19.8 Printing Technologies and Systems
19.8.1 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.
19.8.2 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.
19.8.3 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
FIGURE 19.66 Character printing with continuous ink jet. The deflection plate applies an analog voltage to steer the drop to the desired location; unwanted droplets are undeflected and captured by the return gutter. (Source: Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA. With permission.)
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 ac- complished 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 vari- able deflection field to direct it to a specific spot on the paper, enabling full height characters to be printed in one pass (see Fig. 19.66). 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. 19.67). 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 address- abilities 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.
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.
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. 19.68). 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 fea- tures 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.
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. 19.69). 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. 19.70). 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. 19.71.
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. 19.72). 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. 19.73). 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. 19.74). 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.
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 19.75 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 cir- cular 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 from sharp 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.
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 reli- able 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.