microcontrollers

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.

Electrophotographic Printing Technology

Electrophotography is a well established and versatile printing technology. Its first application was in 1960 when it was embodied in an office copier. The process itself bears a strong resemblance to offset lithography. The role of the printing plate is played by a cylindrical drum or belt coated with a photoconductor (PC) on which is formed a printing image consisting of charged and uncharged areas. Depending on the implementation of the technology either the charged or uncharged areas will be inked with a charged, pigmented powder known as toner. The image is offset to the paper either by direct contact or indirectly via a silicone-based transfer drum or belt (similar to the blanket cylinder in offset lithography). Early copiers imaged the material to be copied onto the photoconductor by means of geometrical optics. Replacing this optical system with a scanning laser beam, or a linear array of LEDs, which could be electronically modulated, formed the basis of today’s laser printers. As a technology it spans the range from desktop office printers (4–10 ppm) to high-speed commercial printers (exceeding 100 ppm). Although capable of E-size printing its broadest application has been in the range of 8 1 in to 17 in wide, in color and in black and white.

Printing Process Steps

Electrophotographic printing involves a sequence of interacting processes which must be optimized collectively if quality printing is to be achieved. With respect to Fig. 23.12 they are as follows.

1. Charging of the photoconductor to achieve a uniform electrostatic surface charge can be done by means of a corona in the form of a thin, partially shielded wire maintained at several kilovolts with respect to ground (corotron). For positive voltages, a positive surface charge results from ionization in the vicinity of the wire. For negative voltages, negative surface charge is produced but by a more complex process involving secondary emission, ion impact, etc., that makes for a less uniform discharge. The precise design of the grounded shield for the corona can have a significant effect on the charge uniformity produced. To limit ozone production, many office printers (<20 ppm) employ a charge roller in contact with the

FIGURE 23.12 Schematic of the electrophotographic process. Dual component evelopment is shown with hot roll fusing and coronas for charging and cleaning. (Source: Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA. With permission.)

photoconductor. A localized, smaller discharge occurs in the gap between the roller and photoconductor, reducing ozone production between two and three orders of magnitude.

2. The charged photoconductor is exposed as described previously to form an image that will be at a significant voltage difference with respect to the background. The particular properties of the photoconductor in this step relate to electron hole generation by means of the light and the transport of either electron or hole to the surface to form the image. This process is photographic in nature and has a transfer curve reminiscent of the H and D curves for silver halide. The discharge must be swift and as complete as possible to produce a significant difference in voltage between charged and uncharged areas if optimum print quality is to be achieved. Dark decay must be held to a minimum and the PC must be able to sustain repeated voltage cycling without fatigue. In addition to having adequate sensitivity to the dominant wavelength to the exposing light, the PC must also have a wear-resistant surface, be insensitive to fluctuations in temperature and humidity, and release the toner completely to the paper at transfer. It is possible for either the discharged or the charged region to serve as the image to be printed. Widespread practice today, particularly in laser printers, makes use of the discharged area.

Early PCs were sensitive to visible wavelengths and relied on sulfur, selenium, and tellurium alloys. With the use of diode laser scanners, the need for sensitivity in the near infrared has given rise to organic photoconductors (OPC), which in their implementation consist of multiple layers, including a submicron- thick charge generation layer and a charge transport layer in the range of 30 µm thick. This enables the optimization of both processes and is in wide-spread use today. A passivation or wear layer is used for OPCs, which are too soft to resist abrasion at the transfer stage. In many desktop devices the photoconductive drum is embodied in a replaceable cartridge containing enough toner for the life of the photoconductor. This provides a level of user servicing similar to that for thermal ink jet printers having replaceable printheads.

3. Image formation is achieved by bringing the exposed photoconductor surface in contact with toner particles, which are themselves charged. Electrostatic attraction attaches these particles to form the image. Once again, uniformity is vital, as well as a ready supply of toner particles to keep pace with the development process. Two methods are in widespread use today: dual component, popular for high-speed printing and monocomponent toners commonly found in desktop printers. Dual component methods employ magnetic toner particles in the 10-µm range and magnetizable carrier beads whose characteristic dimension is around 100 µm. Mechanical agitation of the mixture triboelectrically charges the toner particles, and the combination is made to form threadlike chains by means of imbedded magnets in the development roller. This dense array of threads extending from the development roller is called a magnet brush and is rotated in contact with the charged photoconductor (Fig. 23.10). The toner is then attracted to regions of opposite charge and a sensor-controlled replenishment system is used to maintain the appropriate ratio of toner to carrier beads.

