June 6, 2011, 6:16 p.m.
posted by mv
On paper, you may be challenged to determine what printer technology created an image. Although an old dot-matrix printout is distinctive, modern printers have converged on a uniform, high-quality level of output. That's not to say that printer technologies are interchangeable. Laser printers excel at everyday black-and-white business work, producing pages at high speed and low cost. Inkjets trade their low acquisition cost for a higher cost per page (mostly because of the absurdly high cost of ink) with the ancillary benefit of color. They are consequently the top choice for home printing.
But at a deeper level, printers differ in a number of aspects. These include printing speed, on-paper quality, color capabilities, the print engine, media-handling, and the cost of various consumables. Although many of these issues seem obvious, even trivial, current printer technologies—and hype—add some strange twists. For example, speed ratings may be given in units that can't quite be compared. Quality is more than a matter of dots per inch. And color can take on an added dimension—black.
No one wants to wait. When you finish creating a report or editing a picture, you want your hardcopy immediately—you've finished, and so should your computer. The ideal printer would be one that produces its work as soon as you give the "print" command—all fifty thousand pages of your monthly report in one big belch.
No printer yet has achieved instantaneous operation. In fact, practical machines span a wide range of printing speeds.
Engineers divide computer printers into two basic types: line printers and page printers. A line printer, as its name implies, works on text one line at a time. It usually has a printhead that scans across the paper one line of characters at a time. These printers earn their name because they think of printing in terms of the line. Most line printers start printing each line as soon as all the character data to appear in that line is received. Inkjet printers are today's preeminent line printers. On the other hand, a page printer rasterizes a full page image in its own internal memory and prints it one line of dots at a time. It must receive all the data for a full page before it begins to print the page. Laser printers are page printers.
Line printers and page printers produce equivalent results on paper. For engineers, the chief difference is what gets held in the printer's memory. For you, the chief difference is how the engineers specify the speed of the two kinds of printer.
The two most common measurements of printer speed are characters per second and pages per minute. Both should be straightforward measures. The first represents the number of characters a printer can peck out every second, and the second represents the number of completed pages that roll into the output tray every minute. In printer specification sheets, however, both are theoretical measures that may have little bearing on how fast a given job gets printed.
Line printer speed is usually measured in characters per second. Most printer manufacturers derive this figure theoretically. They then take the time the printhead requires to move from the left side of a page to the right side and divide it into the number of characters that might print on the line. This speed is consequently dependent on the width of the characters, and manufacturers often choose the most favorable value.
The highest speed does not always result from using the narrowest characters, however. The rate at which the printhead can spray dots of ink (or hammer print wires) is often fixed, so to print narrower characters the printer slows its printhead.
Characters per second does not directly translate into pages per minute, but you can determine a rough correspondence. On a standard sheet of paper, most line printers render 80 characters per line and 60 lines per page, a total of 4800 characters per page. Because there are 60 seconds in every minute, each page per minute of speed translates into 80 characters per second. Or you can divide the number of characters per second by 80 to get an approximate page per minute rating.
This conversion can never be exact, particularly on real-world printing chores. Few documents you print will fill every line from left to right with dense text. A line printer uses time only to work on lines that are actually printed. Modern printers have sufficient built-in intelligence to recognize areas that will appear blank on the printed sheet and don't bother moving their printheads over these empty spaces. Page printers, on the other hand, must scan an entire page even if it only has a single line of text on it. A line printer, on the other hand, dispenses with a single-line page in a few seconds.
Engine Speed Versus Throughput
Even within a given family of printers, ratings do not reflect real-world performance. The characters per second or pages per minute rating usually given for a printer does not indicate the rate at which you can expect printed sheets to dribble into the output tray. These speed measurements indicate the engine speed, the absolute fastest the mechanism of the printer allows paper to flow through its path. A number of outside factors slow the actual throughput of a printer to a rate lower—often substantially so—than its engine speed.
With line printers, the speed ratings can come close to actual throughput. The major slowdowns for line printers occur only when lines are short and the printhead changes direction often and when the printhead travels long distances down the sheet without printing.
