Laser printing

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History of printing
Woodblock printing 200 Movable type 1040 Printing press c. 1440 Etching c. 1515 Mezzotint 1642 Aquatint 1772 Lithography 1796 Chromolithography 1837 Rotary press 1843 Hectograph 1869 Offset printing 1875 Hot metal typesetting 1884 Mimeograph 1886 Photostat and Rectigraph 1907 Screen printing 1910 Spirit duplicator 1923 Xerography 1938 Phototypesetting 1949 Inkjet printing 1951 Dye-sublimation 1957 Dot matrix printing 1968 Laser printing 1969 Thermal printing c. 1972 3D printing 1984 Digital printing 1993

Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics (and moderate-quality photographs) by repeatedly passing a laser beam back and forth over a negatively charged cylindrical drum to define a differentially-charged image.^[1] The drum then selectively collects electrically charged powdered ink (toner), and transfers the image to paper, which is then heated in order to permanently fuse the text and/or imagery. As with digital photocopiers and multifunction/all-in-one inkjet printers, laser printers employ a xerographic printing process. However, laser printing differs from analog photocopiers in that the image is produced by the direct scanning of the medium across the printer's photoreceptor. This enables laser printing to copy images more quickly than most photocopiers.

Invented at Xerox PARC in the 1970s, laser printers were introduced for the office and then home markets in subsequent years by IBM, Canon, Xerox, Apple, Hewlett-Packard and many others. Over the decades, quality and speed have increased as price has fallen, and the once cutting-edge printing devices are now ubiquitous.

History

In the 1960s, the Xerox Corporation held a dominant position in the photocopier market. In 1969, Gary Starkweather, who worked in Xerox's product development department, had the idea of using a laser beam to 'draw' an image of what was to be copied directly onto the copier drum. After transferring to the recently formed Palo Alto Research Center (Xerox PARC) in 1971, Starkweather adapted a Xerox 7000 copier to create SLOT (Scanned Laser Output Terminal). In 1972, Starkweather worked with Butler Lampson and Ronald Rider to add a control system and character generator, resulting in a printer called EARS (Ethernet, Alto Research character generator, Scanned laser output terminal) -- which later became the Xerox 9700 laser printer.^[2][3][4]

The first commercial implementation of a laser printer was the IBM 3800 in 1976. It was designed for data centers, where it replaced line printers attached to mainframe computers. The IBM 3800 was used for high-volume printing on continuous stationery, and achieved speeds of 215 pages per minute (ppm), at a resolution of 240 dots per inch (dpi). Over 8,000 of these printers were sold.^[5] The Xerox 9700 was brought to market in 1977. Unlike the IBM 3800, the Xerox 9700 was not targeted to replace any particular existing printers; but, it did have limited support for the loading of fonts. The Xerox 9700 excelled at printing high-value documents on cut-sheet paper with varying content (e.g., insurance policies).^[5]

In 1979,^[6] inspired by the Xerox 9700's commercial success, Japanese camera and optics company, Canon, developed a low-cost, desktop laser printer: the Canon LBP-10. Canon then began work on a much-improved print engine, the Canon CX, resulting in the LBP-CX printer. Lacking experience in selling to computer users, Canon sought partnerships with three Silicon Valley companies: Diablo Data Systems (who turned them down), Hewlett-Packard (HP), and Apple Computer.^[7]

The first laser printer designed for office use reached market in 1981: the Xerox Star 8010. The system used a desktop metaphor that was unsurpassed in commercial sales, until the Apple Macintosh. Although it was innovative, the Star workstation was a prohibitively expensive (US$17,000) system, affordable only to a fraction of the businesses and institutions at which it was targeted.^[8]

The first laser printer intended for mass-market sales was the HP LaserJet, released in 1984; it used the Canon CX engine, controlled by HP software. The LaserJet was quickly followed by printers from Brother Industries, IBM, and others. First-generation machines had large photosensitive drums, of circumference greater than the loaded paper's length. Once faster-recovery coatings were developed, the drums could touch the paper multiple times in a pass, and therefore be smaller in diameter.

In 1985, Apple introduced the LaserWriter (also based on the Canon CX engine),^[9] but used the newly released PostScript page-description language. Up until this point, each manufacturer used its own page-description language, making the supporting software complex and expensive. PostScript allowed the use of text, fonts, graphics, images, and color largely independent of the printer's brand or resolution. PageMaker, written by Aldus for the Macintosh and LaserWriter, was also released in 1985 and the combination became very popular for desktop publishing.^{[4]:13/23[5]:364} Laser printers brought exceptionally fast and high-quality text printing, with multiple fonts on a page, to the business and consumer markets. No other commonly-available printer during this era could also offer this combination of features.

