Print Engines

The challenge faced by any printer is how to get ink (or something that looks like ink but isn't) onto paper—and keep it there. The actual mechanism that handles this job and forms an image on paper is called the print engine. Engineers have developed a number of technologies for print engines, and each uses a somewhat different physical principle to form an image on paper. Although each technology has its strengths, weaknesses, and idiosyncrasies, you might not be able to tell the difference between the pages they print. Careful attention to detail has pushed quality up to a level where the paper rather than the printer is the chief limit on resolution, and color comes close to photographic, falling short only on the depth that only a thick gelatin coating makes possible.

In making those images, however, the various print engine technologies work differently at different speeds, at different noise levels, and with different requirements. These differences can make one style of print engine a better choice for your particular application than the others.

Impact Dot-Matrix

Today's computer printer evolved from a brash, noisy creation called the impact dot-matrix printer. Although only a few models persist on the market, impact printers once were the mainstay of the industry, and their method of forming characters, from a matrix of dots one line at a time, survives to this day in nearly all low-cost printers.

The heart of the dot-matrix printer is a mechanical printhead that shuttles back and forth across the width of the paper. A number of thin print wires act as the hammers that squeeze ink from a fabric or Mylar ribbon to paper. Once the mainstay of computer printing, the classic dot-matrix printer is now an endangered species. It remains noteworthy, however, as the progenitor of the computer printer.

In most impact dot-matrix printers, a seemingly complex but efficient mechanism controls each of the print wires. The print wire normally is held away from the ribbon and paper, and against the force of a spring, by a strong permanent magnet. The magnet is wrapped with a coil of wire that forms an electromagnet, wound so that its polarity is the opposite of that of the permanent magnet. To fire the print wire against the ribbon and paper, this electromagnet is energized (under computer control, of course), and its field neutralizes that of the permanent magnet. Without the force of the permanent magnet holding the print wire back, the spring forcefully jabs the print wire out against the ribbon, squeezing ink onto the paper. After the print wire makes its dot, the electromagnet is de-energized and the permanent magnet pulls the print wire back to its idle position, ready to fire again. Figure 26.1 shows a conceptual view of the mechanism associated with one printhead wire.

The two-magnets-and-spring approach is designed with one primary purpose—to hold the print wire away from the paper (and out of harm's way) when no power is supplied to the printer and the printhead. The complexity is justified by the protection it affords the delicate print wires.

The printhead of a dot-matrix printer is made from a number of these print wire mechanisms. Most first-generation personal computer printers and many current machines use nine wires arrayed in a vertical column. To produce high quality, the second generation of these machines increased the number of print wires to 18 or 24. These often are arranged in parallel rows with the print wires vertically staggered, although some machines use different arrangements. Because the larger number of print wires fit into the same space (and print at the same character height), they can pack more detail into what they print. Because they are often finer than the print wires of lesser endowed machines, the multitude of print wires also promises higher resolution.

No matter the number of print wires, the printhead moves horizontally as a unit across the paper to print a line of characters or graphics. Each wire fires as necessary to form the individual characters or the appropriate dots for the graphic image. The impact of each wire is precisely timed so that it falls on exactly the right position in the matrix. The wires fire on the fly—the printhead never pauses until it reaches the other side of the paper.

A major factor in determining the printing speed of dot-matrix machines is the time required between successive strikes of each print wire. Physical laws of motion limit the acceleration that each print wire can achieve in ramming toward the paper and back. Therefore, the time needed to retract and reactuate each print wire puts a physical limit on how rapidly the printhead can travel across the paper. It cannot sweep past the next dot position before each of the print wires inside it is ready to fire. If the printhead travels too fast, dot-positioning (and character shapes) would become rather haphazard.

Adding color to an impact dot-matrix printer is relatively straightforward. The color that the impact printer actually prints is governed by the ink in or on its ribbon. Although some manufacturers build color impact printers using multiple ribbons, the most successful (and least expensive) design uses special multicolored ribbons lined with three or four bands corresponding to the primary colors. To change colors, the printer shifts the ribbon vertically so that a differently hued band lies in front of the print wires. Most of the time the printer will render a row in one color, shift ribbon colors, and then go across the same row in a different color. The extra mechanism required is simple and inexpensive, costing as little as $50 extra. (Of course, the color ribbon costs more and does not last as long as its monochrome equivalent.)

