[argyllcms] Re: Color Calibration

On Friday 26 September 2008 12:08:02 Frédéric wrote:
> On vendredi 26 septembre 2008, Pascal de Bruijn wrote:
> > Hi,
> >
> > ArgyllCMS works well with my new Huey (my previous one
> > was defective).
> >
> > However, I'm wondering whether it's possible to adjust my
> > screen settings to get it to be closer to sRGB.
> >
> > I'm currently profiling using my screen's default settings.
> >
> > Does ArgyllCMS have a tool which can read a few spots,
> > and then give some hints, for example to increase/decrease
> > brightness/contrast/temp, and then read the few spot again.
> > Then repeat this a couple of times to get the screen as
> > close to sRGB as possible in hardware. And then do
> > the normal calibration run, to compensate for the
> > remaining difference...
> >
> > Does such a tool exist? Or is there a way to deduce that
> > from a generated profile?
>
> Yes, dispcal can guide you using the 1-6 menu...
>
> But as far as I know, the only thing you are able to adjust to target sRGB
> are the color temp. (6500K) and gamma (2.2). Primaries can't be
> adjusted...

Dispcal does have the ability to calibrate to an sRGB gamma curve.

>
> Brightness sets the black level, and contrast the white level (at least on
> a CRT monitor). On LCD, you can only adjust the backlight level ;
> modifiying other settings can lead to problems, because it will modify the
> internal LUT of the screen, which is not very accurate (6 bits).

This may be true for older low end LCD monitors and perhaps some current 
laptop displays but in general virtually all current LCD monitors have 10 or 
12 bit/channel internal LUTs.   

A bigger issue for LCD displays is that, unlike CRTs, there are no standards 
or industry norms for what the controls actually do (the exception being the 
brightness control) and this will vary from vendor to vendor and model to 
model for the same vendor.  In addition what controls are available varies 
from vendor to vendor and model to model.  Some monitors only have a 
brightness control for example.   

As a result any instructions on how to go about adjusting the display controls 
for an LCD has to be at the very least vendor specific and more likely model 
specific.  In addition, the only way to know for sure what the various 
controls for a particular display model do you need take a lot of measurements 
while testing each control to see what it does.  For example I have a Samsung 
245BW monitor and after a lot of testing I have found the following:

The brightness control adjusts the back light intensity (fairly standard 
behavior that is true for most LCDs).

The contrast control alters the LUT so that increasing the contrast setting 
causes the light end of the tone curve to get brighter.  In this respect it 
sort of mimics the contrast control for a CRT.  At settings near 100 it allows 
all of the back light through when displaying a white color but it also causes 
light tones to be blown out and it pulls up the dark tones near black too 
much.  I used a gray step wedge to set this by finding the setting that gave 
the most visually even distribution of gray tones while still getting near 
full brightness for the white level.  For my monitor this was a setting of 88; 
the default setting is 75.

The Color controls actually set the gamma of the individual channels but have 
almost no affect on the white point.  Higher settings decrease the gamma and 
lower settings increase the gamma.    The lowest setting will result in a 
gamma of around 3.0 and the highest setting will result in a gamma near 1.0.  
The default setting (50) has a gamma near 1.8 (with the gamma control set to 0 
see below).  I ended up using settings near 20-25 for these controls.

The gamma control does change the overall gamma (IE. it affects all three 
channels) but there are only three settings and these change the gamma through 
about a +- 0.2 range.  I left this at the default settings.

There are no controls that have more than a slight affect on the monitors 
white point and even then only when set to fairly extreme settings.  Since the 
back light is around 6600K I concluded that this monitor is best calibrated 
using it's native white point in part because it is close to 6500K and mostly 
because there is no way to change it.    I used the Color controls to set both 
the gamma and black point.  I was able to get the black point to with in 1.5 
dE of matching the native white point with these controls and the gamma ended 
up being very close to 2.2 while the contrast ratio was still >900.  The 
resulting vcgt curves were fairly flat and are nearly on top of each other 
through most of the tonal range indicating that these adjustments had come 
fairly close to the desired state.

Again the above is specific to a Samsung 245BW and may not even be correct for 
some other Samsung models and is likely completely incorrect for monitors from 
other vendors.  YMMV, use at your own risk.  My main point is that if you 
spend some time and do a lot of measurements you should be able to figure out 
what the controls on your LCD actually do and then use them to get the best 
possible calibration.

>
> But if you have a high-end LCD monitor, things can be a little bit
> different...

Some very high end LCD monitors actually allow you to set the white point of 
the back light.  In most cases these will be monitors with an LED back light 
(these actually use red, green and blue LEDs and the white point adjustment 
changes the intensity of the various colored lights).

Hal

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