Monocomponent development simplifies this process by not requiring carrier beads, replenishment system, and attendant sensors. A much more compact development system results, and there are two implementations: magnetic and nonmagnetic. Magnetic methods still form a magnetic brush but it consists of toner particles only. A technique of widespread application is to apply an oscillating voltage to a metal sleeve on the development roller. The toner brush is not held in contact with the photoconductor but, rather, a cloud of toner particles is induced by the oscillating voltage as particles detach and reattach depending on the direction of the electric field. Nonmagnetic monocomponent development is equally popular in currently available printers. There are challenges in supplying these toners in charged condition and at rates sufficient to provide uniform development at the required print speed. Their desirability derives from lower cost and inherent greater transparency (for color printing applications) due to the absence of magnetic additives.

One way of circumventing the limitations on particle size and the need for some form of brush technique is to use liquid development. The toner is dispersed in a hydrocarbon-based carrier and is charged by means of the electrical double layer that is produced when the toner is taken into solution. Typically, the liquid toner is brought into contact with the photoconductor via a roller. Upon contact, particle transport mechanisms, such as electrophoresis, supply toner to the image regions. Fluid carryout is a major challenge

for these printers. To date this has meant commercial use where complex fluid containment systems can be employed. The technique is capable of competing with offset lithography and has also been used for color proofing.

4. The transfer and fuse stage imposes yet more demands on the toner and photoconductor. The toner must be released to the paper cleanly and then fixed to make a durable image (fusing). The majority of fusing techniques employ heat and pressure, although some commercial systems make use of radiant fusing by means of zenon flash tubes. The toner particles must be melted sufficiently to blend together and form a thin film, which will adhere firmly to the substrate. The viscosity of the melted toner, its surface tension, and particle size influence this process. The design challenge for this process step is to avoid excessive use of heat and to limit the pressure so as to avoid smoothing, that is, calendering and/or curling of the paper. Hot-roll fusing passes the toned paper through a nip formed by a heated elastomer-coated roller in contact with an unheated opposing roller that may or may not have an elastomer composition. Some designs also apply a thin film of silicone oil to the heated roller to aid in release of the melted toner from its surface. There is inevitably some fluid carryout under these conditions, as well as a tendency for the silicone oil to permeate the elastomer and degrade its physical properties. Once again materials innovation plays a major role in electrophotography.

5. The final phase involves removal of any remaining toner from the photoconductor prior to charging and imaging for the next impression. Common techniques involve fiber brushes, magnetic brushes, and scraper blades. Coronas to neutralize any residual charge on the PC or background toner are also typical components of the cleaning process. The toner removed in this step is placed in a waste toner hopper to be discarded. The surface hardness of the PC plays a key role in the efficiency of this step. Successful cleaning is especially important for color laser printers since color contrast can make background scatter particularly visible, for example, magenta toner background in a uniform yellow area.

Dot Microstructure

With respect to image microstructure, the design of the toner material, the development technique, and the properties of the photoconductor play key roles. It is desirable to have toner particles as small as possible and in a tightly grouped distribution about their nominal diameter. Composition of toner is the subject of a vast array of publications and patents. Fundamental goals for toner are a high and consistent charge-to-mass ratio, transparency in the case of color, a tightly grouped distribution, and a minimum, preferably no, wrong-sign particles. The latter are primarily responsible for the undesirable background scatter that degrades the print. Recent developments in toner manufacture seek to control this by means of charge control additives which aid in obtaining the appropriate magnitude of charging and its sign. Grayscale in laser printers is achieved by modulating the pulse width of the diode laser. The shape and steepness of the transfer curve, which relates exposure to developed density, is a function of photoconductor properties, development process, and toner properties. It is possible to produce transfer curves of low or high gradient. For text, a steep gradient curve is desirable, but for images a flatter gradient curve provides more control. Since the stability of the development process is subject to ambient temperature and humidity, the production of a stable grayscale color laser printer without print artifacts is most challenging.

Thermal Printing Technologies

Printing technologies that employ the controlled application of thermal energy via a contacting printhead to activate either physical or chemical image formation processes come under this general classification. There are four thermal technologies in current use: direct thermal, direct thermal transfer, dye diffusion thermal transfer, and resistive ribbon thermal transfer.