Although the ribbons used by most of these color printers are soaked with three or four colors of ink, they can achieve seven colors on paper by combining color pairs. For example, laying a layer of blue over a layer of yellow results in an approximation of green.

Most impact printers can spread their output across any medium that ink has an affinity for, including any paper you might have lying around your home, from onion skin to thin cardstock. Although both impact and non-impact technologies have been developed to the point that either can produce high-quality or high-speed output, impact technology takes the lead when you share one of the most common business needs: making multipart forms. Impact printers can hammer an impression not just through a ribbon but through several sheets of paper as well. Slide a carbon between the sheets, or better yet, treat the paper for noncarbon duplicates, and you get multiple, guaranteed-identical copies with a single pass through the mechanism. For a number of business applications—for example, the generation of charge receipts—exact carbon copies are a necessity and impact printing is an absolute requirement.

But impact printers have fallen from favor for several reasons. The primary one is, as always, cost. That marvelous printhead is a complicated mechanism with many parts to manufacturer and many areas to develop problems. Today's leading printer technologies have cut the number of moving parts in a printer (and thus its cost) dramatically. Moreover, because the wires are mechanical constructions, making them smaller is a tough engineering problem. Compared to other technologies, the size of the dots made by impact printers is huge and the quality is low. Impact printouts look like, well, old-fashioned computer printouts.

Impact printers reveal their mechanical heritage in other ways. The hammer bashing against the ribbon and paper makes noise, a sharp staccato rattle that is high in amplitude and rich in high frequency components, penetrating and bothersome as a dental drill or angry horde of giant, hungry mosquitoes. Typically, the impact printer rattles and prattles louder than most normal conversational tones, and it is more obnoxious than an argument. The higher speed the impact printer, the higher the pitch of the noise and the more penetrating it becomes. What's more, printheads wear out. Nothing can take a constant beating without suffering, and tiny printhead wires are no different.

Engineers discovered how to duplicate the work of the impact dot-matrix printer, only do it better, sharper, and with fewer moving parts and noise. The result was the inkjet printer, today's low-cost, mass-market leader.

Inkjets

Today's most popular personal printers use inkjet print engines. The odd name, inkjet, actually describes the printing technology. If it conjures up images of the Nautilus and giant squid or a B-52 spraying out blue fluid instead of a fluffy white contrail, your mind is on the right track. Inkjet printers are electronic squids that squirt out ink like miniature jet engines fueled in full color. Although this technology sounds unlikely—a printer that sprays droplets of ink onto paper—it works well enough to deliver image sharpness on par with most other output technologies.

In essence, the inkjet printer is a line printer, little more than a dot-matrix printer with the hammer impact removed. Instead of a hammer pounding ink onto paper, the inkjet flings it into place from tiny nozzles, each one corresponding to a print wire of the impact dot-matrix printer. The motive force can be an electromagnet or, as is more likely today, a piezoelectric crystal (a thin crystal that bends when electricity is applied across it). A sharp, digital pulse of electricity causes the crystal to twitch and force ink through the nozzle in its flight to paper.

Today's inkjet printers are able to make sharper images than impact dot-matrix technology because they do not use ribbons, which would blur their images. The on-paper quality of an inkjet can equal and often better that of more expensive laser printers. Even inexpensive models claim resolution as high or higher than laser printers, say about 1200 or 1440 dots per inch.

Another advantage of the inkjet is color. Adding color is another simple elaboration. Most color impact printers race their printheads across each line several times, shifting between different ribbon colors on each pass—for example, printing a yellow row, then magenta, then cyan, and finally black. Inkjet printers typically handle three or four colors in a single pass of the printhead, although the height of colored columns often is shorter.

The liquid ink of inkjet printers can be a virtue when it comes to color. The inks remain fluid enough even after they have been sprayed on paper to physically blend together. This gives color inkjet printers the ability to actually mix their primary colors together to create intermediary tones. The range of color quality from inkjet printers is wide. The best yield some of the brightest, most saturated colors available from any technology. The vast majority, however, cannot quite produce a True Color palette.

