Encyclopedia of Computer Science and Technology

382 printersarray of magnetically controlled pins (9 pins at first, but 24on later models). Each character is formed by pressing theappropriate pins into a ribbon that pushes into the paper,leaving a pattern of tiny dots.Besides being relatively inexpensive, dot-matrix printersare versatile in that a great variety of character stylesor fonts can be printed (see font), either by loading differentsets of bitmaps. Likewise, graphic images can also beprinted. However, because the characters are made of tinydots, they don’t have the crisp, solid look of printed type.Laser and Ink-jet PrintersThe majority of printers used today use laser or ink-jet technology.Both combine the versatility of dot-matrix with theletter quality of typewriter-style printers. Xerox introducedthe first laser printer in the 1970s, although the technologywas too expensive for most users at first.The laser printer converts data from the computer intosignals that direct the laser beam to hit precise, tiny areasof a revolving drum. The drum is covered with a charged(usually negative) film. The areas hit by the laser, however,A laser printer uses a mirror-controlled laser beam to strike smallspots on a rotating drum (called an OPC or Organic PhotoconductingCartridge) that had been given an electrical charge (usuallypositive) by a corona wire. The spots where the light beam hit aregiven an opposite charge (usually negative). The drum is thencoated with a powdery toner that is charged opposite to the placeswhere the light hit, so the toner clings to the drum to form the patternsof the characters or graphics. A piece of paper is then givena strong negative charge so it can pull the toner off the drum as itpasses under it. Finally, heated rollers called fusers bind the tonerto the paper to form the final image.gain the opposite charge. As the drum continues to revolve,toner (a black powder) is dispensed. Because the toner isgiven a charge opposite to the places where the laser hit, thetoner sticks to those places. Meanwhile, the paper is drawninto the drum. Because the paper is given the same chargeas that produced by the laser beam (but stronger), the toneris pulled from the dots on the drum to the correspondingparts of the paper, forming the characters or graphics. Aheating system then fuses the toner to the paper to makethe image permanent. Meanwhile, the drum is dischargedand the printer is ready for the next sheet of paper.Color laser printers are also available, although they arestill relatively expensive. They work by using four revolutionsof the drum for each sheet of paper, depositing appropriateamounts of black, magenta, cyan, and yellow toner.Laser printers fell in price throughout the 1990s (to$500 or so), but were soon rivaled by a different technology,the ink-jet printer.The ink-jet printer has a print head that contains an inkcartridge for each primary printing color. Each cartridgehas 50 nozzles, each thinner than a human hair. To print,the appropriate nozzles of the appropriate colors are subjectedto electric current, which goes through a tiny resistorin the nozzle. An intense heat results for a few microseconds,long enough to create a tiny bubble that in turn forcesa droplet of ink onto the page.Ink-jet printers are generally slower than lasers, althoughfast enough for most purposes. Although the ink-jet is likethe dot-matrix in producing tiny dots, the dots are muchfiner. With output at up to 2,880 dots per inch, the resultingcharacters are virtually indistinguishable from typeprinting.Using high resolution and special papers, ink-jetprinters can now also produce photo prints comparable tothose created by traditional processes.An interesting offshoot of ink-jet printing technologycan be found in the development by HP of skin patches thatcan deliver controlled doses of drugs using tiny, virtuallypainless needles. The tiny droplets of drugs are transportedin much the same way as ink goes from cartridge reservoirto page.TrendsBy the end of the 1990s, the ink-jet printer was decliningsteeply in price, and today quite capable units canbe purchased for as little as $30 or so. Because of theirgreater speed, however, lasers are still used for higher-volumeprinting operations. “Multifunction” units combiningprinter, scanner, copier, and fax functions are also popularand cost less than a printer alone did only a few years ago.Advocates of office automation have long predicted the“paperless office,” but so far computers and their printershave churned out more paper, not less. However, there aresome trends that might eventually reverse this course. Developmentof practical “electronic books” (page-size displaysthat can hold thousands of pages of text) may reduce theneed for printed output (see e-books and digital libraries).Another possible replacement for printing is “electronicink,” a sheet of paper with charged ink held in suspension.The text or graphics on the page can be changed electroni-