Direct Thermal

This is the oldest and most prolifically applied thermal technology. The imaging process relies on the application of heat to a thermochromic layer of approximately 10 µm in thickness coated onto a paper substrate. The thermally active layer contains a leuco dye dispersed along with an acid substance in a binder. Upon heating fusion melting occurs resulting in a chemical reaction and conversion of the leuco dye into a visible deeply colored mark. Key to this process is the design of the printhead, which can be either a page-wide array or a vertical array or a scanning printhead. Two technologies are in use, thick film and thin film. Thick-film printheads have resistor material between 10 and 70 µm. The resistive material is covered with a glass layer approximately 10 µm thick for wear resistance. The thin-film printheads bear a strong resemblance to those found in thermal ink jet printheads. They employ resistive material, such as tantalum nitride, at 1 µm thickness and a 7-µm-thick silicon dioxide wear layer. Thin-film heads are manufactured in resolutions up to 400 dpi. In each case the resistors are cycled via electrical heating pulses through temperature ranges from ambient (25◦C) up to 400◦C. Overall, the thin-film printheads excel in energy efficiency conversion, print quality, response time, and resolution. For these reasons the thin-film printheads are used when high resolution is required, whereas the thick-film printhead excels in commercial applications such as bar coding, airline tickets, fax, etc.

FIGURE 23.11 Schematic of wax transfer process: (a) Intimate contact between printhead, ribbon, and paper is required for successful transfer, (b) design elements of thin-film thermal printhead. Thermal barrier insulates heater for the duration of the heat pulse but allows relation of heater temperature between pulses.

Direct Thermal Transfer

These printers transfer melted wax directly to the paper (Fig. 23.11(a) and Fig. 23.11(b)). The wax that contains the colorant is typically coated at 4 µm thickness onto a polyester film, which, in common implementations, is approximately 6 µm thick. A thermal printhead of the kind described previously presses this ribbon, wax side down, onto the paper. As the individual heating elements are pulsed, the wax melts and transfers by adhesion to the paper. The process is binary in nature; but, by the use of shaped resistors, which produce current crowding via an hourglass shape, for example, the area of wax transferred can be modulated. Common implementations employ page width arrays at 300 dpi with some providing vertical addressability of 600 dpi. The thermal ribbons are also packaged in cassettes for scanning printhead designs in desktop and portable printers. Power consumption is an issue for all thermal printers, and efforts to reduce this for direct thermal transfer have focused on reducing the thickness of the ribbon.

Dye Diffusion Thermal Transfer

This technology involves the transfer of dye from a coated donor ribbon to a receiver sheet via sublimation and diffusion, separately or in combination. The amount of dye transferred is proportional to the amount of heat energy supplied; therefore, this is a continuous tone technology. It has found application as an alternative to silver halide photography, graphics, and prepress proofing. As with all thermal printers the energy is transferred via a transient heating process. This is governed by a diffusion equation and depending on the length of the heating pulse will produce either large temperature gradients over very short distances or lesser gradients extending well outside the perimeter of the resistor. Much of the design, therefore, focuses on the thicknesses of the various layers through which the heat is to be conducted. In the case of thermal dye sublimation transfer a soft-edged dot results, which is suitable for images but not for text. Shorter heating pulses will lead to sharper dots.

Resistive Ribbon Thermal Transfer

This technology is similar to direct thermal transfer in that a thermoplastic ink is imaged via thermal energy onto the substrate. The ribbon is composed of three layers: An electrically conductive substrate of polycarbonate and carbon black (16 µm thick), an aluminum layer 1000–2000 A˚ , and an ink layer which is typically 5 µm. The aluminum layer serves as a ground return plane. Heat is generated by passing current from an electrode in the printhead in contact with the ribbon substrate through the polycarbonate/carbon layer to the aluminum layer. The high pressure applied through the printhead ensures intimate contact with the paper, which does not have to be especially smooth for successful transfer. Printed characters can be removed by turning on all electrodes at a reduced energy level and heating the ink to the point that it

bonds to the character to be corrected but not to the paper. The unwanted character is removed as the printhead passes over it. This technology does not adapt to color printing in a straightforward way.

Nonimpact Printing Technologies

Ink Jet

The transfer process of ink jet printing is one of removing a drop of liquid ink from the bulk and giving it a velocity of sufficient precision and magnitude to place it on a substrate in close proximity to but not touching the printhead. There are three broad techniques: continuous, electrostatic, and drop on demand. Continuous ink jet printing, because of its intrinsic high drop rate, has tended to find more applications in commercial systems; electrostatic methods have yet to find widespread application, but have been used for facsimile recording; drop on demand, because of its simplicity and ease of implementation of color, has been widely accepted in the office and home market.

Continuous Ink Jet

The basic principle of continuous ink jet is to take advantage of the natural breakup process due to an instability in the jet that is formed when fluid is forced under pressure through a small orifice. This results from the interplay of surface tension and viscosity and takes place in a quasirandom manner unless external stimulation is applied. This breakup process was first studied by Rayleigh who characterized it via a growth rate for the instability, which depended on the jet diameter D, its velocity V , and the frequency F of any external stimulation. Rayleigh showed that the frequency for maximum growth rate of the instability was

F = V/4.5D. By stimulating the jet at this frequency it is possible to obtain a uniform stream of droplets. The typical method of providing this stimulation today is via a piezoelectric transducer as an integral part of the printhead.