Because inkjets are non-impact printers, they are much quieter than ordinary dot-matrix engines. Without hammers pounding ink onto paper like a bunch of myopic carpenters chasing elusive nails, inkjet printers sound almost serene in their everyday work. The tiny droplets of ink rustle so little air they make not a whisper. About the only sound you hear from them is the carriage coursing back and forth.

As mechanical line printers, however, inkjet engines have an inherent speed disadvantage when compared to page printers. Although they deliver comparable speeds on text when they use only black ink, color printing slows them considerably, to one-third speed or less.

The underlying reason for this slowdown is that most color inkjets don't treat colors equally and favor black. After all, you'll likely print black more often than any color or blend. A common Lexmark color inkjet printhead illustrates the point. It prints columns of color only 16 dots high while printing black columns 56 dots high (see Figure 26.2). Printing a line of color the same height as one in black requires multiple passes, even though the printer can spray all three colors with each pass.

Inkjet technology also has disadvantages. Although for general use you can consider them to be plain-paper printers, able to make satisfactory images on any kind of stock that will feed through the mechanism, to yield their highest quality inkjets require special paper with controlled absorbency. Although plain paper produces printouts adequate for business letters and other public disclosures, inkjet paper delivers the last iota of sharpness. You also have to be careful to print on the correct side of the paper because most paper stocks are treated for absorption only on one side. If you try to get by using cheap paper that is too porous, the inks wick away into a blur. If the paper is too glossy, the wet ink can smudge.

Early inkjet printers also had the reputation, often deserved, of clogging regularly. To avoid such problems, better inkjets have built-in routines that clean the nozzles with each use. These cleaning procedures do, however, waste expensive ink. Most nozzles now are self-sealing, so when they are not used, air cannot get to the ink. Some manufacturers even combine the inkjet and ink supply into one easily changeable module. If, however, you pack an inkjet away without properly purging and cleaning it first, it is not likely to work when you resurrect it months later.

Inkjet printers commonly use two different technologies, thermal and piezo-electric. (A third inkjet technology, phase-change, is distinct enough to have entirely different printing qualities.) At heart, the basic technology of both kinds of inkjets is the same. The machines rely on the combination of the small orifice in the nozzle and the surface tension of liquid ink to prevent a constant dribble from the jets. Instead of oozing out, the ink puckers around the hole in the inkjet the same way that droplets of water bead up on a waxy surface. The tiny ink droplets scrunch together rather than spread out or flow out the nozzle, because the attraction of the molecules in the ink (or water) is stronger than the force of gravity. The inkjet engine needs to apply some force to break the surface tension and force the ink out, and that's where the differences in inkjet technologies arise.

Thermal Inkjets

The most common inkjet technology is called thermal because it uses heat inside its printhead to boil a tiny quantity of water-based ink. Boiling produces tiny bubbles of steam that can balloon out from the nozzle orifices of the printhead. The thermal mechanism carefully controls the bubble formation. It can hold the temperature in the nozzle at just the right point to keep the ink bubble from bursting. Then, when it needs to make a dot on the paper, the printhead warms the nozzle, the bubble bursts, and the ink sprays from the nozzle to the paper to make a dot. Because the bubbles are so tiny, little heat or time is required to make and burst the bubbles—the printhead can do it hundreds of times in a second.

This obscure process was discovered by a research specialist at Canon way back in 1977, but developing it into a practical printer took about seven years. The first mass-marketed computer inkjet printer was the Hewlett-Packard ThinkJet, introduced in May, 1984, which used the thermal inkjet process (which HP traces back to a 1979 discovery by HP researcher John Vaught). This single-color printer delivered 96 dot per inch resolution at a speed of 150 characters per second, about on par with the impact dot-matrix printers available at the same time. The technology—not to mention the speed and resolution—has improved substantially since then. The proprietary name, BubbleJet, used by Canon for its inkjet printer derives from this technology, although thermal-bubble design is also used in printers manufactured by Hewlett-Packard, Lexmark, and Texas Instruments.

The heat that makes the bubbles is the primary disadvantage of the thermal inkjet system. It slowly wears out the printhead, requiring that you periodically replace it to keep the printer working at its best. Some manufacturers minimize this problem by combining their printers' nozzles with their ink cartridges so that when you add more ink you automatically replace the nozzles. With this design you never have to replace the nozzles, at least independently, because you do it every time you add more ink.

