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| Reference Series |
Table of Contents For This Issue
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| How Computers Work, Part I | |
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August 2001• Vol.5 Issue 3 Page(s) 72-77 in print issue | |
The Eminent Inkjet A Behind-The-Scenes Look At Printer Technology |
It wasn’t too long ago that creating quality text documents at home was a frustrating ordeal. You either employed an ear-
rending dot matrix printer that created hazy text or spent your nest egg to purchase a laser printer. Even if you splurged on a laser,
good graphics output was a pipedream, and the idea of photo-quality pictures was laughable. Fortunately for our ears and eyes, inkjet printers that are capable of both sharp text and stunning color have revolutionized the printer market.Inkjet printer shipments grew explosively in the 1990s. IDC research analyst Riley McNulty says, “Annual unit shipments in the U.S. increased 60% from 1997 to 2000, while prices declined 44% in that time.” But the inkjet market has matured. In 2000, 19.5 million units were shipped, just a 1% increase over 1999. Nevertheless, McNulty expects the inkjet market to continue growing and forecasts a five-year compounded annual growth rate of 6%. The phenomenal success that inkjets have enjoyed is due to their versatility and cost efficiency. Not only can these printers create near laser-quality text, but their color output is second to none, and it’s all done for mere pennies per page. It hasn’t always been this way, of course. The genesis of the modern inkjet occurred back in the days when Hewlett-Packard was known more for its handheld scientific calculators than its PCs.  Pioneering Printers.
The birth of inkjet technology is something of a modern legend in technology circles, the result of a laboratory mishap. In 1979, a Canon employee unintentionally pressed a hot soldering iron to an
ink-filled syringe. The contact caused a bubble to form in the ink, creating enough pressure to push ink from the needle. This discovery led to Canon’s development of its Bubble Jet printer line and prompted other companies to tinker with the new technology as well. Yet it wasn’t until the early 1990s that inkjets began to create text crisp enough to measure up to laser printer output, and unfortunately, they were still too expensive for most consumers, at an average price of nearly $1,000. What’s more, these were simple black-and-white printers with low resolutions, about 360dpi (dots per inch). These days, a reasonably equipped inkjet costs less than $200. For that price, you can buy a printer with capabilities virtually unimaginable less than 10 years ago. The technological advances made in ink dispersion and even the actual inks are responsible for the quick price declines. We’ll discuss the breakthroughs that have been made in the inkjet industry later in this article; for now, the technology at work inside your inkjet is a story in itself.
The Process.
The process of printing what you see on your PC monitor to a printer seems simple: click Print, and a minute or two later the document is in your hands. In reality, that one click engages a flurry of software and hardware activity. The program you are using to print a file sends data to the printer driver, which is the software used to interpret the data into information that the printer can understand and use. The same data is put through a half- toning process. Half-toning involves composing text or images with a series of tiny dots and also dictates how the printer will create certain color combinations. For instance, half-toning determines that a yellow dot placed next to a blue dot creates the impression of the color green. After the elements of half-toning have been completed, the data forms a bit map, which is a graphics file that indicates, bit by bit, the color of each dot that will be used to print the text or image. Each bit prompts the printer with a command to use a certain color at a certain area on the paper. The darker the image, the larger the half-toned dots become, sometimes to the point of overlapping. The regular pattern of the dots creates the illusion of a continuous tone when put to paper. The PC’s processor then compresses the bit map created by the software. This minimizes the amount of information that must make the journey through the cable to the printer. An analogy for compression would be to take a sentence of English, which could be composed of dozens of letters, and then translate it into a Chinese character. The latter conveys as much information as the former, but does so more quickly. After the driver has translated the data and ensured that the printer is online (ready to print), it routes the information from the PC to the printer. The printer begins to buffer the information using its RAM. The more RAM a printer has, the faster the PC can finish sending the document and move on to other tasks. The printer’s controller, which coordinates the printer’s functions, then decompresses the data and prepares the nozzles in the printhead for the job. If the printer hasn’t been used recently, it may need to engage a cleaning cycle to ensure that its printheads aren’t fouled. After this process is complete, printing can begin.