To make use of these droplets for printing it is necessary to charge them at breakoff. This is accomplished by placing electrodes in proximity to the breakup region of the jet. Deflection voltages are then applied farther downstream to direct the droplet to the substrate or into a collector for re- circulation and reuse. The earliest techniques in- volved charging the droplet and applying a variable deflection field to direct it to a specific spot on the paper, enabling full height characters to be printed in one pass (see Fig. 23.1). Later methods focused on producing a stream of charged droplets and using the printing (high-voltage) electrode to deflect unwanted drops for recirculation and reuse (Fig. 23.2). This technique, known as binary charged continuous ink jet, lends itself to the construction of multiple nozzle arrays, and there are a number of page-wide implementations in use.

Binary charged continuous ink jet with its high droplet rate makes a simple gray scaling technique possible. The dot on the paper is modulated in size by printing from one up to N droplets at the same location, where N is the number of different dot sizes desired. By operating at high frequencies and small drop volumes it is possible to produce sufficient gray levels such that full grayscale printing is achieved at reasonable print speeds. The most recent implementation of this method offers 512 gray levels at addressabilities between 200–300 pixels/in. To achieve the small fundamental droplet size, typical implementations employ glass capillaries with diameters of the order of 10 µm and are pressurized from 500–700 lb/in2. A color printhead will contain four capillaries, one for each color ink plus black.

FIGURE 23.2 Binary charged continuous ink jet. Droplets, charged at breakoff, are directed to the paper on a rotating drum. Droplets not selected for printing are diverted by the high-voltage electrode to be collected by a return gutter. (Source: Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA. With permission.

Drop On Demand (DOD) Ink Jet

For office and home applications the complexities of continuous ink jet technology, such as startup and shutdown procedures, ink recirculation, and the limited nozzle count, have led to the development of drop on demand ink jet technology. These devices employ unpressurized ink delivery systems and, as implied by their name, supply a drop only when requested. The basic technique employed is to produce a volume change in either the ink supply channel or an ink chamber adjacent to the nozzle such that the resulting pressure wave causes drop ejection. Refill is achieved by capillary forces and most DOD systems operate with a slight negative pressure at the ink reservoir. The mechanism for generating the pressure wave dominates the design of these devices, and there are two techniques extant in common DOD printers. One employs the pressure pulse derived from the vaporization of superheated fluid, and the other makes use of piezoelectric materials, which can be deformed by the application of electric potentials.

Devices employing the vaporization of superheated fluid are known concurrently as thermal ink jet or bubble jet printers, the choice of name depending on the manufacturer. Since drop on demand ink jets rely on capillary refill, their operational frequencies are much lower than for continuous ink jet devices. This stresses the importance of the compactness of the actuating system so as to achieve reasonable printing speeds via multiple nozzle printheads. The nozzles must also be precisely registered with respect to each other if systematic print artifacts are to be avoided.

FIGURE 23.3 Drop ejection sequence for thermal ink jet. Nominal time frames and values for various parameters are given to indicate the scale of the three processes of nucleation, bubble growth, and jet formation followed by drop ejection and refill.

Thermal Ink Jet/Bubble Jet DOD Printers

When fluids are heated at extreme rates (e.g., 500 × 106 W/m2), they enter a short-lived metastable state where temperatures can far exceed the boiling point at atmospheric pressure. The difference between the elevated temperature and the boiling point is known as the degree of superheat. This process does not continue indefinitely, and all fluids have what is known as a superheat limit. At this point nucleation and vaporization will occur in the bulk of the fluid. These devices employ an electrically driven planar heater (typically, 50–60 µm2) in contact with the fluid. Under these conditions vaporization commences at the surface of the heater due to the presence of nucleation sites such as microscopic roughness. With correctly chosen heating rates this can be made very reliable. These heating rates lead to electrical pulse widths of 3–5 µs. In this time frame only a submicron portion of the fluid will be superheated. The net result is a vaporization pulse well in excess of atmospheric pressure and of approximately 3/4-µs duration. By locating a nozzle directly over or alongside the resistor this pressure pulse will eject a droplet (Fig. 23.3).