Because nozzles ordinarily last much longer than the supply in any reasonable inkjet reservoir, other manufacturers make the nozzles a separately replaceable part. The principal difference between these two systems amounts to nothing more than how you do the maintenance. Although the combined nozzles-and-ink approach would seem to be more expensive, the difference in the ultimate cost of using either system is negligible.

Piezoelectric Inkjets

The alternative inkjet design uses the squirt gun approach—mechanical pressure to squeeze the ink from the printhead nozzles. Instead of a plunger pump, however, these printers generally use special nozzles that squash down and squeeze out the ink. These nozzles are made from a piezoelectric crystal, a material that bends when a voltage is applied across it. When the printer zaps the piezoelectric nozzle with a voltage jolt, the entire nozzle flexes inward, squeezing the ink from inside and out the nozzle, spraying it out to the paper. This piezoelectric nozzle mechanism is used primarily by Epson in its Stylus line of inkjet printers.

The chief benefit of this design, according to Epson, is a longer-lived printhead. The company also claims it yields cleaner dots on paper. Bursting bubbles may make halos of ink splatter, whereas the liquid droplets from a piezoelectric printer form more solid dots.

Phase-Change Inkjets

Closely related to inkjet machines are phase-change printers. These printers are actually a derivation on inkjet technology that concentrates on the ink more than its motion. Instead of using solvent-based inks that are fixed (that is, that dry) by evaporation or adsorption into the print medium, the phase-change printer uses inks that harden, changing phase from liquid to solid. Scientifically speaking, the hardening process is a change in the state or phase of the ink, hence the name of the technology.

The ink of the phase-change printer starts as solid sticks or chunks of specially dyed wax. The printhead melts the ink into a thin liquid that is retained in a reservoir inside the printhead. The nozzles mechanically force out the liquid and spray it on paper. The tiny droplets, no longer heated, rapidly cool on the medium, returning to its solid state. Because of the use of solid ink, this kind of printer is sometimes called a solid inkjet printer.

The first printer to use phase-change technology was the Howtek Pixelmaster in the late 1980s. Marketed mostly as a specialty machine, the Howtek made little impression in the industry. Phase-change technology received its major push from Tektronix with its introduction of its Phaser III PXi in 1991. Tektronix, which was acquired by Xerox in 2001, refined phase-change technology to achieve smoother images and operation. Whereas the Pixelmaster used plastic-based inks that left little lumps on paper and sometimes clogged the printhead, the Phaser III used wax-based inks and a final processing step—a cold fuser—that flattened the cold ink droplets with a steel roller as the paper rolled out of the printer.

Phase-change printers are less sensitive to media than ordinary inkjet printers. They are also renown for the bright, saturated colors of their wax-based inks.

Laser

The one revolution that has changed the faces of both offices and forests around the world is the photocopier. Trees plummet by the millions to provide fodder for the duplicate, triplicate, megaplicate. Today's non-impact, bit-image laser printer owes its life to this technology.

At heart, the laser printer principle is simple. Some materials react to light in strange ways. Selenium and some complex organic compounds modify their electrical conductivity in response to exposure to light. Both copiers and laser printers capitalize on this photoelectric effect by focusing an optical image on a photoconductive drum that has been given a static electrical charge. The charge drains away from the conductive areas that have been struck by light but persists in the dark areas. A special pigment called a toner is then spread across the drum, and the toner sticks to the charged areas. A roller squeezes paper against the drum to transfer the pigment to the paper. The pigment gets bonded to the paper by heating or "fusing" it.

The laser printer actually evolved from the photocopier. Rather than the familiar electrostatic Xerox machine, however, the true ancestor of the laser printer was a similar competing process called electrophotography, which used a bright light to capture an image and make it visible with a fine carbon-based toner. The process was developed during the 1960s by Keizo Yamaji at Canon. The first commercial application of the technology, called New Process to distinguish it from the old process (xerography), was a Canon photocopier released in 1968.

The first true laser printer was a demonstration unit made by Canon in 1975 based on a modified photocopier. The first commercial computer laser printer came in 1984 when Hewlett-Packard introduced its first LaserJet, which was based on the Canon CX engine. At heart, it and all later laser printers use the same process, a kind of heat-set, light-inspired offset printing.