With page printers, the difference between theory and reality can be dramatic. Because of their high resolutions, line printers require huge amounts of data to make bit-image graphics. The transfer time alone for this information can be substantial. Page printers suffer this penalty most severely when your computer rasterizes the image and sends the entire page as a bit-image to the printer. On the other hand, if the printer rasterizes the image, the processing time for the rasterization process adds to the print time. In either case it is rare, indeed, for pages to be prepared as quickly as the engine can print them when graphics are involved. Instead of pages per minute, throughput may shift to minutes per page.
Older impact printers often had several other modes. In draft mode, they blackened only every other dot. The thinly laid dots gave text and graphics a characteristic gray look. Near letter quality mode slowed the printhead so that text characters could be rendered without the machine-gun separated-dots look of draft mode. Because the dot density is higher, characters appear fully black and are easier to read. Letter quality mode slowed printing further, often using two or more passes to give as much detail as possible to each individual character.
Inkjet printers aren't bothered by the mechanical limitations of impact printers, so you need not worry so much about dot density at higher speeds. Nevertheless, the time required to form each jet of ink they print constrains the speed of their printheads. Most inkjet printers operate at the maximum speed the jet-forming process allows at the time. However, some offer the choice of higher quality modes keyed to different paper types. An inkjet printer may slow down to render characters with greater quality on better paper. Although it may not indicate separate printing modes, you can often see a speed difference when you shift between types of paper and resolution levels.
When it comes to choosing between color and black-and-white printing, speed differences can be substantial. Most color inkjet designs have fewer nozzles for color ink than they do for black ink. Typically, each of the three primary colors will have one-third the number of nozzles as black. As a result, the printhead must make three times as many passes to render color in the same detail as black, so color mode is often one-third the speed as black-and-white printing.
The speed relationship between color and black-and-white printing varies widely, however. In comparing the speeds of two printers, you must be careful to compare the same mode. The most relevant mode is the one you're likely to use most. If you do mostly text, then black-and-white speed should be the most important measure to you. If you plan to do extensive color printing, compare color speeds.
Although many bit-image printers don't allow you to directly alter their resolution, you can accelerate printing by making judicious choices through software. A lower resolution requires less time for rendering the individual dots, so in graphics mode, choosing a lower resolution can dramatically accelerate print speed. The speed gain isn't only mechanical. When you select a lower resolution, you also reduce rendering time. Fewer dots means less data to manipulate. At low resolutions, graphics printing speed can approach engine speed. The downside is, of course, you might not like the rough look of what you print.
The look of what you get on paper isn't completely in your control. By selecting the resolution your printer uses, you can raise speed and lower quality. But every printer faces a limit as to the maximum quality it can produce. This limit is enforced by the design of the printer and its mechanical construction. The cause and measurement of these constraints vary with the printer technology.
As with computer displays, the resolution and addressability of any kind of printer are often confused. Resolution indicates the reality of what you see on paper; addressability indicates the more abstract notion of dot placement. When resolution is mentioned, particularly with impact dot-matrix printers, most of the time addressability is intended. A printer may be able to address any position on the paper with an accuracy of, say, 1/120th of an inch. If an impact print wire is larger than 1/120th of an inch in diameter, however, the machine never is able to render detail as small as 1/120th of an inch. Inkjet printer mechanisms do a good job of matching addressability and resolution, but those efforts easily get undone when you use the wrong printing medium. If inkjet ink gets absorbed into paper fibers, it spreads out and obscures the excellent resolution many of these machines can produce.
Getting addressability to approach resolution was often a challenge for the designers of the impact dot-matrix printer (and one of the many reasons this technology has fallen from favor). The big dots made by the wide print wires blurred out the detail. Better quality impact dot-matrix printers had more print wires, and they were smaller. Also, the ribbon that is inserted between the wires and paper blurred each dot hammered out by an impact dot-matrix printer. Mechanical limits also constrained the on-paper resolution of impact machines.
With non-impact bit-image printers, resolution and addressability usually are the same, although some use techniques to improve apparent resolution without altering the number of dots they put in a given area.
Resolution Enhancement Technology (ReT) improves the apparent quality of on-paper printing within the limits of resolution—it can make printing look sharper than would ordinarily be possible. The enhancement technology, introduced by Hewlett-Packard in March, 1990, with its LaserJet III line of printers, works by altering the size of toner dots at the edges of characters and diagonal lines to reduce the jagged steps inherent in any matrix bit-image printing technique. Using ReT, the actual on-paper resolution remains at the rated value of the print engine—for example, 300 or 600 dpi—but the optimized dot size makes the printing appear sharper.
