ICC Color Management

Color management systems were first developed by Kodak and EFI (Electronics for Imaging).

They included profiles for Kodak and other output devices including monitors. • Other vendors such as Agfa, Lintotype (later Linotype-Hell and now part of Heidelberg) followed suit with profiles mainly for their own devices.

The lack of standards and compatibility between systems prevented these systems from being effective in a real workflow having devices from multiple vendors.

This spawned the formation of the International Color Consortium (ICC) in 1993 by Adobe, Agfa, Apple, Kodak, Microsoft, Sun Microsystems and Silicon Graphics.

They established a common profile format and a common transformation framework for exchanging color data.

Because of the composition of the consortium, ICC color management is now built into the operating systems (MacOS, Windows 98/2000, Solaris, Irix, etc.)

The CMS must be able to deal with the different color spaces used by the various devices.

RGB is the native color model used by computers and desktop scanners.

Part of the problem of matching scanner, monitor and print is that RGB must be converted to CMYK before printing.

Another, is that the RGB of the scanner generally does not match that of the monitor because the color filters in the scanner do not match the output of the monitor's phosphors.

Translating RGB to CMY may be well defined, but when some proportion of CMY is replaced by black to get CMYK (GCR or UCR), ambiguities can result.

Thus, there are many equally valid CMYK values for a given RGB.

But, RGB and CMYK are device dependent.

Different monitors may require different RGB's to display the same color and different presses and digital printers may require different CMYK's to print the same color.

Calibration - Calibration is a necessary precursor to profiling to ensure that a device is performing the same as when it was profiled.

Devices should be recalibrated periodically.

The general steps for calibration are

1. Identify variables to be calibrated.

2. Capture or output a test image.

3. Evaluate the results.

4. Adjust the device accordingly.

For accurate measurement, a colorimeter or spectrophotometer must be used.

Sometimes a densitometer may also be used to compare densities with target values.

Calibration utilities may be available from device manufacturers or third parties. The ICC color management architecture consists of 4 major elements. These are:

1. Profiles communicate device characteristics to the Color Management Module (CMM).

2. The CMM connects profiles together to produce transformations for any group of devices, using the Profile Connection Space (PCS).

3. The application calls on the CMM to handle color transformations.

4. The operating framework enables the application to access profiles and CMMs, providing a default CMM when a device specific one is not installed. A default CMM is provided by the operating system.

The Macintosh CMM is known as ColorSync and the Windows CMM is known as ICM (Integrated Color Management).

A generic CMM is being developed by the ICC. The Profile Connection Space defines a reference medium and a viewing environment.

The PCS operates a virtual space defining the relationship between color in the different device spaces such that a unique transformation can be generated for any pair of devices.

The ICC defines PCS as "The CIE colorimetry required to achieve the desired color appearance on reference media and in a viewing environment.