Within limits of the drop volume desired, it is found that the linear dimensions of the nozzle diameter and planar resistor are comparable. The actuator is therefore optimally compact, and this enables high- nozzle count printheads. The fabrication of the resistors is accomplished by photolithographic techniques common to the IC industry and the resistor substrates are silicon with a thin layer of insulating silicon dioxide. Precise registration from nozzle to nozzle is guaranteed under these circumstances, and electrical drive circuits may be integrated into the head to provide multiplexing capability. This is a valuable attribute for scanning printheads, which employ a flexible printed circuit for interconnect. These features have produced printheads currently numbering 300 or more nozzles for a single color. An additional benefit of the compactness of this technology is that the ink supply can be fully integrated with the print- head. This provides the user with virtually maintenance free operation as the printhead is replaced when the ink supply is consumed. Since the majority of problems arise from paper dust particles finding their way into a nozzle and occasionally becoming lodged there, printhead replacement provides for user service at reasonable cost. Some implementations feature a semipermanent printhead, which is supplied by ink from replaceable cartridges. The design choice is then a tradeoff involving many factors: frequency of maintenance, cost of operation, how often the printer is to be used, type of material printed, etc. The important subject of ink and paper for these printers will be taken up at the end of the section on DOD technologies.

FIGURE 23.4 Squeeze tube piezoelectric ink jet. Early implementation of piezoelectric transducers for drop ejection. (Source: Advanced Technology Resources Corporation.)

Piezoelectric DOD Printers

Crystalline structures, which develop a spontaneous dipole moment when mechanically strained, thereby distorting their crystal structures, are called piezoelectric. These materials may conversely be caused to be distorted via electrical potentials applied to the appropriate planes of the cyrstal. Piezoceramics have a polarization direction established during the manufacturing process, and the applied fields then interact with this internal polarization to produce mechanical displacement. Depending on the direction of the applied fields, the material can compress or extend longitudinally or transversely. These materials have found widespread use as transducers for DOD printers. An early form was that of a sleeve over a glass capillary, which terminated in a nozzle (Fig. 23.4). Depending on the location of the electrodes either a radial or longitudinal compression could be applied leading to a pressure wave in the enclosed ink sufficient to eject a droplet. Using the diameter of the nozzle as a unit of linear dimension, this approach placed the transducer well upstream from the nozzle (Fig. 23.5). Implementation of this design in a multinozzle printhead required careful matching of transducers and fluid impedance of the individual channels feeding each nozzle. This was a challenging task, and most designs bond a planar transducer to an ink chamber adjacent to a nozzle, as shown in Fig. 23.6.

FIGURE 23.5 Drop ejection sequence for piezoelectric printhead. Schematic of drop ejection via deflection of piezocrystal bonded to an ink capillary. In practice the piezodrivers were located well upstream of the nozzle due their size. (Source: Advanced Technology Resources Corporation.)

FIGURE 23.6 Design of Stemme Larson electric-driven DOD ink jet. Note the direct coupling of the pressure pulse to the ink chamber at the nozzle. (Source: Advanced Technology Resources Corporation.)

The method of directly coupling the piezoelectric transducer through an ink chamber to an exit nozzle has seen many enhancements and developments since its invention. A feature of some designs is that of air flow channeled at the orifice in such a way as to entrain the droplet as it exits the nozzle and to improve its directional stability, as well as to accelerate the droplet. This enables the device to be operated at lower transducer deflections and, therefore, at higher droplet rate since the settling time of the device has been reduced. Piezodevices can operate at elevated temperatures and are used to eject inks that are solid at room temperature. For solid inks the material is melted to a design temperature for appropriate viscosity and surface tension and then supplied to the piezoelectric-driven ink chamber. The ink then solidifies instantly on contact with the substrate.

A more recent innovation employs piezoelectric transducers operated in the longitudinal mode. The transducers are formed from a single block of piezoceramic material in the form of an array of rods. Suitably placed potentials excite the rods to extend in the longitudinal direction. By bonding one end of the rod in contact with a thin membrane forming the base of an ink chamber, a pressure pulse is generated similar to that of the previous design (Fig. 23.7). To achieve sufficient pressure amplitude a diaphragm is used that is substantially larger than the orifice exit diameter. The consequence of this is that high nozzle

density printheads will require multiple rows of nozzles (Fig. 23.8). This design has been implemented to date with liquid inks only.

Grayscale Methods for DOD Ink Jet Printers

The drop rates for DOD devices are typically an order of magnitude less than those of continuous, pressurized systems. This dictates different strategies for the achievement of grayscale. Techniques are based on the generation of a few gray levels that, when incorporated into digital halftoning algorithms, such as error diffusion, clustered, dispersed dot, or blue-noise dither, produce a satisfactory grayscale. The number of levels necessary, their position relative to the maximum modulation achievable (i.e., maximum dot size or maximum intensity), and the specialized techniques employed in digital halftoning are an area of active research. There are many patents in the literature governing these techniques, and manufacturers seek to distinguish their devices by the technique offered. When combined with resolution enhancement methods mentioned in the section on print quality, printers with medium resolution, such as 300 dpi and 2 bits of grayscale, can produce remarkable results for both images, text, and graphics.