Increasing resolution is more than a matter of refining the design of print engine mechanics. The printer's electronics must be adapted to match, including adding more memory—substantially more. Memory requirements increase as the square of the linear dot density. Doubling the number of dots per inch quadruples memory needs. At high resolutions, the memory needs for rasterizing the image can become prodigious—about 14MB for a 1200 dpi image. Figure lists the raster memory needs for common monochrome printer resolutions.
Adding color, of course, increases the memory requirements. Fortunately, the color bit-depth used by common printer technologies doesn't impose the same extreme demands as monitors. A printer has only a few colors corresponding to the hues of its inks and, except for continuous-tone technologies such as dye-diffusion, the range of each color usually is limited to on or off. Thankfully, color resolutions are generally substantially lower than monochrome, defined by the size of the color super-pixels rather than individual dots. In any case, the raster memory requirements of a color printer are substantially higher than monochrome.
Note that when printing text, page printers may operate in a character-mapped mode, so memory usage is not as great. Even with minimal memory, a printer can store a full-page image in ASCII or a similar code, one byte per letter, as well as the definitions for the characters of several fonts. In this mode, it generates the individual dots of each character as the page is scanned through the printer.
Moving to higher resolutions makes other demands on a printer as well. For example, in laser printers, finer resolutions require 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.
Printers start with the primaries when it comes to color. They start with inks corresponding to the three primary colors—red, yellow, and blue. If you want anything beyond those, the printer must find some way of mixing them together. This mixing can be physical or optical.
The physical mixing of colors requires that two or more colors of ink actually mix together while they are wet. Printer inks are, however, designed to dry rapidly, so the colors to be mixed must be applied simultaneously or in quick succession. Few printers rely on the physical mixing of inks to increase the number of colors they produce.
Optical mixing takes place in either of two ways. One color of ink can be applied over another (that has already dried) or the colors can be applied adjacent to one another.
Appling multiple layers of color requires that the inks be to some degree transparent, because a truly opaque ink would obscure the first color to be applied. Most modern printer inks are transparent, which allows them to be used on transparencies for overhead projection as well as on paper. The exact hue of a transparent ink is, of course, dependent on the color of the medium it is applied to.
Optical mixing also takes place when dots of two or more colors are intermixed. If the dots are so close together that the eye cannot individually resolve each one, their colors blend together on the retina, mixing the individual hues together. Most computer color printers take advantage of this kind of optical mixing by dithering.
Three-, Four-, and Six-Color Printers
Color primaries in printing aren't so simple as the familiar threesome. To achieve better color reproduction, printers use a skewed set of primary colors—magenta instead of red, cyan instead of blue, and plain-old ordinary yellow. Even this mix is so far from perfect that, when all are combined, they yield something that's often far from black. Consequently, better printers include black in their primary colors.
Black, in fact, may play two roles in a color printer. A few inkjet printers allow you to choose between black-and-white and color operation as simply as swapping ink cartridges. In these machines, black is treated as a separate hue that cannot be mixed in blends with the three color primaries. These three-color printers render colors only from the three primaries even though some machines can print pure black when using a suitable black-only ink cartridge. The approximation of black made from the three primaries is termed composite black and often has a off-color cast. Four-color printers put black on the same footing as the three primary hues and mix with all four together. This four-color printing technique gives superior blacks, purer grays, and greater depth to all darker shades.
To further increase the range of pure colors possible with a printer, manufacturers are adding more colors of ink. The most common addition is two extra ink colors beyond the standard four, generally orange and violet, which extend the spectrum that the printer can render. This greater range in primaries translates into more realistic reproduction of photographs with less need for other color-enhancing techniques, such as dithering.
Color televisions do an excellent job with their three primaries and paint a nearly infinite spectrum. But the television tube has a luxury most printers lack. The television can modulate its electron beam and change its intensity. Most printers are stuck with a single intensity for each color. As a result, the basic range of most printers is four pure colors and seven when using mixtures, blending magenta and blue to make violet, magenta and yellow for orange, and blue and yellow for green. Count the background color of the paper being printed upon, and the basic range of most color printers is eight hues.
