Laser printer

From Wikipedia, the free encyclopedia

Image:Apple LaserWriter Pro 630.jpg A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. Like photocopiers, laser printers employ a xerographic printing process but differ from analog photocopiers in that the image is produced by the direct scanning of a laser beam across the printer's photoreceptor.

1 Overview
2 History
3 How it works
4 Steganographic anti-counterfeiting ("secret") marks
5 See also
6 External links

[edit] Overview

Laser printers have significant advantages over other types of printers. Unlike impact printers, laser printers can mix text and graphics, including varied fonts and character sizes, and halftone images. Laser printers use toner, a black or colored powder, rather than liquid ink. Unlike thermal printers, whose images can be smeared if the paper comes in contact with a source of heat, laser printers do not require special paper. Advantages of laser printers over inkjet printers include higher resolution, no smearing, lower cost per page, and faster print speed, since the entire page is imaged at one time, while an inkjet typically prints a series of narrow strips. However, laser printers always produce raster images, and except in the highest-quality versions, are less able to reproduce continuous tone images such as photographs.

The slowest laser printers produce about four pages per minute (ppm), and are relatively inexpensive. Printer speed can vary widely, however, and depends on many factors, including the graphic-intensity of the job being processed. The fastest models can print over 200 monochrome pages per minute (12,000 pages per hour). The fastest color laser printers can print over 100 pages per minute (6000 pages per hour). Very high-speed laser printers are used for mass mailings of personalized documents, such as credit card or utility bills, and are competing with photolithography in some commercial applications.

The cost of this technology depends on a combination of costs of paper, toner replacement, and infrequent drum replacement, as well as the replacement of other consumables such as the fuser assembly and transfer assembly. Often printers with soft plastic drums can have a very high cost of ownership that does not become apparent until the drum requires replacement.

A duplexing printer (one that prints on both sides of the paper) can halve paper costs and reduce filing volumes and floor weight. Formerly only available on high-end printers, duplexers are now common on mid-range office printers, though not all printers can accommodate a duplexing unit. Duplexing can also result in a slower page-printing rate, because of the more complicated paper path.

[edit] History

The first laser printer was produced by Xerox when Xerox researcher Gary Starkweather modified a Xerox copier in 1971.[1] Laser printing eventually became a multibillion-dollar business for Xerox.

The first commercial implementation of a laser printer was the IBM model 3800 in 1976, used for high-volume printing of documents such as invoices and mailing labels. It is often cited in technology writing as "taking up a whole room," implying that it was a primitive version of the later familiar device used with a personal computer; however, while large, it was designed for an entirely different purpose. Many 3800s are still in use.

Image:Pa 1977 9700-medium.jpg

The first laser printer designed for use with an individual computer was released with the Xerox Star 8010 in 1981; however, although it was highly innovative, the Star was an expensive ($17,000) system that was only purchased by a small number of laboratories and institutions. After personal computers became more widespread, the first laser printer intended for a mass market was the HP Laserjet 8ppm, released in 1984, which used a Canon Inc. developed engine controlled by HP developed software. The HP Laserjet printer was quickly followed by other laser printers from Brother Industries, IBM, and others.

Most noteworthy, however, was the role the laser printer played in popularizing desktop publishing with the introduction of the Apple LaserWriter for the Apple Macintosh, along with Aldus PageMaker software, in 1985. With these products, users could create documents that would previously have required professional typesetting.

As with most electronic devices, the cost of laser printers has fallen markedly over the years. In 1985 the HP LaserJet sold for $2995.00 and weighed a hefty 71 pounds (32.2 kg). The Apple LaserWriter (which shipped with a more powerful processor and the Postscript page description language) cost almost $7000.00. Today a comparable laser printer that boasts more memory, a higher print speed and duplexing capability can be had for about $300.00. A bare-bones laser printer costs less than $100.00.

[edit] How it works

Image:Laser print head.jpg

Main article: Xerography

There are typically six steps involved in the laser printing process:

Charging: A corona wire (in older printers) or a primary charge roller projects an electrostatic charge onto the photoreceptor, a revolving photosensitive drum or belt which is capable of holding an electrostatic charge on its surface as long as it hasn't been exposed to certain wavelengths of electromagnetic radiation.
Writing: A Raster Image Processor (RIP) chip converts incoming images to a raster image suitable for scanning onto the photoreceptor. The laser is aimed at a moving mirror which directs the laser beam through a system of lenses and mirrors onto the photoreceptor. Lasers (now typically laser diodes) are used because they generate a coherent beam of light for a high degree of accuracy. Wherever the laser strikes the photoreceptor the charge is reversed, thus creating a latent electro-photographic image on the photoreceptor surface;
Developing: The surface containing the latent image is exposed to toner, very fine particles of dry plastic powder mixed with carbon black or coloring agents. The charged toner particles are electrostatically attracted to the photoreceptor where the laser wrote the latent image;
Transferring: The photoreceptor is pressed or rolled over paper, transferring the image. Higher end machines use a positively charged transfer roller on the back-side of the paper to pull the toner from the photoreceptor to the paper.
Fusing: The paper passes through a fuser assembly, which, having rollers that provide heat and pressure (up to 200 degrees Celsius), bonds the plastic powder to the paper;
Cleaning: When the print is complete an electrically neutral rubber blade cleans any excess toner from the photoreceptor and deposits it into a waste reservoir, and a discharge lamp removes the remaining charge from the photoreceptor.

Different printers implement these steps in distinct ways. Some "laser" printers actually use a linear array of light-emitting diodes to write the light on the drum (see LED printer). The toner is based on either wax or plastic, so that when the paper passes through the fuser assembly, the particles of toner melt. The paper may or may not be oppositely charged. The fuser can be an infrared oven, a heated pressure roller, or (on some very fast, expensive printers) a xenon flash lamp. The Warm Up process that a laser printer goes through when power is initially applied to the printer consists mainly of heating the fuser element. Many printers have a toner-conservation mode or economode, which can be substantially more economical at the price of slightly lower contrast. Color laser printers add colored toner (typically but not always cyan, yellow, and magenta -- see CMYK) in three additional steps or passes.

Aside from these components, typical maintenance is to vacuum the mechanism, and eventually clean or replace the paper-handling rollers. The rollers have a thick rubber coating which eventually become covered with slippery paper dust and suffer wear. They can usually be cleaned with a damp lint-free rag and there are chemical solutions that can help restore the traction of the rubber.

[edit] Steganographic anti-counterfeiting ("secret") marks

Image:Printermarkrp.jpg

Main article: Printer steganography

Modern color laser printers mark printouts by a nearly invisible dot raster, for the purpose of identification. The dots are yellow and about 0.1 mm in size, with a raster of about 1 mm. This is purportedly the result of a deal between the US government and printer manufacturers to help track counterfeiters.

The dots encode the printing date, time, and printer serial number in binary-coded decimal on every sheet of paper printed, which allows pieces of paper to be traced by the manufacturer to identify the place of purchase, and sometimes the buyer. Some are concerned that this is a threat to the privacy and anonymity of those who print.