There are several methods available for DOD devices to modulate either the size or intensity of the dot. For piezodevices, pulse width modulation has been shown to produce droplets of different volumes and, therefore, dot sizes. All DOD ink jet devices have the option of ejecting a droplet repeatedly at the same location by passing over the same swath as many times as desired but this will affect throughput rates. Printheads with sufficient nozzle count can do this and still keep throughput rates within reason. For vapor bubble driven devices, a unique option exits by virtue of the short duration of the actuating bubble. Typical lifetime of bubbles in these devices, from vaporization through to bubble collapse, is of the order of 20 µs. If the resistor is pulsed shortly after bubble collapse, a second droplet can be ejected virtually on the tail of the initial droplet. This technique has been called multidrop in the literature. The ink chamber is fired under partial refill conditions, but with proper design several droplets can be ejected by this method at drop rates at around 40 kHz and having substantially the same volume (Fig. 23.9). These merge on the substrate to produce different dot sizes according to the number of droplets ejected for the location. This is not an option for most piezodevices due to the slower settling time of the actuator. Dye dilution methods have also been demonstrated as a way of modulating the intensity of the dot. If no halftone algorithm is employed, this will require many sets of nozzles to accommodate the different dye dilutions.

FIGURE 23.9 Stylized representation of multidrop process. Each input pulse consists of a group of drive pulses chosen for the final dot size desired. (Source: Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA. With permission.)

Ink and Paper for Ink Jet Devices

When liquid inks are employed the paper properties have a major impact on the print quality. The ink droplets will be absorbed by a substrate whose internal structure and surface energy will determine the size, shape, and overall microstructure of the drop. Paper, being a interlocking mesh of cellulose fibers with sizing and binding chemistry, is quite variable in nature. Figure 23.10 is a schematic indication of the response of paper to different volumes of ink. Note that it can be very nonlinear at low drop volumes and either flat or high gain at large volumes. The implication is that by simply changing the paper the print quality is altered. To control this variablity some papers are given a thin coat of claylike material containing whiteners, which are often fluorescent.

This coating presents a microporous structure that is more uniform than the cellulose fibers.

Dot formation on coated papers is therefore circular and more stable than on uncoated stock. Un-coated papers allow the ink to wick down the fibers producing an effect known as feathering of the dot.  In this case, microscopic tendrils of dye appear at the edge of the dot giving it and the overall print quality a blurred effect. This is particularly serious in the case of text printing, which benefits most fromsharp dot edges. Feathering is common for papers used in xerographic copiers. Bond paper, which is a popular office stock, is a partially coated paper and exhibits little feathering.

FIGURE 23.10 Paper response curves. The lower curve is typical of coated stock, which minimizes dot spread. (Source: Durbeck, R.C. and Sherr, S. 1988. Hardcopy Out- put Devices. Academic Press, San Diego, CA. With per- mission.)

Depending on the manufacturer, several techniques are employed to minimize the impact of paper variability on print quality. One method, the use of a heater, takes advantage of the fact that absorption into the paper does not commence immediately upon contact. There is a period known as the wetting time, which can be as long as 80 µs during which no absorption takes place. The application of heat immediately in the vicinity of the printhead swath can effectively “freeze the dot” by vaporizing carrier. This makes the printer insensitive to change in paper stock and provides uniformly high print quality regardless of substrate. Other methods make use of altering the fluid chemistry by means of surfactants, which increase penetration rate, and high vapor pressure additives to increase removal of fluid into the atmosphere. If the ink penetrates quickly then it is less likely to spread sideways and, thereby, dot size variation is lessened. When choosing a drop on demand ink jet printer, it is advisable to test the performance over the range of paper stocks to be used. In some cases it will be found that high-quality printing can only be obtained when a paper specified by the manufacturer is chosen.