After the printhead makes a single pass across a page, it may return to the other side of the page and begin printing again. In contrast, bi-directional printers print from side to side, making them much faster, though some bi-directional printers set to the highest resolution may only print in one direction. That’s the route your novella takes from a word processing program to the printed page, and it’s essentially the same for all inkjets. However, the technique that an inkjet’s printhead uses to squirt ink onto the paper can differ depending on the model. Printhead Technologies.
If spraying ink at a piece of paper doesn’t sound like a precise printing method, think again. Inkjets produce some of the highest quality text and images available to the home user. But not all inkjets work the same.Older inkjet technology was continuous flow, which, as its name implies, sends a steady surge of ink through a nozzle in the printhead to the destination medium. Modern printers use drop-on-demand technology, which instantly squeezes ink drops in intervals dictated by the printer’s controller. The extreme precision required for drop-on-demand printing is performed by one of two printhead technologies: thermal or Piezo-electric. Thermal printing employs a miniscule resistor that warms ink in a chamber beneath the main ink reservoir. Heat causes the ink to develop a vapor bubble that forces a drop of ink from the printhead nozzle. The bubble eventually bursts, and its demise creates a vacuum that sucks the next ink drop in. This process occurs thousands of times every second through the hundreds of nozzles used for each color. Piezo-electric technology differs in that it does not require heat. These printers use a Piezo-electric crystal that changes shape when an electrical charge is applied. A negative charge deflects the crystal away from the ink chamber, creating suction and pulling in ink. A positive charge warps the crystal in the opposite direction, pushing a plate into the ink chamber and expelling a droplet onto the paper. Piezo-electric technology has its advantages. For one, it’s a little faster. Thermal inkjets must pause for a moment to let ink cool before continuing, whereas Piezo-electric printers do not. Piezo-electric also creates more consistent drop sizes and more uniformly round shapes, with fewer “satellite” droplets that can cause “tails” or other imperfections (though these are often visible upon close scrutiny for both printhead types). The smaller droplets also allow for greater printhead nozzle density, and thus, higher resolutions. Epson is currently the only major inkjet manufacturer to use Piezo-electric technology because it owns the patent on this technology. The other major companies in the inkjet market are Hewlett-Packard, Lexmark, and Canon; they make thermal inkjets. In terms of quality, thermal and Piezo-electric inkjets produce remarkably similar output. Most users who compare documents from these methods would find few differences. In most cases, it’s simply a matter of which output you prefer. The technologies do produce some differences in resolution, however. Resolution Revolution.
Resolution is the term used to describe how clearly images appear. Whether you print text or graphics, all inkjet output is made up of infinitesimal dots reminiscent of the dots that make up a TV or computer monitor’s picture. Printers with higher resolution
can create more dots per square inch, and thus, create sharper output. When you’re shopping for a printer, you determine its resolution by the dpi rating.Resolution is determined by the number of nozzles in a printhead. A color printhead has roughly 300 to 600 nozzles. Monochrome (or black-and-white) printheads typically have fewer nozzles. For example, the monochrome printhead for the Epson Color Stylus Photo 890 has 48 nozzles. The resulting ink drops from these nozzles are extremely small, at volumes from three to 10 picoliters (a picoliter is a trillionth of a liter, or one millionth the size of a raindrop). On paper, the dot isn’t much bigger, at about 50 to 60 microns in diameter. This means that the dots an inkjet produces are nearly imperceptible to the naked eye, which is