't know the real meaning of this curve. Especially I don't under stand why, if its meaning should be "right, the Fujitsu is tinted red" why this wasn't compensated through the display. Or could it be the other way round: Red is intensified by the profile because the instrument read a smaller brightness for red and wants to compensate for that.

But then, if the latter should be the case: I did calibrate the displays with the colorimeters before each profiling, so the colors brightnesses at maximum brightness should have been essentially the same to begin with - at least that was the colorimeter readout during calibration.

Could it be, that the instruments read the three colors differently during calibration vs. profiling?

I really don't understand what's happening here. My expectation would have been two essentially equal color reproductions on both displays.

Would it help to buy a spectrometer (e.g. Colormunki Photo)? Or what else could I do?

EDIT: MirekE suggested in an answer to try and set the standard observer on dispcal (-Q 1964_10). Unfortunately this doesn't change much and the problem still exists.

Answer 1

In DispCalcGui go to Options->Set additional command line arguments... New dialog box should open with a list of tools. In the box that says "dispcal" enter -Q 1964_10 and see if it makes any difference. This parameter sets standard observer to CIE 1964 10º, which reportedly improves the results when matching multiple displays with different backlighting technology.

Answer 2

To answer my own question: As it turns out, the effect I was seeing was created by some weird color management behaviour in darktable. Even though darktable-cmstest reported for both screens that "the X atom and colord returned the same profile", the weird behaviour went away with a configuration setting: I've now set the GUI option how darktable locates the color profile from all to Colord, and everything works fine, so far. :-)

I achieved this thanks to help on the dispcalgui-users ML. (Follow the link for the thread there.

Answer 3

I'v get a Spyder4Elite where you can choose an option to match multiple screens called "StudioMatch". Don't you have the same with the ColorMunki software? I suppose it's named differently.