[argyllcms] Re: Difficulty profiling new iMac
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 14 Sep 2007 18:07:26 +1000
OK, here's a bit of a clue. If I put up a black test square on
my MacBook with a linear Video LUT at maximum brightness, I get
the following with several different instruments:
X Y Z L* a*
b*
Spectrolino: 0.366415 0.352546 0.692548 3.184534 1.069715
-7.584697
Eye-One Pro: 0.405160 0.391266 0.731828 3.534291 1.126687
-7.723253
Eye-One Display: 0.909546 0.913935 1.476462 8.252869 1.108462
-10.501008
DTP94: 1.010000 1.010000 1.850000 9.074471 1.321416
-13.169153
White (Eye-One Pro) 207.192652 213.696473 231.775628 133.413707 1.192318
-24.608663
I'm, guessing that this machine uses LED backlighting.
So the colorimeters seem to be rather different to the spectrometers, as well
as the difference between white and black color being more extreme. An
explanation
for a "kink" in the blue is that by default for an LCD display, dispcal
doesn't aim to make the whole curve neutral (ie. b* = -24, dE to black in b* of
17)
since this would lighten up the black too much, but it does so down to a
point where it then crosses over to the native black (ie. it does -k 0.0).
This isn't spread evenly over the whole curve, on the basis that it is
desirable to get a good neutral for more of the whole curve. The differences
between the instruments may not mean much as far as the visual effect goes,
although it's another complicating factor.
Another complicating factor with a test ramp starting at
L* = 0, may be the clipping/gamut mapping behaviour of the
CMM when the L* min of the display is 8. This depends on whether
a true gamut mapped perceptual mapping is being used, or
black point compensation, or not.
I can think of two approaches to improve the situation. One would be
to aim for a white point that is more consistent with the color of the
black. (You'd have to convert the black to an x,y value and feed it into
dispcal -w. Hmm. I should add an xy reading mode to spotread!)
The other thing I could do is to make the transition from neutralizing
to not neutralizing more gradual and spread out. I'm not sure if
another parameter is needed, or whether all LCD's will be better off
with a more gradual transition.
Cutting the MacBook brightness back gives:
X Y Z L* a*
b*
Eye-One Pro: 0.201529 0.195416 0.362346 1.765181 0.529371
-3.797652
Eye-One Display: 0.397316 0.394413 0.658303 3.562717 0.687412
-6.286115
DTP94: 0.450000 0.440000 0.820000 3.974504 1.039887
-8.564408
White (Eye-One Pro) 97.101597 100.017926 108.225388 100.006931 1.145532
-18.935191
So the disparity betweem instruments remains, and it is still quite blue (dE 15
in b*).
For comparison, here is the same setup for my older Titanium PowerBook G4:
X Y Z L* a*
b*
Spectrolino: 0.399623 0.401044 0.684995 3.622620 0.522360
-6.686786
Eye-One Pro: 0.402123 0.409760 0.670781 3.701349 0.283949
-6.282682
Eye-One Display: 0.655930 0.685013 1.005142 6.187695 -0.184091
-7.768043
DTP94: 0.640000 0.680000 1.080000 6.142415 -0.632202
-8.961386
White (Spectrolino) 94.719391 102.122330 104.044060 100.814896 -6.469523
-14.685333
I think this machine uses electroluminescent lighting.
The display is half the brightness, and half as blue. The disparity between
the spectrometers and the colorimeters is smaller, and the difference in b*
is dE 8, much more moderate.
Graeme Gill.
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