Disclaimer: the information on this page is based on my personal tests, performed on a single system. Because my findings correlate with the experiences of others, I have written this page in its current, general form in the hope that it may help others. However, I cannot guarantee that the results are not determined by other factors as well, such as video card drivers.
Whether or not it is possible to use color management in conjunction with various versions of Windows has been the subject of much discussion. At the moment of writing, I am only working with Windows XP, so I will restrict myself to that operating system. On this page, I describe how to configure color management for multiple monitors, and point out the limitations as well.
Calibration vs. profiling
Before getting into details, it is important to establish the difference between display calibration and profiling.
Display calibration means getting the display chain into a specified state. This involves setting the brightness and color balance on the monitor, but also setting the look-up table (LUT) on the video card or, on more expensive models, in the monitor itself. Having a calibrated display chain means that you always get the same output color for a triplet of RGB input values.
Profiling is the process of measuring and describin the properties of this calibrated display chain. The display profile describes (assuming a matrix profile) the tone curve (gamma), and the chromaticities of the red, green and blue primaries (how red is red, etc.).
The distinction between calibration and profiling is subtle, and is usually glossed over. In fact, many display calibration/profiling tools use the term ‘calibration’ for the combined process of calibration and profiling. Complicating things further, the ICC profile that is created also contains the LUT data that is needed to properly calibrate the display chain. Although the difference between calibration and profiling can often be glossed over, it will turn out to be very relevant to the topic of multi-monitor use in Windows XP.
Microsoft Color Applet
Assigning display profiles to individual displays using the Microsoft Color Applet
With the release of the Microsoft Color Control Panel Applet it has become possible relate ICC profiles to individual monitors in (nearly) all configurations. Follow the link for download and installation instructions. After installation, a new icon has been installed in the control panel: Color. On the ‘devices’ tab, the Color Applet allows the user to assign a number of profiles (with one default) to each individual monitor.
Perhaps the most useful function of the Microsoft Color Applet is its function as a multi-monitor LUT loader. In this guise, it can replace the third-party LUT loaders like Adobe Gamma, MonacoGamma, etc. that may or may not be multi-monitor capable. To use this functionality replace your current LUT loader in the startup folder by a shortcut to the MS Color Applet, with a /L switch. Assuming default paths, the shortcut command is (without the line break)
"C:\Program Files\Pro Imaging Powertoys\ Microsoft Color Control Panel Applet for Windows XP\WinColor.exe" /L
One thing to watch out for is that some display calibration profiles will try to re-enable their own profiling software after every calibration run. If this is the case, remove the shortcut to the proprietary LUT loader after every calibration to prevent possible conflicts/confusion.
The catch: only a single profile
When setting up my own dual monitor system, I initially thought this was all
there was to it. Unfortunately, this is where the distinction between
calibration and profiling rears its ugly head. The Color Applet allows users to
assign profiles (with LUT data) to individual monitors, and the LUT data can be used to correctly calibrate both multiple display chains.
Selecting the system-wide monitor profile that will be used by applications to convert colors to the monitor color space.
However, the profiles themselves are not used by any of the color managed software packages that I tried. Whether this is a limitation of those packages or of the operating system I do not know, but the fact is that at this moment you have to select a single profile that will be used for both monitors (only for the profiling aspect, not for the LUT calibration). The profile that will be used for color conversions is listed as the default monitor profile on the color management tab of the advanced display properties (control panel->display->settings->advanced). This profile will often be set by the calibration/profiling software and point to the most recent profile you have created.
Knowing that we can calibrate multiple monitors, but we can use only a single profile for the actual profile conversions, what are our options? Below, I will describe two possible methods to deal with this restriction.
Option 1: using a single calibrated monitor
The first option is to simply accept that only one of the displays can be used for color-critical work. This may not be much of a problem when one of your monitors is much more capable than the other, as is the case for laptops with external monitors. In this scenario, you may still calibrate both displays to match their brightness, color temperature and gamma, but the chromaticities will generally differ. Special care should be taken to ensure that the system-wide monitor profile matches that of the monitor that will be used for color-critical work.
Option 2: calibrating monitors to a common standard
The other option is to make sure that the profiles of both monitors match, either exactly or very closely. The simple way to ensure a relatively good match is to use two monitors of the exact same type and calibrate them to the same parameters (brightness, temperature, gamma). It is very likely that their profiles will be near-identical, so that either profile will do.
Another method, that will also work for dissimilar monitors, is to have the display calibration software calibrate multiple monitors to a common profile. This is generally only available in higher-end calibration packages, such as the X-Rite i1 Display 2 (workgroup match). Of course, it has to be noted that not even calibration can force a monitor to do something it can’t, so you will be forced to calibrate to the lowest common denominator.