Printing process

A laser beam (typically, an aluminium gallium arsenide (AlGaAs) semiconductor laser) projects an image of the page to be printed onto an electrically-charged, selenium-coated, rotating, cylindrical drum^[10] (or, more commonly in subsequent versions, organic photoconductors). Photoconductivity allows the charged electrons to fall away from the areas exposed to light. Powdered ink (toner) particles are then electrostatically attracted to the charged areas of the drum that have not been laser-beamed. The drum then transfers the image onto paper (which is passed through the machine) by direct contact. Finally the paper is passed onto a finisher, which uses intense heat to instantly fuse the toner/image onto the paper.

There are typically seven steps involved in the process:

Raster image processing

The document to be printed is encoded in a page description language such as PostScript, Printer Command Language (PCL), or Open XML Paper Specification (OpenXPS). The raster image processor converts the page description into a bitmap which is stored in the printer's raster memory. Each horizontal strip of dots across the page is known as a raster line or scan line.

Laser printing differs from other printing technologies in that each page is always rendered in a single continuous process without any pausing in the middle, while other technologies like inkjet can pause every few lines. To avoid a buffer underrun (where the laser reaches a point on the page before it has the dots to draw there), a laser printer typically needs enough raster memory to hold the bitmap image of an entire page.

Memory requirements increase with the square of the dots per inch, so 600 dpi requires a minimum of 4 megabytes for monochrome, and 16 megabytes for color at 600 dpi. For fully graphical output using a page description language, a minimum of 1 megabyte of memory is needed to store an entire monochrome letter/A4 sized page of dots at 300 dpi. At 300 dpi, there are 90,000 dots per square inch (300 dots per linear inch). A typical 8.5 × 11 sheet of paper has 0.25-inch (6.4 mm) margins, reducing the printable area to 8.0 by 10.5 inches (200 mm × 270 mm), or 84 square inches. 84 sq/in × 90,000 dots per sq/in = 7,560,000 dots. 1 megabyte = 1,048,576 bytes, or 8,388,608 bits, which is just large enough to hold the entire page at 300 dpi, leaving about 100 kilobytes to spare for use by the raster image processor.

In a color printer, each of the four CMYK toner layers is stored as a separate bitmap, and all four layers are typically preprocessed before printing begins, so a minimum of 4 megabytes is needed for a full-color letter-size page at 300 dpi.

During the 1980s, memory chips were still very expensive, which is why entry-level laser printers in that era always came with four-digit suggested retail prices in U.S. dollars. Memory prices later plunged, and 1200 dpi printers have been widely available in the consumer market since 2008. 2400 dpi electrophotographic printing plate makers, essentially laser printers that print on plastic sheets, are also available.

Charging

In older printers, a corona wire positioned parallel to the drum, or in more recent printers, a primary charge roller, projects an electrostatic charge onto the photoreceptor (otherwise named the photo conductor unit), a revolving photosensitive drum or belt, which is capable of holding an electrostatic charge on its surface while it is in the dark.

An AC bias is applied to the primary charge roller to remove any residual charges left by previous images. The roller will also apply a DC bias on the drum surface to ensure a uniform negative potential.

Numerous patents describe the photosensitive drum coating as a silicon sandwich with a photocharging layer, a charge leakage barrier layer, as well as a surface layer. One version uses amorphous silicon containing hydrogen as the light receiving layer, Boron nitride as a charge leakage barrier layer, as well as a surface layer of doped silicon, notably silicon with oxygen or nitrogen which at sufficient concentration resembles machining silicon nitride.

Exposing

The laser is aimed at a rotating polygonal mirror, which directs the laser beam through a system of lenses and mirrors onto the photoreceptor. The cylinder continues to rotate during the sweep and the angle of sweep compensates for this motion. The stream of rasterized data held in memory turns the laser on and off to form the dots on the cylinder. Lasers are used because they generate a narrow beam over great distances. The laser beam neutralizes (or reverses) the charge on the black parts of the image, leaving a static electric negative image on the photoreceptor surface to lift the toner particles.

Some non-laser printers (LED printers) expose by an array of light emitting diodes spanning the width of the page, rather than by a laser ("exposing" is also known as "writing" in some documentation).

Developing

The surface with the latent image is exposed to toner, fine particles of dry plastic powder mixed with carbon black or coloring agents. The toner particles are given a negative charge, and are electrostatically attracted to the photoreceptor's latent image, the areas touched by the laser. Because like charges repel, the negatively charged toner will not touch the drum where the negative charge remains.

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 rollers in the fuser assembly where heat of up to 200 °C (392 °F) and pressure bonds the plastic powder to the paper.

One roller is usually a hollow tube (heat roller) and the other is a rubber backed roller (pressure roller). A radiant heat lamp is suspended in the centre of the hollow tube, and its inferred energy uniformly heats the roller from the inside. For proper bonding of the toner, the fuser roller must be uniformly hot.

Some printers use a very thin flexible metal fuser roller, so there is less mass to be heated and the fuser can more quickly reach operating temperature. If paper moves through the fuser more slowly, there is more roller contact time for the toner to melt, and the fuser can operate at a lower temperature. Smaller, inexpensive laser printers typically print slowly, due to this energy-saving design, compared to large high speed printers where paper moves more rapidly through a high-temperature fuser with a very short contact time.

