Magnetographic and Ionographic Technologies
These two technologies are toner based but utilize different addressing and writing media. The photoconductor is replaced by a thin magnetizable medium or hard dielectric layer, such as anodized aluminum, which is used in ionographic printers. Magnetographic printers employ a printhead that produces magnetic flux transitions in the magnetizable media by changing the field direction in the gap between the poles of the printhead. These magnetic transitions are sources of strong field gradient and field strength. Development is accomplished by means of magnetic toner applied via a magnetic brush. The toner parti- cles are magnetized and attracted to the medium by virtue of the strong field gradient. Transfer and fusing
proceed in a similar manner to that of electrophotography. Ionographic printers write onto a dielectric coated drum by means of a printhead containing individual electron sources. The electrons are generated in a miniature cavity by means of air breakdown under the influence of an RF field. The electron beam is focused by a screen electrode, and the cavity functions in a manner similar to that of vacuum tube valves. The role of the plate is played by the dielectric coated metal drum held at ground potential. The charge image is typically developed by monocomponent toner followed by a transfix, that is, transfer and fuse operation, often without the influence of heat. Both systems require a cleaning process: mechanical scraping for ionography and magnetic scavenging for magnetography.
System Issues
Processing and communicating data to control today’s printers raises significant system issues in view of the material to be printed. Hardcopy output may contain typography, computer-generated graphics, and natural or synthetic images in both color and black and white. The complexity of this information can require a large amount of processing, either in the host computer or in the printer itself. Applications software programs can communicate with the printer in two ways: via a page description language (PDL), or through a printer command set. The choice is driven by the scope of the printed material. If full page layout with text, graphics, and images is the goal, then PDL communication will be needed. For computer generated graphics a graphical language interface will often suffice. However, many graphics programs also provide PDL output capability. Many options exist and a careful analysis of the intended printed material is necessary to determine if a PDL interface is required.
When processing is done in the host computer, it is the function of the printer driver to convert the outline fonts, graphical objects, and images into a stream of bits to be sent to the printer. Functions that the driver may have to perform include digital halftoning, rescaling, color data transformations, and color appearance adjustments among other image processing operations, all designed to enable the printer to deliver its best print quality. Data compression in the host and decompression in the printer may be used to prevent the print speed being limited by the data rate. Printers that do their own internal data processing contain a hardware formatter board whose properties are often quoted as part of the overall specification for the printer. This is typical for printers with a PDL-based interface. Some of the advantages for this approach include speed of communication with the printer and relieving of the host computer of the processing burden, which can be significant for complex documents.
The increase in complexity of printed documents has emphasized several practical system aspects that relate to user needs: visibility and control of the printed process, font management, quick return to the software application, and printer configuration. The degree of visibility and control in the printing process depends on the choice of application and/or operating environment. Fonts, either outline or bit map, may reside on disk, on computer, or printer read-only memory (ROM). To increase speed, outline fonts in use are rasterized and stored in formatter random-access memory (RAM) or computer RAM. Worst cases exist when outline fonts are retrieved at printing and rasterization occurs on a demand basis. This can result in unacceptably slow printing. If quickness of return to the application is important, printers containing their own formatter are an obvious choice. It is necessary, therefore, to take a system view and evaluate the entire configuration (computer hardware; operating system; application program; interconnect; printer formatter, and its CPU, memory, and font storage) to determine if the user needs will be met.