limited to detecting dots about 30 microns in size. In comparison, a human hair is about 70 microns in diameter. The ability of printers to manipulate dots into tight patterns has increased significantly in recent years. Some inexpensive printers have resolutions as high as 2,400 x 1,200 dpi, which means they can pack 2,880,000 dots into one square inch. That capability makes a big difference in overall image quality, especially when compared to the 360dpi resolutions of early inkjets, which in contrast laid down only 129,600dpi. Generally, higher resolutions mean smoother color transitions and crisper details. To visualize the difference, imagine that a high- and low-resolution printer must paint a straight line at a 45-degree angle, keeping in mind that the printer works horizontally. Upon close inspection, the line from the high-resolution printer looks straight. The line from the lower-resolution printer looks more like a staircase; the jagged line is a result of the printer’s fewer ink nozzles and the limited area into which it can insert ink drops. From a practical point of view, more nozzles mean straighter lines, which in turn means the details of your favorite photos are much clearer. As you browse a printer’s dpi capability, you’ll want to keep in mind that some printers tout high resolutions as a result of special software controls. HP uses PhotoREt III technology on some of its printers. The technique lets a printer shoot as many as 16 droplets into a pixel, where they partially bleed together, creating very smooth color gradations. HP claims this process has greater bearing on quality than increasing the number of dots per inch. Keep this in mind when you’re looking for a new inkjet—sometimes a high dpi rating won’t guarantee output superior to a printer with ink-layering techniques. You’ll have to actually try a printer at the store to find which look you prefer. Resolution is just one key aspect of a printer’s specifications. Whether you use a high-tech, high-resolution printer, or just an inexpensive inkjet, they all use an amazingly simple coloring technique. Four-Color Power.
Most inkjets use four colors to produce their dazzling pictures: cyan, magenta, yellow, and black, referred to as CMYK (the K stands for black). The colors in CMYK cartridges can be combined to make up the spectrum called true color, which includes 16.7 million
different tones. Older printers use a three-color CMY cartridge. To produce black, these printers spew all three colors at once, creating a “composite” black, which often appears muddier than a true black. Other CMY printers solve this problem by supplying a black ink cartridge; unfortunately, you have to swap out the color cartridge and insert the black ink cartridge for black images. This is the reason four-color inkjets are becoming popular. Four-color printers often have two cartridges, a tri-color cartridge paired with a black cartridge. But this technique has a drawback as well. If only one color in a tri-color cartridge runs dry, you have to replace it even though the other inks remain, a fact that may bother cost-conscious consumers. Manufacturers are addressing this concern by equipping newer printers with four replaceable cartridges instead of one or two. If you run out of one color, simply replace one (significantly cheaper) cartridge. Higher-end inkjets use light cyan and light magenta in addition to the CMYK inks used in four-color printers. Such six-color printers can create more subtle flesh tones and even smoother color transitions. HP is one company that has pushed six-color printers as the wave of the future. Such printers, it claims, can produce more realistic color and better detail while avoiding the technological restrictions of attempting to increase resolution by packing evermore nozzles into a printhead. For now, six-color printers are usually photo printers specialized for detailed color production. The fact that a small printer can blend four or six colors into millions of variations is astounding. To accomplish this, developers rely on ink chemistry to aid their efforts. Nearly as much work goes into improving ink characteristics as building a printer. Ink & Paper.
