Hardware Issues for Color Management

 

Microsoft Corporation

November 1998

Summary: Integrated Color Management version 2 (ICM 2.0) is a set of software APIs and built-in software modules that permit end-to-end management of color imaging on the Microsoft® Windows® 98 and Windows 2000 operating systems. The goal of color management is to ensure that users experience consistent quality color from their color peripherals.

This article describes the technical issues for hardware and the implementation of color management on the Windows 98 and Windows 2000 platforms. The issues discussed relate to future versions of color management as well as operating systems.

Color Management

If an image is scanned with a color scanner, saved as a file, and then used for desktop publishing, the user expects the printed output to closely resemble the input image. When colors are modified, the user expects the output to resemble the selected colors. ICM ensures quality results by knowing the color imaging characteristics of the color peripherals and managing the color data.

As more color peripheral devices are used, the difficulties of getting good color output on the display screen or on paper become more apparent. Color devices such as scanners, digital cameras, displays, and printers are now affordable for many families. Not everyone who uses these devices understands the degree of art and science required in achieving the results in color photography and in color magazines, yet users often expect similar results from their color peripheral devices.

The purpose of color management is to conveniently tie together diverse technologies that scan, capture, display, and print color pictures to yield consistent, high-quality color images without the attendant skills required in the past. Color management solutions are often perceived as software-only solutions. Although software certainly provides the integrating portion of the color management solution, the color device data or profiles provide the operative information that enables color management software to prepare a color imaging solution.

The color system shown in Figure 1 illustrates the typical peripherals that users encounter. Today, the PC manages all communication among peripherals. For the PC to provide a color management solution, it must know the color characteristics of the peripherals.

Figure 1. Typical color system

Three terms important to color management are calibration, characterization, and profile:

  • Calibration of a device brings it to a known state of operation so that a signal of known value yields a predictable result. For example, a reflectance of 0.65 might result from sending red, green, and blue byte values of 255, 127, 0 to a printer.
  • Characterization of a device means sending a series of signals to a device and measuring the output. This differs from calibration in that measurement is taken without an expectation of a particular result. Send the printer a value of 127, 127, 127 and you might expect a gray, but the output is another color. Device characterization measures what you get from the byte values that are input. Color characterization gives you the color result from a particular device stimulus.
  • Profile can be considered as the inverse of the characterization. This is not strictly inverting and smoothing of the characterization data, but can be thought of in this way. The profile is the opposite of a characterization: when one requests a midtone gray from a color device, instead of sending equal red, green, and blue signals such as 127, 127, 127 to get it, the profile would tell you that you need to send 131,122, 119 (for example) because the data in the characterization tells you what is produced from what is sent. The profile enables you to ask for a particular result and have the required stimulus returned. This is how the majority of imaging applications work.

Figure 2 is a diagram of a simple color management system. Device profiles are used by the Color Management Module (CMM), which in turn uses the attached device profiles to provide the correct output device drive signals. If the color devices are properly profiled, then the CMM can take the user's desired result—for example, what they see on the screen—and make the printed output look like the screen, based on the screen and printer profile.

Figure 2. Simple color management system

Color Management Implications for Hardware

Color imaging devices are not necessarily stable in their color characteristics. Although progress has been made, variations in color output or detection still occur with the same stimulus. For example, over time, the display on your desk does not necessarily give the same color even though the input signals are identical. Color printers can vary with different paper types, changes of ink or toner, humidity, number of images made, and so on.

When a color printer is characterized and a profile is generated, this profile might have a limited life. A change of paper, consumables, and adjustment might cause the printer to deliver unpredicted results. Except for the professional publisher, users will not use color instruments in their systems. These color instruments are expensive and require skill for proper results. The typical user just wants color to work.

Electron gun life and convergence of the red, green, and blue beams can change color display output. The many controls on the front of most displays, especially the cathode ray tube (CRT), enable users to change the brightness, contrast, and sometimes even the color balance. The user can also set gammas of the display. Aging and abuse add to the potential difficulties. These changes alter the characterization and hence the data that the profile relies upon without operating system notification or often without changing available data available.

Color Management Implications for Displays

Color displays are the easiest to characterize and profile. The display and graphics adapter communicate with each other and with the operating system to control the system color results. For practical reasons, the user must be able to adjust the display. However, changes to the brightness, contrast, or color balance of the display, and any changes in the display or graphics adapter must be available to the operating system.

Future display systems will need simple sensors in their bezels that provide R, G, and B data indicating the local illumination level and its color balance or "white" point. In the future, displays should communicate the state of their color performance so that any shipped profile can be modified to reflect the current state. Display makers and graphics adapter suppliers must permit the operating system to interrogate the display system to determine new settings. Then the operating system can update the display profile for proper color management operation.

Flat panels use fluorescent backlights or similar systems to illuminate the liquid crystal display structure. The displays have dye-based filter structures to filter the backlight illumination. The filters reside behind the liquid crystal shutter. The dye filters do not vary and have a long life in most cases. The backlight can age, but flat panels are fairly stable in color. With the advent of desktop liquid crystal display (LCD) panels, graphics adapters play a role as they do with CRTs. Any changes to LCD operating characteristics need to be capable of being "read" by the operating system just as with CRTs. Other types of flat panel technologies, such as field emission displays (FEDs), plasma panels, digital mirror devices, and so on, require similar feedback to the operating system.

Design Points to Consider for Displays

  • Changes in the physical display operating characteristics, such as brightness or contrast, need to be available to the operating system for proper color management.
  • Changes to the video card data, such as gamma and so on, must also be readable by the operating system.
  • Display and graphics adapter operating characteristics that affect the display system colorimetric characteristics should be capable of being set by the operating system.
  • Display system ability to determine approximate user working space luminance and illuminant white point should be built into the monitor and the data made available to the operating system.