The need to print color images and complex color shaded graphics has brought issues such as color matching, color appearance, and color print quality to the fore. Color printer configuration now includes choices as to halftoning algorithm, color matching method, and, in some cases, smart processing. The latter refers to customized color processing based on whether the object is a text character, image, or graphic. A further complication arises when input devices and software applications also provide some of these services, and it is possible to have color objects suffer redundant processing before being printed. This can severely degrade the print quality and emphasizes the importance of examining the entire image processing chain and turning off the redundant color processing. Color printer configuration choices focus on a tradeoff between print speed and print quality. Halftoning algorithms that minimize visible texture
and high print quality modes that require overprinting consume more processing time. For color images and graphics, the relationship between the CRT image and hard copy is a matter of choice and taste. For color graphics, it is common practice to sacrifice accuracy of the hue in the interests of colorfulness or saturation of the print. In the case of natural images, hue accuracy, particularly for flesh tones, is more important, and a different tradeoff is made. Some software and hardware vendors provide a default configuration that seeks to make the best processing choice based on a knowledge of the content to be printed. If more precise control is desired, some understanding of the color reproduction issues represented by the combination of color input and output devices linked by a PC having a color monitor is required. This is the domain of color management.
Color Management
The fundamental issue to be addressed by color management is that of enabling the three broad classes of color devices (input, display, output) to communicate with each other in a system configuration. The technical issue in this is one of data representation. Each device has an internal representation of color information that is directly related to the nature in which it either represents or records that information. For printers it is typically amounts of cyan, magenta, yellow, and often black (CMY, K) ink; for displays, digital counts of red, green, and blue (RGB); and for many input devices, digitized values of RGB. These internal spaces are called device spaces and map out the volume in three-dimensional color space that can be accessed by the device. To communicate between the devices these internal spaces are converted either by analytical models or three-dimensional lookup tables (LUTs) into a device-independent space. Current practice is to use Commision Internationale d’Eclairage (CIE) colorimetric spaces, based on the CIE 1931 standard observer for this purpose. This enables the device space to be related to coordinates that are derived from measurements on human color perception. These conversions are known as device profiles, and the common device independent color space is referred to as the profile connection space (PCS). When this is done it is found that each device accesses a different volume in human color space. For example, a CRT cannot display yellows at the level of saturation available on most color printers. This problem, in addition to issues relating to viewing conditions and the user state of adaptation, makes it necessary to perform a significant amount of color processing if satisfactory results are to be obtained. Solutions to this problem are known as color management methods (CMM) (Fig. 23.13). It is the goal of color management systems to coordinate and perform these operations.
The purpose of a color management system is, therefore, to provide the best possible color preference matching, color editing, and color file transfer capabilities with minimal performance and ease of use penalties. Three levels of color-management solutions are common available, point solutions, applica- tion solutions, and operating system solutions. Point solutions perform all processing operations in the device driver and fit transparently into the system. If color matching to the CRT is desired, either infor- mation as to the make of CRT or visual calibration tools are provided to calibrate the CRT to the driver.
FIGURE 23.13 Proposed ICC color management using ICC profiles. Note fundamental parts: CM framework inter- face, CMM, third party CMMs, and profiles (which may be resident or embedded in the document).
Application solutions contain libraries of device profiles and associated CMMs. This approach is intended to be transparent to the peripheral and application vendor. Operating system solutions embed the same functionality within the operating system. These systems provide a default color matching method but also allow vendor-specific CMMs to be used.
Although the creation of a device profile involves straightforward measurement processes, there is much to be done if successful color rendition is to be achieved. It is the property of CIE colorimetry that two colors will match when evaluated under the same viewing conditions. It is rarely the case that viewing conditions are identical and it is necessary to perform a number of adaptations commonly called color appearance transformations to allow for this. A simple example is to note that the illuminant in a color scanner will have a different color chromaticity than the white point of the CRT, which will also differ from the white point of the ambient viewing illuminant. In addition, as has been mentioned, different devices access different regions of color space; that is, they have different color gamuts. Colors outside the gamut of a destination device such as a printer must therefore be moved to lie within the printer gamut. This will also apply if the dynamic ranges are mismatched between source and destination. Techniques for performing all of the processes are sophisticated and proprietary and reside in vendor specific CMMs.
Defining Terms
Addressability: The spacing of the dots on the page, specified in dots per unit length. This may be different in horizontal and vertical axes and does not imply a given dot diameter.
CIE 1931 standard observer: Set of curves obtained by averaging the results of color matching experiments performed in 1931 for noncolor defective observers. The relative luminances of the colors of the spectrum were matched by mixtures of three spectral stimuli. The curves are often called color matching curves.