The magic in a laser printer is forming the image by making a laser beam scan back and forth across the imaging drum. The trick, well known to stage magicians, is to use mirrors. A small, rotating mirror reflects the laser across the drum, tracing each scan line across it. The drum rotates to advance to the next scan line, synchronized to the flying beam of laser light. To make the light-and-dark pattern of the image, the laser beam is modulated on and off. It's rapidly switched on for light areas, off for dark areas, one minuscule dot at a time to form a bit-image.

The major variations on laser printing differ only in the light beam and how it is modulated. LCD-shutter printers, for example, put an electronic shutter (or an array of them) between a constant light source (which need not be a laser) and the imaging drum to modulate the beam. LED printers modulate ordinary light-emitting diodes (LEDs) as their optical source. In any case, these machines rely on the same electrophotographic process as the laser printer to carry out the actual printing process.

The basic laser printer mechanism requires more than just a beam and a drum. In fact, it involves several drums or rollers, as many as six in a single-color printer and more in color machines. Each has a specific role in the printing process. Figure 26.3 shows the layout of the various rollers.

The imaging drum, often termed the OPC for optical photoconductor, first must be charged before it will accept an image. A special roller called the charging roller applies the electrostatic charge uniformly across the OPC.

After the full width of an area of the OPC gets its charge, it rotates in front of the modulated light beam. As the beam scans across the OPC drum and the drum turns, the system creates an electrostatic replica of the page to be printed.

To form a visible image, a developing roller then dusts the OPC drum with particles of toner. The light-struck areas with an electrostatic charge attract and hold the toner against the drum. The unexposed parts of the drum do not.

The printer rolls the paper between the OPC drum and a transfer roller, which has a strong electrostatic charge that attracts the toner off the drum. Because the paper is in between the transfer roller and the OPC drum, the toner collects on the paper in accord with the pattern that was formed by the modulated laser. At this point, only a slight electrostatic charge holds the toner to the paper.

To make the image permanent, the printer squeezes the paper between a fuser and backup roller. As the paper passes through, the printer heats the fuser to a high temperature—on the order of 350 degrees Fahrenheit (200 degrees Celsius). The heat of the fuser and the pressure from the backup roller melt the toner and stick it permanently on the paper. The completed page rolls out the printer.

Meanwhile, the OPC drum continues to spin, wiping the already-printed area against a cleaning blade that scrapes any leftover toner from it. As the drum rotates around to the charging roller again, the process repeats. Early lasers used OPC drums large enough to hold an entire single-page image. Modern machines use smaller rollers that form the image as a continuous process.

Although individual manufacturers may alter this basic layout to fit a particular package or to refine the process, the technology used by all laser machines is essentially the same. At a given resolution level, the results produced by most mechanisms is about the same, too. You need an eye loupe to see the differences. The major difference is that manufacturers have progressively refined both the mechanism and electronics to produce higher resolutions. Basic laser printer resolution starts at 300 dots per inch. The mainstream is now at the 600 dpi level. The best computer-oriented laser printers boast 1200 dpi resolution.

In most laser printers, the resolution level is fixed primarily by the electronics inside the printer. The most important part of the control circuitry is the raster image processor, also known as the RIP. The job of the RIP is to translate the string of characters or other printing commands into the bit-image that the printer transfers to paper. In effect, the RIP works like a video board, interpreting drawing commands (a single letter in a print stream is actually a drawing command to print that letter), computing the position of each dot on the page and pushing the appropriate value into the printer's memory. The memory of the printer is arranged in a raster just like the raster of a video screen, and one memory cell—a single bit in the typical black-and-white laser printer—corresponds to each dot position on paper.

The RIP itself may, by design, limit a laser printer to a given resolution. Some early laser printers made this constraint into an advantage, allowing resolution upgrades through after-market products that replaced the printer's internal RIP and controlled the printer and its laser through a video input. The video input earns its name because its signal is applied directly to the light source in the laser in raster scanned form (like a television image), bypassing most of the printer's electronics. The add-in processor can modulate the laser at higher rates to create higher resolutions.

