Friday, December 10, 2010
A type of printer that utilizes a laser beam to produce an image on a drum. The light of the laser alters the electrical charge on the drum wherever it hits. The drum is then rolled through a reservoir of toner, which is picked up by the charged portions of the drum. Finally, the toner is transferred to the paper through a combination of heat and pressure. This is also the way copy machines work.
Because an entire page is transmitted to a drum before the toner is applied, laser printers are sometimes called page printers. There are two other types of page printers that fall under the category of laser printers even though they do not use lasers at all. One uses an array of LEDs to expose the drum, and the other uses LCDs. Once the drum is charged, however, they both operate like a real laser printer.
In most cases, your PC talks with controller circuitry (1) in your laser printer to queue up and translate printing data; a raster image processor (RIP) converts images and text into a virtual matrix of tiny dots.
The main actor, however, is the photoconduction drum (2), a specially coated cylinder that receives a positive or negative charge from a charging roller (3) (or, in some printers, a corona wire). A laser beam(4), switching rapidly on and off and deflected off a rotating mirror(5), scans the charged drum horizontally in precise lines. When the beam flashes on, it reverses the charge of tiny spots on the drum, corresponding to dots that are to be printed black. After the laser scans a line, a stepper motor advances the drum, and the laser repeats the process—all, of course, blindingly fast.
Next, the drum's laser-kissed portion encounters the developer roller(6), which is coated in charged toner particles from the toner hopper(7), part of the toner cartridge. Charged toner clings to the discharged areas of the drum, reproducing, in reverse, your images and text.
Meanwhile, a belt or roller assembly (8) draws paper inside from the paper tray (9), past a transfer roller or charging wire(10) that applies a charge opposite the toner's to the paper. As the paper sheet meets the drum, the drum-borne toner transfers to paper. A cleaning blade (11) then cleans the drum, and the process continues in a smooth, circular flow. (Color lasers work similarly, but the paper may require four passes by the drum for four toner colors. Alternately, the printer may transfer each color layer to an intermediate belt before applying it to the paper, or employ four drum/toner assemblies.)
Last, your page, with its imprint of tenuously anchored toner, reaches the fuser (12)—a heat roller and a pressure roller. It melts the toner, which contains resins and sometimes wax, onto the page. Voila, pages in your out tray.
In 1975, IBM introduced the first laser printer, the model 3800. Later, Siemens came out with the ND 2 and Xerox with the 9700. These self-contained printing presses were online to a mainframe or offline, accepting print image data on tape or disk.
In 1984, HP introduced the LaserJet, the first desktop laser printer, which rapidly became a huge success and a major part of the company's business. Desktop lasers made the clackety daisy wheel printers obsolete, but not dot matrix printers, which are still widely used for labels and multipart forms.
The Laser Mechanism
The laser printer uses electrostatic charges to (1) create an image on the drum, (2) adhere toner to the image, (3) transfer the toned image to the paper, and (4) fuse the toner to the paper. The laser creates the image by "painting" a negative of the page to be printed on the charged drum. Where light falls, the charge is dissipated, leaving a positive image to be printed.
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