Commercial color printing faces the same problem of trying to render a wide spectrum from four primary colors. To extend the range of printing presses, graphic artists make color halftones. They break an image into dots photographically using a screen. Using special photographic techniques (or, more often today, a computer), they can vary the size of the dot with the intensity of the color.
Most computer printers cannot vary the size of their dots. To achieve a halftone effect, they use dithering. In dithering, colors beyond the range of pure hues of which a printer is capable are rendered in patterns of primary-colored dots. Instead of each printed dot representing a single pixel of an image, dithering uses a small array of dots to make a single pixel. These multiple-dot pixels are termed super-pixels. By varying the number of dots that actually get printed with a given color of ink in the super-pixel matrix, the printer can vary the perceived intensity of the color.
The problem with dithering is that it degrades the perceived resolution of the color image. The resolution is limited by the size of the super-pixels rather than the individual dots. For example, to attempt to render an image in True Color (eight bits per primary), the printer must use super-pixels measuring eight by eight dots. The resolution falls by an equivalent factor. A printer with 600 dpi resolution yields a color image with 75 dpi resolution.
Getting good color with dithering is more art than science. The choice of dithering pattern determines how smooth colors can be rendered. A bad choice of dithering pattern often results in a moiré pattern overlaid on your printed images or wide gaps between super-pixels. Moreover, colors don't mix the same on screen and on paper. The two media often use entirely different color spaces (RGB for your monitor, CYMK for your printer), thus requiring a translation step between them. Inks only aspire to be pure colors. The primary colors may land far from the mark, and a color blended from them may be strange, indeed.
Your printer driver can adjust for all these issues. How well the programmer charged with writing the driver does his job is the final determinant in the color quality your printer produces. A good driver can create photo-quality images from an inkjet printer, whereas a bad driver can make deplorable pictures even when using the same underlying print engine. Unfortunately, the quality of a printer's driver isn't quantified on the specifications sheet. You can only judge it by looking at the output of a printer. For highest quality, however, you'll always want driver software written for your particular model of printer, not one that your printer emulates. Moreover, you'll want to get the latest driver. You may want to periodically cruise the Web site of your printer-maker to catch driver updates as they come out.
Most inks fade. The ultraviolet energy in daylight attacks many pigments, breaking the molecular bonds that give a dye or pigment its color. With enough light, the color changes, shifting in hue, or fades until it is invisible on paper. Light is not the only culprit. Airborne chemicals (such as pollution) also react with dyes and pigments and can change their colors. Pigments sometimes even react with each other when they are mixed—a problem for painters that's rarely encountered in printing.
Of all ink colors, black is usually the most permanent, at least when it is made using carbon as a pigment. This black does not fade in light and, except for fire, rarely reacts with chemicals in the air. The black toner used by most laser printers is therefore among the most color-permanent of printing. The conventional commercial printing ink that's used for books usually is based on carbon pigments.
Colors, however, vary in their permanence. Many of the brightest hues fade quickly, while a few hues are immutable as black. The color inks used by most computer printers are designed to last only for a few years. For most business documents, this impermanence is no problem. It may even save you a trip to the shredder. But if you want to print artwork or a family photo, having it turn green or fade to white after a few months of display is far from ideal. To help prevent problems with fading, some printer manufacturers (particularly those offering photo printers) have begun to manufacture permanent inks rated for more than 100 years of display.
Permanence has another aspect. Some inks are water soluble. When a printout made using soluble ink get soaked or even damp, the colors can run like a bad tie-dye job. The somewhat psychedelic effect can be interesting, but it also interferes with legibility. Most inkjet printer inks are water soluble. For more permanent printouts, some manufacturers offer nonsoluble inks and special papers that help fix the soluble inks and prevent their running.
Key to the design of all printers is that their imaging systems operate in only one dimension, one line at a time, be it the text-like line of the line printer or the single raster line of the page printer. To create a full two-dimensional image, all printers require that the printing medium—typically paper—move past the print mechanism. With page printers, this motion must be smooth and continuous. With line printers, the paper must cog forward, hold its position, and then cog forward to the next line. Achieving high resolutions without distortion requires precision paper movement with a tolerance of variation far smaller than the number of dots per inch the printer is to produce.