Moving from 300 dpi to 600 dpi and 1200 dpi means more than changing the RIP and adding memory, however. The higher resolutions also demand improved toner formulations because, at high resolutions, the size of toner particles limits sharpness, much as the size of print wires limits impact dot-matrix resolution. With higher-resolution laser printers, it becomes increasingly important to get the right toner, particularly if you have toner cartridges refilled. The wrong toner limits resolution just as a fuzzy ribbon limits the quality of impact printer output.

Adding color to a laser printer is more than dumping a few more colors of toner. The laser must separately image each of its three or four primary colors and transfer the toner corresponding to each to the paper. The imaging process for each color requires forming an entire image by passing it past the OPC drum. Forming a complete image consequently requires three or four passes of the drum.

Exactly what constitutes a pass varies among manufacturers. Most color laser printers use three or four distinct passes of each sheet of paper. The paper rolls around the drum and makes four complete turns. Each color gets images separately on the drum, then separately transferred to the sheet. The printer wipes the drum clean between passes.

So-called "one-pass" printing, pioneered by Hewlett-Packard, still requires the drum to make four complete passes as each color gets separately scanned on the drum and toner is dusted on the drum separately for each color. The paper, however, only passes once through the machine to accept the full-color image at once and then to have all four colors fused together onto the paper. The first three colors merely transfer to the drum. After the last color—black—gets coated on the drum, the printer runs the paper through and transfers the toner to it. The paper thus makes a single pass through the printer, hence the "one-pass" name.

This single-pass laser technology yields no real speed advantage. The photoconductor drum still spins around the same number of times as a four-pass printer. The speed at which the drum turns and the number of turns it makes determines engine speed, so the one-pass process doesn't make a significant performance increase.

The advantage to one-pass color laser printing comes in the registration of the separate color images. With conventional color laser systems, the alignment of the paper must be critically maintained for all four passes for all the colors to properly line up. With the one-pass system, paper alignment is not a problem. Only the drum needs to maintain its alignment, which is easy to do because it is part of the mechanism rather than an interloper from the outside world.

No matter the number of passes, adding color in laser printing subtracts speed. In general, color laser speed falls to one-quarter the monochrome speed of a similar engine because of the requirement of four passes. (With three-pass printing, speed falls to one-third the monochrome rate). For example, a printer rated at 12 pages per minute in monochrome will deliver about 3 ppm in color. Even allowing for this slowdown, however, color lasers are usually faster than other color page printers. They are also often quieter. Compared to thermal wax transfer printers, a popular high-quality color technology, they are also economical because a laser uses toner only for the actual printed image. Thermal wax machines need a full page of ink for each page printed regardless of the density of the image.

Direct Thermal

A printer that works on the same principle as a wood-burning set might seem better for a Boy Scout than an on-the-go executive, but today's easiest-to-tote printers do exactly that—the equivalent of charring an image on paper. Thermal printers use the same electrical heating of the wood-burner, a resistance that heats up with the flow of current. In the case of the thermal printer, however, the resistance element is tiny and heats and cools quickly, in a fraction of a second. As with inkjets, the thermal printhead is equivalent to that of a dot-matrix printer, except that it heats rather than hits.

Thermal printers do not, however, actually char the paper on which they print. Getting paper that hot would be dangerous, precariously close to combustion (although it might let the printer do double-duty as a cigarette lighter). Instead, thermal printers use special, thermally sensitive paper that turns from white to near-black at a moderate temperature.

Thermal technology is ideal for portable printers because few moving parts are involved—only the printhead moves, nothing inside it. No springs and wires means no jamming. The tiny, resistive elements require little power to heat, actually less than is needed to fire a wire in an impact printer. Thermal printers can be lightweight, quiet, and reliable. They can even run on batteries.

The special paper they require is one drawback. Not only is it costly (because it is, after all, special paper), but it feels funny and it is prone to discoloration if it is inadvertently heated to too high a temperature. Paper cannot tell the difference between a hot printhead and a cozy corner in the sun.

Thermal printers have become special application machines. Inkjets have many of the same virtues and more reasonable paper; therefore, low-cost inkjets dominate the printer market.

