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Elements of the Color Management System

Components of the Color Management System

1. Colorimeter
2. Device profiling
3. Device calibration
4. Computer operating system
5. Color management system
6. Color matching method
7. Application system

(1) Colorimeter
The colorimeter is a basic device for color management, because it works similar to a human's eye. There are three different types of colorimeters. In order of price, they include; spectrophotometer, color photometer, and color densitometer. The price of color management systems is largely dependent on the price of such colorimeters.
Spectrophotometers, which can analyze spectrum characteristics as well as spectral distribution, show color values as " lab values". Until recently, the price range for many of them fell between several characteristics nor spectral distribution can be measured with these devices. The price of color photometers is less than one million yen, but many of at them are still rather too expensive for consideration at this time.
On the other hand, color densitometers have no function for measuring color values. Since these devices can only measure the color density of primary color such as CMYK or RGB, they are mostly used for the calibration of devices or the like. The price range for color densitometers is between 200,000 and 400,000 yen.

(2) Device Profiling
Device profiling is a system for making device profiles, which describes color reproduction characteristics of input/output devices such as CRT monitors, printers, color scanners, etc. On this system, after RGB values are measured on the appropriate device such as a CRT color display, their actual color values on the printed sheet are measured.
Then, the characteristics of the device (for example, monitors having a tendency to be bluish in comparison with the output on paper) are recorded so that the result can be referred to for subsequent jobs. Such a quality of device profile greatly effects the reliability of color management. The unit, lab values, which are not used on devices, is adopted to represent color value. It is measured and represented as lab values of RGB and CMYK is compiled. It is most desirable to produce a full conversion table, which is produced by measuring every possible combination of color swatches, but this is unrealistic, because there are 16,700,000 different combinations of colors IRGB alone.
As a practical method, therefore, 200 to 1,000 key color swatches are measured, and based on a standardized format, a device profile is made by interpolating values for other colors.

(3) Calibration of devices
When we want to establish a color management system, it is not enough to prepare a profile only. Because conditions of devices such as scanners, monitors and printers change with time and due to their inherent characteristics, the output of color cannot be even and always fluctuates. For instance, suppose someone tries to measure color values on a color swatch, which was output through a color printer, in order to prepare a profile. Then, if another operator outputs many pages during this process simultaneously, the measured color values may turn out to be useless as the condition of the printer had been changed from the time when the profile was completed.
A method to solve this trouble is called device calibration. This calibration system checks a device to determine whether its condition has deviated from the standard set when the profile was first created. Whenever the system detects any deviation, it makes an adjustment to restore the standardized condition of the device. Once each device is stabilized with such a calibration system, profiles can be effectively utilized.

(4) Operating systems
Color management systems have been incorporated into functions of computer operating systems (OSs). For example, the OS of Macintosh employs ColorSync 2.0; Windows 95 uses ICM; and SUN Solaris and SGI adopt KCMS. These four OSs can conduct a color management by using the standard ICC profile, while some other special OSs may not be able to process ICC profiles appropriately. Therefore, it is always necessary to consider types of computer operating systems when employing a color management system.

(5) Color management system
The real purpose of color management systems is not to make conversion of colors. They are, as described above, applied as a part of the OS, and then provide device profiles (ICC profiles) and a color matching method (CMM) as well as an interface for exchanging applications within a framework to be incorporated into the OS.
It is often pointed out that ColorSync 2.0 does not work properly, or that the level of its accuracy is a problem. However, it is both the accuracy of a device profile (ICC profile) and the accuracy of a color matching module that control actual color qualities, and a color management system itself is not involved in the process.

(6) Color matching method
The actual processes for color conversion and color matching are conducted through a color matching method or a color matching module.
The key issue here is how to deal with a color that cannot be reproduced on a certain device in order to show it through a different device as if it were visually the same color, just as when the reproducible color range between color transparencies and printed matters are different. There is no common method for this process, and the performance depends on the capability of each color matching method, which will be one of the key points to be discussed in the near future.

(7) Application software
Wherever an application software corresponds to a certain color management system, the software instructs the OS to execute color conversion (e.g. conversion of RGB signals to CMYK signals). An operating system calls out a color management program (e.g. Macintosh calls out ColorSync 2.0). Then, it brings a profile and a color matching method, which are stored within each device, into the system in order to conduct color conversion and to output the result. The application software itself does nothing, basically, but its function is to transmit the optimal CMYK signals to a printer when RGB data are delivered to the OS. The question of how an application software corresponds to a certain color management system will become a point of the issue to be discussed.

The following items are the components of color management systems, arranged in the order of extents of influence on the overall quality of color management:

1. Accuracy of device profiling
2. Accuracy of colorimeter
3. Functions of color matching method