Adding further complexity, the printer must be able to move paper or other printing media in and out of its mechanism. Most modern printers use sheet feeders that can pull a single page from a stack, route it past the imaging mechanism, and stack it in an output tray. Older printers and some of the highest speed machines use continuous-form paper, which trades a simplified printer mechanism for your trouble in tearing sheets apart. Each paper-handling method has its own complications and refinements.
The basic unit of computer printing is the page, a single sheet of paper, so it is only natural for you to want your printer to work with individual sheets. The computer printing process, however, is one that works with volume—not pages but print jobs, not sheets but reams.
The individual sheet poses problems in printing. To make your ideas, onscreen images, and printed hardcopy agree, each sheet must get properly aligned so that its images appear at the proper place and at the proper angle on every sheet. Getting perfect alignment can be vexing for both human and mechanical hands. Getting thousands into alignment is a project that might please only the Master of the Inquisition. Yet every laser printer, most inkjet printers, and a variety of other machines face that challenge every time you start a print job.
To cope with this hardcopy torture, the printer requires a complex mechanism called the cut-sheet feeder, or simply the sheet feeder. You'll find considerable variation in the designs of the sheet feeders of printers. All are complicated designs involving cogs, gears, rods, and rollers, and every engineer appears to have his own favorite arrangement. The inner complexity of these machines is something to marvel at but not dissect, unless you have too much time on your hands. Differences in the designs of these mechanisms do have a number of practical effects: the capacity of the printer, which relates to how long it can run without your attention; the kinds and sizes of stock that roll through; how sheets are collated and whether you have to spend half an afternoon to get all the pages in order; and duplex printing, which automatically covers both sides of each sheet.
When you load a single sheet of paper into a modern inkjet printer, it reverts to friction feed, which uses the same technology as yesteryear's mechanical typewriter. It moves paper through its mechanism by squeezing the paper against a rubber drive roller. The first inkjet printers actually used the large rubber roller of the typewriter, called a platen, and smaller drive rollers. Friction between the rubber and the paper or other printing media gives the system a positive grip that prevents slipping and ensures each sheet gets where it's suppose to. This friction also gives the technology its name.
The mechanism also relies on the lack of friction to operate properly. To feed individual sheets, the friction feed mechanism must grasp only a single sheet, which must slide against all the others. If there's too much friction between individual sheets of paper, they may stick together. Several may go through the printer at once, or you may seem to print on every other sheet. Many factors in the production and packaging of paper affect whether sheets stick to one another. In addition, the environment exerts its own effect—according to Hewlett-Packard, humidity can cause paper jams. When paper is too damp, sheets can stick together. When it is too dry, paper may develop a static charge, which also makes sheets stick together. Most laser printers have some means of draining the static from paper, but sometimes it is not successful.
If you cannot change the humidity around your printer and its paper, you can ward off the effects of sticky sheets by choosing a different kind or brand of paper. Some people recommend riffling through the paper before you load it into your printer to separate the sheets. Riffling each ream also helps avoid the other causes of sheets sticking together.
Modern printers integrate the feed mechanism with the rest of the printer drive system. You load cut sheets into a bin or removable tray, and the printer takes over from there. The mechanism is reduced to a number of rollers chained, belted, or geared together that pull the paper smoothly through the printer. This integrated design reduces complexity, increases reliability, and often trims versatility. Its chief limitations are in the areas of capacity and stock-handling.
The most obvious difference between sheet feed mechanisms of the printer is capacity. Some machines are made only for light, personal use and have modestly sized paper bins that hold 50 or fewer sheets. In practical terms, this means that every 10 to 15 minutes, you must attend to the needs of the printer, loading and removing the wads of paper that course through it. Larger trays require less intervention. Printers designed for heavy network use may hold several thousand sheets at a time.
The chief enemy of capacity is size. A compact printer must necessarily devote less space—and therefore less capacity—to stocking paper. A tray large enough to accommodate a ream (500 sheets) of paper would double the overall volume of some inkjet printers. In addition, larger tray capacities make building the feed mechanism more difficult. The printer must deal with a larger overall variation in the height of the paper stack, which can challenge both the mechanism and its designer.