Commision Internationale d’Eclairage (CIE): International standards body for lighting and color measurement. Central Bureau of the CIE, a-1033 Vienna, P.O. Box 169, Austria.
Digital halftone: Halftone technique based on patterns of same size dots designed to simulate a shade of gray between white paper and full colorant coverage.
Grayscale: Intrinsic modulation property of the marking technology that enables either dots of different size or intensity to be printed.
Halftone: Technique of simulating continuous tones by varying the amount of area covered by the colorant. Typically accomplished by varying the size of the printed dots in relation to the desired intensity.
H and D curve: Characteristic response curve for a photosensitive material that relates exposure to produced/developed optical density.
Resolution: Spacing of the printer dots such that full ink coverage is just obtained. Calculated from the dot size and represents the fundamental ability of the printer to render fine detail.
Saturation: When applied to color it describes the colorfulness with respect to the achromatic axis. A color is saturated to the degree that it has no achromatic component.
References
Cornsweet, T.N. 1970. Visual Perception. Academic Press, New York.
Diamond, A.S., ed. 1991. Handbook of Imaging Materials. Marcel Dekker, New York.
Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA.
Hunt, R.W.G. 1992. Measuring Color, 2nd ed. Ellis Horwood, England.
Hunt, R.W.G. 1995. The Reproduction of Color, 5th ed. Fountain Press. England.
Scharfe, M. 1984. Electrophotography Principles and Optimization. Research Studies Press Ltd., Letchworth,
Hertfordshire, England.
Schein, L.B. 1992. Electrophotography and Development Physics, 2nd ed. Springer–Verlag, Berlin.
Schreiber, W.F. 1991. Fundamentals of Electronic Imaging Systems, 2nd ed. Springer–Verlag, Berlin. Ulichney, R. 1987. Digital Halftoning. MIT Press, Cambridge, MA.
Williams, E.M. 1984. The Physics and Technology of Xerographic Processes. Wiley-Interscience, New York.
Further Information
Color Business Report: published by Blackstone Research Associates, P.O. Box 345, Uxbridge, MA 01569- 0345. Covers industry issues relating to color, computers, and reprographics.
International Color Consortium: The founding members of this consortium include Adobe Systems Inc., Agfa-Gevaert N.V., Apple Computer, Inc., Eastman Kodak Company, FOGRA (Honorary), Microsoft Corporation, Hewlett-Packard Journal, 1985. 36(5); 1988. 39(4) (Entire issues devoted to Thermal Ink Jet). Journal of Electronic Imaging: co-published by IS&T and SPIE. Publishes papers on the acquisition, display, communication and storage of image data, hardcopy output, image visualization, and related image topics. Source of current research on color processing and digital halftoning for computer printers.
Journal of Imaging Science and Technology: official publication of IS&T, which publishes papers covering a broad range of imaging topics, from silver halide to computer printing technology.
The International Society for Optical Engineering, SPIE, P.O. Box 10, Bellingham, Washington 98227- 0010, sponsors conferences in conjunction with IS&T on electronic imaging and publishes topical proceedings from the conference sessions.
The Hardcopy Observer, published monthly by Lyra Research Services, P.O. Box 9143, Newtonville, MA 02160. An industry watch magazine providing an overview of the printer industry with a focus on the home and office.
The Society for Imaging Science and Technology, IS&T, 7003 Kilworth Lane, Springfield, VA 22151. Phone (703)642 9090, Fax (703)642 9094. Sponsors wide range of technical conferences on imaging and printing technologies. Publishes conference proceedings, books, Journal of Electronic Imaging, Journal of Imaging Science and Technology, IS&T Reporter.
The Society for Information Display, 1526 Brookhollow Drive, Ste 82, Santa Ana, CA 92705-5421, Phone (714)545 1526, Fax (714)545 1547. Cosponsors annual conference on color imaging with IS&T.