Something I don't understand about color management...

[yb98]

I have a pc with windows xp and a spider to calibrate my screen.
When windows start I can see that the profile I have defined with spider is loaded. I can even see that the colors of my desktop background image changes when the profile is loaded. This let me think that the profile is loaded into a low level in windows since the colors of the desktop background image have changed.
The question is : when an image is displayed inside firefox, is this profile active or not ? if no, why is it active on the background desktop image and not inside firefox ? if yes, why is it then necessary to specify it again inside dpp (since there is no need to specify it inside firefox) ?

Yacine.

[René Damkot]

The question is : when an image is displayed inside firefox, is this profile active or not ?
Yes. FF isn't colormanaged however, since it doesn't use the 'input' profile (of the image being displayed).

if yes, why is it then necessary to specify it again inside dpp ?

DPP is silly that way.

[gcogger]

Time to repeat the old explanation again...

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What you're seeing is the difference between 'calibration' of the monitor, and the monitor profile. There are 2 parts to 'profiling' a monitor.

1 - The monitor is calibrated. This sets up things like the approximate white point and gamma of the screen, and is done by a combination of adjusting the monitor by hand and loading a LookUp Table (LUT) to the graphics card. This gets the monitor into a reasonable and, more importantly, known state for profiling.

2 - The monitor is profiled. This involves measuring the actual response of the calibrated monitor (RGB chromaticities, measured white point etc.). This info is stored in the ICC profile ('.icc' or '.icm' file), and is used by Photoshop, DPP (and some other apps, but not all) to correct the RGB values sent to the monitor.

When Windows starts your software will load the appropriate calibration LUT to the graphics card that works with the ICC profile selected. This will change the appearance of the screen in all applications - it's the reason you can see the screen appearance change while Windows is starting.

For accurate colour, however, an application still needs to use the profile to adjust the colours correctly. With things like DPP, that means you must select the monitor profile in order to be colour managed. Some applications (e.g. Photoshop, Elements) will automatically use the profile without being told to. Most applications (e.g. Firefox) will ignore the profile completely, and will therefore not display colour accurately.

(Just to confuse the issue, the ICC profile specification allows people to store additional information in the '.icm/icc' file. While it is not strictly part of the profile itself, most vendors use this facility to store the LUT data in the '.icm/icc' file. This is sensible, since it means you can't lose the calibration data - the profile is only valid after the LUT is applied).

[yb98]

Thanks for the explanation !

[Picture North Carolina]

1 - The monitor is calibrated. ...

2 - The monitor is profiled.

Question: are you referencing CRT or LCD? The reason I ask is this. A tech support guy at greytag/mcbeth told me that CRTs are calibrated, but in the case of LCDs, they recommend placing the panel into a standard mode such as 6500 kelvin then only profiling it. Was he off base?

[gcogger]

Question: are you referencing CRT or LCD? The reason I ask is this. A tech support guy at greytag/mcbeth told me that CRTs are calibrated, but in the case of LCDs, they recommend placing the panel into a standard mode such as 6500 kelvin then only profiling it. Was he off base?

I think that what he's referring to is the calibration adjustments on the monitor itself.

With a CRT, the calibration part is done by a mixture of (analogue) adjustments on the monitor itself (RGB guns, brightness, contrast) and the graphics card LUT I described previously. This makes sense for a CRT, where the adjustments are real physical changes on the monitor.

With an LCD, the only real (analogue) adjustment you can make is to the intensity of the backlight. All other adjustments are actually implemented by an internal lookup table in the monitor, which maps the 256 input values for each colour to 256 output values. Therefore, if you adjust the settings on the monitor when you run the profile software, the RGB values go through 2 separate LUTs before being displayed. This can be bad, since each LUT can give rise to 'steps' in the response (i.e. skipping some of the 256 values). Doing it twice is much worse, and can lead to banding or 'blotchiness' in areas of subtle colour changes. So, for an LCD monitor, it is best to leave the monitor alone (so the internal LUTs do nothing) and let the calibration software create a single LUT to be applied by the graphics card. Of course, it can be difficult to know what the 'unadjusted' setting is on an LCD monitor...

Some of the better LCD monitors (Eizo CG series, for example) can have their internal LUTs created by the calibration software. These internal LUTs are normally 10-bit (i.e. 1024 values instead of 256) or more, so in these cases the software creates a LUT in the monitor but not the graphics card. Note that there is still only one applied.

Phew! I hope you're still awake :)

[Picture North Carolina]

I think that what he's referring to is the calibration adjustments on the monitor itself.

With a CRT, the calibration part is done by a mixture of (analogue) adjustments on the monitor itself (RGB guns, brightness, contrast) and the graphics card LUT I described previously. This makes sense for a CRT, where the adjustments are real physical changes on the monitor.

With an LCD, the only real (analogue) adjustment you can make is to the intensity of the backlight. All other adjustments are actually implemented by an internal lookup table in the monitor, which maps the 256 input values for each colour to 256 output values. Therefore, if you adjust the settings on the monitor when you run the profile software, the RGB values go through 2 separate LUTs before being displayed. This can be bad, since each LUT can give rise to 'steps' in the response (i.e. skipping some of the 256 values). Doing it twice is much worse, and can lead to banding or 'blotchiness' in areas of subtle colour changes. So, for an LCD monitor, it is best to leave the monitor alone (so the internal LUTs do nothing) and let the calibration software create a single LUT to be applied by the graphics card. Of course, it can be difficult to know what the 'unadjusted' setting is on an LCD monitor...

Some of the better LCD monitors (Eizo CG series, for example) can have their internal LUTs created by the calibration software. These internal LUTs are normally 10-bit (i.e. 1024 values instead of 256) or more, so in these cases the software creates a LUT in the monitor but not the graphics card. Note that there is still only one applied.

Phew! I hope you're still awake :)

Yes, awake and appreciative. Thanks for validating that. I had always read what you said, but was surprised about the amount of people in this and other forums that consistently say they calibrate their LCDs with calibration procedures then profile it.