With reference to the section on print quality, it should be kept in mind that the choice of paper will affect the overall dynamic range of print. Text printing, to be pleasing, needs to have an optical density of at least 1.3–1.4. For images, the more dynamic range the better, and special coated stock will always excel over copy paper if image quality is important. Many of the coated papers available still have a matte surface that diffusely reflects the light and limits the dynamic range for reasons previously discussed. Some manufacturers now offer a high-gloss substrate specifically intended for images. These substrates have a plastic base with special coatings to absorb the ink through to the substrate leaving a high gloss finish. This greatly improves the dynamic range to the point of approximating that of photographic paper. These substrates provide ink jet printers with the capability to produce highly saturated brilliant colors with exceptional chromatic and dynamic range and should be used if image printing is the primary objective. Besides excellent print quality there are other demands placed on the ink. It must provide reliable operation of the device and a durable image. By this it is meant that the image does not fade rapidly, that it is mechanically sound, that it cannot be easily removed from the paper, and that it is impervious to liquids. For liquid ink, this is a challenge since solvent-based color highlighter pens are commonly used to mark up printed documents. These solvents can cause the ink to smear depend- ing on the choice of ink chemistry and the manner in which the colorants are fixed to the substrate. These issues focus on colorant chemistry, and much research is applied to this problem. There are fade-proof dyes, but many are either incompatible with the ink vehicle, typically water, or are toxic or

mutagenic.

Printing Technologies

The four basic elements of any printing technology are: addressing, marking substance and its storage and delivery, transfer of the marking substance, and fixing. Addressing refers to the communication of electronic data to the marking unit, typically via electronic or optical means. The marking substance contains the

colorant, vehicle/carrier material for transport, binders to secure the colorants to the paper, stabilizing agents to resist fading, and technology specific additives such as biocides for liquid inks. The transfer process is the fundamental physical mechanism whereby a specific amount of the marking substance is removed from the bulk and transferred to the paper. Fixing embodies the processes of adhesion, drying, or solidification of the material onto the paper to form a durable image. These fundamental subsystems interact with each other to give each printing technology its own unique characteristics. The common classification of printing technologies today begins with the broad separation into two classes: Impact and nonimpact printing technologies. Impact methods achieve transfer via the direct mechanical application of force or pressure via a marking element, which can be either a fine wire or fully formed character onto a colorant carrying ribbon in contact with the paper; the simplest form of this is a typewriter. Nonimpact methods cover a wide range of technologies that achieve transfer through a variety of means that may be either contact or noncontact in nature.

Introduction

The basic parameters of print quality are resolution, addressability, gray scale, and dot microstructure. A real device also has intrinsic variability in the printing process, producing visual artifacts, which come under the general heading of noise. Some of the more common manifestations of this are background scatter, dot placement errors, voids (due to nozzle malfunction in ink jet, for example), and banding in images. The significance of any of these aspects of print quality can only be determined by examining them with respect to the properties of the human visual system. The design choices of the basic print quality parameters are, therefore, guided by the properties of the human visual system to determine where improvement needs to be made or where little is to be gained by increasing any one of the specifications.

Resolution and Addressability

Resolution, the most widely used specification to rate print quality, is sometimes confused with the related term addressability. Fundamentally, resolution refers to the ability of the device to render fine detail. This simple definition is complicated by the fact that detail can be regarded as the fineness of the width of a line, the transition between white paper and printed intensity, and/or the smoothness of the edge of a curved line or a line printed at any arbitrary angle. In the simplest case, the resolution of a printer is defined as

the spacing of the dots such that full coverage is obtained, that is, no white paper can be seen. For circular dots placed on a square grid, this number would be calculated by dividing the diameter by the square root of two and taking its inverse. For example, an ideal 300 dots per inch (dpi) printer would produce 120-µm-diam dots at an 85 µm spacing. In practice, the dot would be made somewhat larger to allow for dot placement errors. This definition is best understood in terms of the finest line that can be printed by the device. At 300 dpi the line would exhibit a perceptible edge waviness, especially when printed at certain sensitive angles. This would also be true of curved lines. In addition, the range of lines of increasing thickness would have discontinuities since they would consist of an integral number of the basic line, each spaced at 85 µm. These issues have an important bearing on text print quality, which depends on the ability to render both curved and straight lines at variable widths.

The preceding definition is related to the specification of resolution with respect to the human visual system. In this case resolution is determined by the closeness of spacing between alternate black and white lines of equal width and defined contrast. These are known as line pairs and for a 300-dpi printer it would result in a value of 150 line pairs per inch. This is not strictly correct since the black lines would be wider than the white spaces due to the roundness of the dot. Since the human visual system has a response that approaches zero near 300 line pairs per inch, gains made in text print quality by increasing resolution alone can be expected to diminish above this value. At this point, issues such as print noise and grayscale enter if further improvement in print quality is desired.

To focus only on the resolution as defined in the previous paragraphs ignores the specific needs of the components of the printed material, that is, text and lines vs. images and area fill. Gains in text print quality may be had if the device can space the dots closer than the fundamental resolution. This can result in substantial dot overlap but allows the line width to be varied more continuously. In addition, at the edge of a curved line, the subpixel adjustments of individual dots increase the perception of smoothness commonly known as getting rid of the jaggies. This ultimate dot spacing of the device is called addressability. For example, printers employing this technique are specified as 300 × 600 dpi indicating a native resolution of 300 dpi in the horizontal direction and a vertical addressability of 600 dpi.