At the core of inkjet technology is the ink itself. Here’s just one example of why ink chemistry is so crucial: the viscosity of an ink affects its velocity as it exits a printhead nozzle. If the drop is moving too slowly, it’s misplaced on the page; if it moves too quickly, it splatters.Developers of thermal inkjets have two other primary problems to contend with as they strive to improve their inks: heat tolerance and drying rates. If an ink breaks down under heat and loses its optimum performance, printer output suffers. And if it doesn’t dry as soon it hits the paper, some degree of smearing is imminent. Smearing is an issue that ink developers are rapidly overcoming. Before, if you were to rub your finger across a freshly printed page, you could run the inks together and ruin the picture. Now, inks dry almost instantly. Contemporary inks are also more durable than their precursors. A completed printout can now withstand some harsh treatment, including a fingernail dragged across the surface. Some inks can even resist fading under harsh ultraviolet light. Because its printers use Piezo-electric technology, Epson can ignore the heat tolerance factor, allowing the company’s ink researchers to concentrate more on other aspects of ink development, such as drying rates and ink durability. Their work has paid off. Epson’s inks are solvent-based and dry very quickly. The drops created by this type of ink penetrate the paper, maintaining their shape. In contrast, thermal inkjet makers use primarily oil-based dyes for inks; these inks tend to spread out more over the surface of the paper, meaning printer designers must contend with inks bleeding into each other and thus plan accordingly. Epson took ink development to an extreme for its top-of-the-line Stylus Photo 2000P, which uses a specially formulated pigment ink. Tiny hunks of pigment are encapsulated in resin, the advantage being that the resin lets light reflect more evenly, giving the final print a smoother finish and a tougher protective coating. Also, while droplet sizes for typical inkjets can range from 3 to 10 picoliters, the 2000P ejects pigment “particles” as small as 0.1 microns, allowing for extremely fine detail. As you can see, inkjet inks are more sophisticated than the stuff that comes in the average ballpoint pen. That’s the reason companies warn against using cheaper refilled ink cartridges. Doing so can cause unsatisfactory results even though the printer is functioning correctly. The best inks in the world can’t save an inkjet if it’s using poor-quality paper. Plain copier paper works fine for text. For color graphics, however, photo paper helps most inkjets perform optimally. Manufacturers also recommend that you use the papers they create, which are designed to work best with their printer lines. Bragging Rights.
As we’ve demonstrated, there are a lot of details for inkjet manufacturers to heed as they design their printers. Each puts its own spin on its products, hoping to convince you that its printers have an advantage.Epson takes advantage of being the sole producer of Piezo-electric printers. It stresses the importance of high resolution in overall image quality, especially in the production of photo-quality graphics. The company makes some of the cheapest high-resolution inkjets on the market; for example, the Stylus Photo Color 777 goes for only $89, works at a resolution of 2,880 x 720, and can create droplets as small as 4 picoliters. HP is another major player in the inkjet market, consistently producing some of the most popular printers with its DeskJet line. Its PhotoREt III resolution enhancement lets its machines compete with printers boasting higher dpi ratings. The difference in droplet size after integrating the PhotoREt technique is dramatic, with drops going from 10 to 5 picoliters in size. Canon’s pride and joy is the S600. The printer is less than $200 yet has one of the highest resolutions available, at 2,400 x 1,200. This printer features the Canon’s Advanced Microfine Droplet technology, featuring a new nozzle and heater system that the company touts as producing faster and more precise ink placement. The higher-resolution printhead has over 1,000 nozzles. The heating mechanism is closer to the tip of each nozzle, and thus, closer to the point of ejection. Lexmark’s newest additions to its popular Z-line have units displaying 7-picoliter droplets and a 2,400 x 1,200 resolution, and the Z33 and Z43 models are less than $100. Lexmark inkjets also are optimized to accommodate cultural color preferences. The company uses regional color tables based on research that color preferences differ by geographical regions and culture. The Asian color table includes warmer tones, and the American table uses more vibrant colors, among other options. As each manufacturer battles it out for its share of the marketplace, inkjets will keep evolving. Don’t wait for the next big technological breakthrough, though, because these printers are happening right now. Your Best Bet Is An Inkjet.
Inkjets are the closest thing to a cure-all solution for home printing needs. Small offices may find them costly, especially models that guzzle a lot of ink. But inkjets sport reasonably quick speeds, produce crisp text, and reign supreme of the photo-production
kingdom. No affordable color laser can come close to the brilliant color and fine detail of a reputable inkjet.As technology advances, smaller droplet sizes will achieve laser-like text and help to improve already fantastic color output. Better inks will ensure more vibrant colors and archival-quality prints. Best of all, the high-tech desktop inkjet you buy will be amazingly affordable, making it a good choice for your home office and as a color supplement for the text you print on your company’s laser printer. 
by Nathan Chandler View the graphics that accompany this article. (NOTE: These pages are PDF (Portable Document Format) files. You will need Adobe Acrobat Reader to view these pages. Download Adobe Acrobat Reader)
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