A printer needs at least two trays or bins—one to hold blank stock waiting to be printed upon and one to hold the results of the printing. These need not be, and often are not, the same size. Most print jobs range from a few to a few dozen sheets, and you will usually want to grab the results as soon as the printing finishes. An output bin large enough to accommodate your typical print job usually is sufficient for a personal printer. The input tray usually holds more so that you need to load it less frequently—you certainly don't want to deal with the chore every time you make a printout.
Most printers are designed to handle a range of printing media, from paper stock and cardboard to transparency acetates. Not all printers handle all types of media. Part of the limitation is in the print engine itself. Many constraints arise from the feed mechanism, however.
With any cut-sheet mechanism, size is an important issue. All printers impose minimum size requirements on the media you feed them. The length of each sheet must be long enough so that one set of drive rollers can push it to the next. When too short sheets slide between rollers, nothing except your intervention can move them out. Similarly, each sheet must be wide enough that the drive rollers can get a proper grip. The maximum width is dictated by the width of the paper path through the printer. The maximum length is enforced by the size of paper trays and the imaging capabilities of the printer engine.
In any case, when selecting a printer you must be certain that it can handle the size of media you want to use. Most modern printers are designed primarily for standard letter-size sheets; some, but not all, accommodate legal-size sheets. If you want to use other sizes, take a close look at the specifications. Figure lists the dimensions of common sizes of paper.
Most sheet-fed printers cannot print to the edges of any sheet. The actual image area is smaller because drive mechanisms may reserve a space to grip the medium, and the engine may be smaller than the sheet to minimize costs. If you want to print to the edge of a sheet, you often need a printer capable of handling larger media and then must trim each page when it is done. Printing to (and beyond) the edge of a sheet is termed full-bleed printing. Only a few sheet-fed printers are capable of managing the task.
Printing media also differ in weight, which roughly corresponds to the thickness of paper. In general, laser printers are the most critical in regard to media weight. The capabilities of a given printer are listed as a range of paper weights the printer can handle—in the case of laser printers, typically from 16 to 24 pounds (most business stationery uses 20- or 24-pound stock). If you want to print heavier covers for reports, your printer needs to be able to handle 70-pound paper. Similarly, printer specifications will reveal whether the mechanism can deal with transparency media and label sheets.
Laser printers impose an additional specification on paper stock—moisture content. The moisture content of paper affects more than whether sheets stick together. It also changes the conductivity of the paper. The laser printing process is based on carefully controlled static charges, including applying a charge to the paper to make toner stick to it. If paper is too moist or conductive, the charge and the toner may drain away before the image is fused to the sheet. In fact, high humidity around a laser printer can affect the quality of its printouts—pale printouts or those with broken characters can often be traced to paper containing too much moisture or operating the printer in a high-humidity environment (which in turn makes the paper moist).
Most modern printers readily accommodate envelopes, again with specific enforced size restrictions. As with paper, envelopes come in standard sizes, the most common of which are listed in Figure.
With a modern computer printer, you should expect to load envelopes in the normal paper tray. Be wary of printers that require some special handling of envelopes—you may find it more vexing than you want to deal with.
When sheet-fed printers disgorge their output, it can fall into the output tray in one of two ways—face up or face down. Although it might be nice to see what horrors you have spread on paper immediately rather than saving up for one massive heart attack, face down is the better choice. When sheets pile on top of one another, face down means you do not need to sort through the stack to put everything in proper order.
A duplex printer is one that automatically prints on both sides of each sheet when you want it to. The chief advantage of double-sided printing is, of course, you use half as much paper, although you usually need thicker, more expensive stock so that one side does not show through to the other.
You can easily simulate duplex printing by printing on one side, turning each sheet over, and printing on the other. When you have a multipage print job, however, it can be daunting to keep the proper pages together. A single jam can ruin the entire job.
With laser printers, you should never try to print on both sides of a sheet except when using a duplex printer. When printing on the second side, the heat of the second fusing process can melt the toner from the first pass. This toner may stick to the fuser and contaminate later pages. With sufficient build-up, the printer may jam. Duplex printers eliminate the problem by fusing both sides of the sheet at once.