Grayscale

The ability of a printing technology to modulate the printed intensity on the page is referred to as its grayscale capability. There are three ways in which this may be accomplished: variation of the dot size, variation of the intensity of the printed dot, and digital halftoning techniques. The first two depend on the intrinsic properties of the technology, whereas digital halftoning can be employed by any printer. A printer that can continuously vary its intensity from white paper through to maximum colorant density is described as having continuous tone capability. Other technologies produce a modest number of intensity levels and make use of digital halftoning techniques to create a continuous tone effect. The manner in which gray scale is achieved is of obvious importance in image printing, particularly in the case of color. In recent years considerable effort has gone into the development of sophisticated digital halftoning algorithms to enable binary (single dot size and no intensity modulation) printers to render images. The resulting image quality depends more strongly on resolution than addressability. But the impact of even a few intrinsic gray levels on the print quality achieved by these algorithms can be dramatic.

An important parameter in grayscale considerations is that of the dynamic range, which is simply called range in the graphic arts. This is measured in terms of optical density, the negative logarithm of the reflectance. An optical density of 1.0 represents 10% of reflected light per instant flux, an optical density of 2.0 corresponds to 1% reflectance, and so on. For printed material the smoothness of the printed surface limits the maximum optical density obtainable. If the surface is smooth and mirrorlike, then the print appears glossy and can have optical densities approaching 2.4. The smooth surface reflects light in a specular manner and, therefore, scatters little stray light from the surface into the eye, and the color intensity is not desaturated. It is most noticeable in the case of photographic paper that has a high gloss finish. If the optical density range of the print is high, it is said to have high dynamic range and a very

pleasing image will result. Not all papers, however, are designed to have a glossy finish. Papers used in the office are also used in copiers and have a surface which produces diffuse reflection at the interface between the air and the paper. For most uncoated, nonglossy papers this will be between 3–4% and limits the maximum optical density to around 1.4. Image quality on these stocks will depend on the fixing of the colorant to the substrate to produce a smooth surface. The potential image quality for a printer is therefore a complex tradeoff involving the design choices of resolution, addressability, grayscale method, digital halftoning algorithm, paper stock, colorant, and fixing technology. Conclusion: for images, resolution alone is not a predictor of print quality.

Dot Microstructure

The microscopic nature of the dot produced by a given technology also has a bearing on final print quality. The most important parameter here relates to the edge gradient of the dot. Known as the normal-edge profile, it characterizes the transition between white paper and maximum colorant intensity, that is, the gradient of optical density that occurs at the edge of the dot and measures the steepness of the transition from white paper to full optical density. Some technologies, such as electrophotography, can vary this profile by adjusting various parameters in the imaging and developing process. For ink jet, various paper types will produce different normal-edge profiles. If the profile is very steep, that is, the transition occurs in a very small distance such as 5 µm, then the dot is described as being a hard dot or having a very sharp edge. This is desirable when printing lines and text that benefit from very sharp transitions between black and white. If this transition is gradual, the dot is described as being soft and produces a blurring of the edge, which can degrade the text quality. In the case of images, where smooth tones and tonal changes are desired, a soft dot can be very beneficial.

Hybrid Methods

From what has been said it should not be inferred that the needs of texts and images are in opposition. In recent years the intrinsic grayscale capability has been used to advantage in improving text print quality. The removal of jaggies can be greatly assisted by the combination of increased addressability and a few gray levels. By the use of gray levels in the region of the jagged stairstep, the transition can be made to take place over several pixels. This is, in essence, blurring the transition to make it less visible to the eye. In the case of certain fonts, there is fine detail requiring resolutions greater than the native resolution of the printer. This fine detail can be rendered through a combination of gray levels and regular pixels. The implementation of these methods requires a complex set of rules to be applied to the data bit stream before it is sent to the marking level of the printer. These rules draw heavily on image processing techniques and a knowledge of the human visual system and are proprietary. Skillfully applied they can have a dramatic effect on the text and line quality. There are a variety of trademarked names for these technologies designed to convey the sense of enhancement of the print quality.

The application of image processing techniques to manipulate the intrinsic properties of electronic printing technologies have made resolution an insufficient measure of print quality. A more comprehensive measure is needed to simplify the identification of the printing technology to serve the design goals for final output quality. Until such a metric is devised, the tradeoff analysis just described, implemented by means of industry standard test charts that separately probe the printer properties, will provide a predictive measure of print quality. Such test charts must also contain test images, which will be subject to the proprietary subjective image enhancement algorithms offered by the manufacturer.