Scanners and Digital Cameras and Color Management

Input devices such as scanners and digital cameras offer a different set of problems. Flatbed and film input scanners usually have software that directly controls their characteristics. If any characteristics are changed, they must be made available to the operating system and, subsequently, the color management system. Scanner data setting and setup values must be made available to the operating system and the color management system.

Digital cameras use outside sources of light. These sources may be bright sun, a cloudy sky, tungsten lights in a home, fluorescent lighting in an office, and so on. The scene illuminant can vary both in spectral characteristics and intensity. All of these issues affect the digital picture. Knowing the approximate white point of the scene illumination is of great assistance to the color management system in improving the picture through color management techniques. Perhaps digital cameras should be equipped with a simple but effective RGB sensor in the camera body that records the light falling on the camera itself as a general indication of the scene white point. Furthermore, all of the data available about the scene should be bundled with the picture itself so it can be used by the color management system.

Design Points to Consider for Scanners and Digital Cameras

  • Bidirectional communication between scanner and operating system must make scanner or camera operational settings, such as brightness and contrast, available to the color management system.
  • The white point of the illumination for digital camera scenes should be communicated to the operating system if the camera can assess it.
  • Both cameras and scanners should be controllable from the operating system. Fundamentally, any device settings that affect the colorimetry of the scanned image or captured picture should be capable of being sent to the device.
  • It would be convenient if the device could be set from the color management system. Then, when the user picks a scanner with a known profile, the scanner settings should be set in the device itself.

Printers and Color Management

Because of a number of device factors, printers represent the most difficult portion of color management. Color is generated in an entirely different way on printer than it is for displays or digital cameras. Typically, the user starts with white paper. The actual spectrum of the white can vary depending on the illumination being used to view the output. In general, the spectrum is broad. Printers use what is termed a subtractive process. The three colorants used are cyan (C), magenta (M), and yellow (Y). Printers use dyes or pigments in the inks or toners to produce CMY. In most quality printers, a fourth color—black (K)—is used for a number of reasons, including better quality and lower operating costs.

Various types of printer technologies exist, such as photographic, thermal, electrostatic, and ink-jet. Printers can be strongly affected by the type of paper used, because they have complex surface interactions with the dyes and paper. Whether the printer is continuous tone or needs to be half-toned also makes a large difference. In CRTs, a color profile can be generated with anywhere from a dozen to 140 measurements. Scanners and digital cameras might require up to 200 or so measurements in their color spaces. Printers often require that hundreds of color patches be measured, and sometimes well over 1,000 patches. This is often a painful process, requiring instruments costing thousands of dollars and taking at least an hour or so to complete the characterization task. To average several readings might require a number of hours.

Printer manufacturers should consider internal measurement capabilities in their devices. Such capabilities would not necessarily require remeasuring hundreds of patches, but merely providing updated information about a few key patches. The updated information could then be used by a more sophisticated color management system to update the profile, based on current printer characteristics. Work is underway to look at this issue from a color management system perspective, and printer vendors should be aware of the upcoming need to communicate closely with their device.

Design Points to Consider for Printers

  • Printers should be prepared to communicate any change in colorimetric effects caused by user adjustments.
  • Internal measurement capabilities should be implemented to permit color parameter measurement, such as color density or chroma changes, based on the paper the user has chosen.
  • Printers could at some point download simple spectra of the paper and current colorants for use by the operating system and color management system, to tune the output by way of advanced color management software.
  • The vast majority of users should not be expected to do their own profiles or independently measure the state of their devices. Prepress and professional users will likely do this, but the typical end user should not have to.

Summary

For quality imaging—and especially color imaging—remember: "If you cannot measure it, you cannot control it." Command and control is the secret to good imaging. Devices that do not vary in their color characteristics are a noble design goal, but in practicality are not fully realizable. Furthermore, if the user can adjust a device aspect that affects colorimetric performance, the color management system must know about it to permit quality output. Displays, scanners, digital cameras, and printers need to be more closely coupled with the operating system so that users' expectations can indeed be delivered. For system color management to become a valuable user solution, software and hardware must fully cooperate and communicate.

If hardware vendors enable proper communication and control of their products, the operating system and color management software can be designed to compensate for device variations or intended changes. Looking to future color management tools in the operating system, the driving vision is the goal of consistent quality color with minimal user intervention.

  • Manufacturers should use the information in this article to design their color products to fully communicate with the color management system or operating system regarding any status or settings that affect their colorimetric performance.
  • Developed color devices should also allow user-permitted changes to be accessible by the color management system for updating color profiles or other key imaging data.
  • Device parameters should be settable by the color management system by way of the operating system to bring a device to a predictable state of operation, based on previous colorimetric data.
  • Windows 98 and Windows 2000 support using color profiles that comply with the ICC Profile Format specification. The ICM APIs and functionality for Windows and Windows NT are described in the Microsoft Platform SDK and the current version of the Windows 2000 DDK.

References

Color Separation on the Desktop, Miles and Donna Southworth, Graphic Arts Publishing, Livonia, New York 14487-9716, 1993.

Color in Business, Science and Industry, Deane B. Judd and Gunter Wyszecki, John Wiley and Sons, 1967.

Digital Color Management, Edward Giorgianni and Thomas Madden, Addison-Wesley, 1997.

ICC Profile Format Specification, available from the International Color Consortium Web site at http://www.color.org.

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