Thermal Transfer

Engineers have made thermal technology more independent of the paper or printing medium by moving the image-forming substance from the paper to a carrier or ribbon. Instead of changing a characteristic of the paper, these machines transfer pigment or dyes from the carrier to the paper. The heat from the printhead melts the binder holding the ink to the carrier, allowing the ink to transfer to the paper. On the cool paper, the binder again binds the ink in place. In that the binder is often a wax, these machines are often called thermal wax transfer printers.

These machines produce the richest, purest, most even and saturated color of any color print technology. Because the thermal elements have no moving parts, they can be made almost arbitrarily small to yield high resolutions. Current thermal wax print engines achieve resolutions similar to those of laser printers. However, due to exigencies of printhead designs, the top resolution of these printers extends only in one dimension (vertical). Top thermal wax printers achieve 300 dots per inch horizontally and 600 dots per inch vertically.

Compared to other technologies, however, thermal wax engines are slow and wasteful. They are slow because the thermal printing elements must have a chance to cool off before advancing the 1/300th of an inch to the next line on the paper. And they are wasteful because they use wide ink transfer sheets, pure colors supported in a wax-based medium clinging to a plastic film base—sort of like a Mylar typewriter ribbon with a gland condition. Each of the primary colors to be printed on each page requires a swath of inked transfer sheet as large as the sheet of paper to be printed—that is, nearly four feet of transfer sheet for one page. Consequently, printing a full-color page can be expensive, typically measured in dollars rather than cents per page.

Because thermal wax printers are not a mass market item and each manufacturer uses its own designs for both mechanism and supplies, you usually are restricted to one source for ink sheets—the printer manufacturer. Although that helps ensure quality (printer-makers pride themselves on the color and saturation of their inks), it also keeps prices higher than they might be in a more directly competitive environment.

For color work, some thermal wax printers give you the choice of three- or four-pass transfer sheets and printing. A three-pass transfer sheet holds the three primary colors of ink—red, yellow, and blue—whereas a four-color sheet adds black. Although black can be made by overlaying the three primary colors, a separate black ink gives richer, deeper tones. It also imposes a higher cost and extends printing time by one-third.

From these three primary colors, thermal wax printers claim to be able to make anywhere from seven to nearly 17 million colors. That prestidigitation requires a mixture of transparent inks, dithering, and ingenuity. Because the inks used by thermal wax printers are transparent, they can be laid one atop another to create simple secondary colors. They do not, however, actually mix.

Expanding the thermal wax palette further requires pointillistic mixing, laying different color dots next to each other and relying on them to visually blend together in a distant blur. Instead of each dot of ink constituting a picture element, a group of several dots effectively forms a super pixel of an intermediate color.

The penalty for this wider palette is a loss of resolution. For example, super pixels measuring five by five dots would trim the resolution of a thermal wax printer to 60 dots per inch. Image quality looks like a color halftone—a magazine reproduction—rather than a real photograph. Although the quality is shy of perfection, it is certainly good enough for a proof of what is going to a film recorder or the service bureau to be made into color separations.

A variation of the thermal wax design combines the sharpness available from the technology with a versatility and cost more in line with ordinary dot-matrix printers. Instead of using a page-wide printhead and equally wide transfer sheets, some thermal wax machines use a line-high printhead and a thin transfer sheet that resembles a Mylar typewriter ribbon. These machines print one sharp line of text or graphics at a time, usually in one color—black. They are quiet as inkjets but produce sharper, darker images.

Dye-Diffusion

For true photo-quality output from a printer, today's stellar technology is the thermal dye-diffusion process, sometimes called thermal dye-sublimation. Using a mechanism similar to that of the thermal wax process, dye-diffusion printers are designed to use penetrating dyes rather than inks. Instead of a dot merely being present or absent, as in the case of a thermal wax printer, diffusion allows the depth of the color of each dot to vary. The diffusion of the dyes can be carefully controlled by the printhead. Because each of the three primary colors can have a huge number of intensities (most makers claim 256), the palette of the dye-diffusion printer is essentially unlimited.

What is limited is the size of the printed area in some printers. The output of most dye-diffusion printers looks like photographs in size as well as color. In fact, dye-diffusion printing doesn't merely look like a photograph, many of your photos are actually made using a dye-diffusion printer. Kodak, for example, uses dye-diffusion for the color prints that it processes.