Cleaning

When the print is complete, an electrically neutral soft plastic 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.

Toner may occasionally be left on the photoreceptor when unexpected events such as a paper jam occur. The toner is on the photoconductor ready to apply, but the operation failed before it could be applied. The toner must be wiped off and the process restarted.

Multiple steps occurring at once

Once the raster image generation is complete all steps of the printing process can occur one after the other in rapid succession. This permits the use of a very small and compact unit, where the photoreceptor is charged, rotates a few degrees and is scanned, rotates a few more degrees and is developed, and so forth. The entire process can be completed before the drum completes one revolution.

Different printers implement these steps in distinct ways. LED printers actually use a linear array of light-emitting diodes to "write" the light on the drum. 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 warmup process that a laser printer goes through when power is initially applied to the printer consists mainly of heating the fuser element.

Performance

As with most electronic devices, the cost of laser printers has fallen markedly over the years. In 1984, the HP LaserJet sold for $3500,^[11] had trouble with even small, low resolution graphics, and weighed 32 kg (71 lb). As of 2008, low-end monochrome laser printers often sell for less than $75. These printers tend to lack onboard processing and rely on the host computer to generate a raster image, but outperform the 1984 LaserJet in nearly all situations.

Laser printer speed can vary widely, 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 lithography in some commercial applications.^[12]

The cost of this technology depends on a combination of factors, including the cost of paper, toner, drum replacement, as well as the replacement of other items 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.

Duplex printing (printing on both sides of the paper) can halve paper costs and reduce filing volumes. 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 give a slower page-printing speed, because of the longer paper path.

Color laser printers

Color laser printers use colored toner (dry ink), typically cyan, magenta, yellow, and black (CMYK). While monochrome printers only use one laser scanner assembly, color printers often have two or more.

Color printing adds complexity to the printing process because very slight misalignments known as registration errors can occur between printing each color, causing unintended color fringing, blurring, or light/dark streaking along the edges of colored regions. To permit a high registration accuracy, some color laser printers use a large rotating belt called a "transfer belt". The transfer belt passes in front of all the toner cartridges and each of the toner layers are precisely applied to the belt. The combined layers are then applied to the paper in a uniform single step.

Color printers usually have a higher cost per page than monochrome printers (even if printing monochrome-only pages).

Comparison with inkjet printers

Manufacturers use a similar business model for both low-cost color laser printers and inkjet printers: the printers are sold cheaply while replacement toners and inks are relatively expensive. Color laser printers are much quicker than inkjet printers and their running cost per page is usually slightly less. The print quality of color lasers is limited by their resolution, typically 600–1200 dpi, and their use of just four color toners. They often have trouble printing large areas of the same or gradually changing color. Inkjet printers designed for printing photos can produce much higher quality color images.^[13]

Anti-counterfeiting marks

Many 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 (0.0039 in) in size, with a raster of about 1 mm (0.039 in). This is purportedly the result of a deal between the U.S. government and printer manufacturers to help track counterfeiters.^[14]

The dots encode data such as 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.

Digital rights advocacy groups such as the Electronic Frontier Foundation are concerned about this erosion of the privacy and anonymity of those who print.^[15]

Smart chips in toner cartridges

Similar to inkjet printers, toner cartridges may contain smart chips that reduce the number of pages that can be printed with it (reducing the amount of usable ink in the cartridge to sometimes only 50%^[16]), in an effort to increase sales of the toner cartridges.^[17] Besides being more expensive to the consumer, this technique also increases waste, and thus increases pressure on the environment. For these toner cartridges (as with inkjet cartridges), reset devices can be used to override the limitation set by the smart chip. Also, for some printers, online walk-throughs have been posted to demonstrate how to use up all the ink in the cartridge.^[18]

Safety hazards, health risks, and precautions

Toner clean-up

Toner particles are designed to have electrostatic properties and can develop static electric charges when they rub against other particles, objects, or the interiors of transport systems and vacuum hoses. Static discharge from charged toner particles can ignite dust in a vacuum cleaner bag or create a small explosion if sufficient toner is airborne. Toner particles are so fine that they are poorly filtered by conventional household vacuum cleaner filter bags and blow through the motor or back into the room.

If toner spills into the laser printer, a special type of vacuum cleaner with an electrically conductive hose and a high efficiency (HEPA) filter may be needed for effective cleaning. These are called ESD-safe (Electrostatic Discharge-safe) or toner vacuums. Similar HEPA-filter equipped vacuums should be used for clean-up of larger toner spills.

Ozone hazards

As a normal part of the printing process, the high voltages inside the printer can produce a corona discharge that generates a small amount of ionized oxygen and nitrogen, forming ozone and nitrogen oxides. In larger commercial printers and copiers, a carbon filter in the air exhaust stream breaks down these oxides to prevent pollution of the office environment.

However, some ozone escapes the filtering process in commercial printers, and ozone filters are not used in many smaller consumer printers. When a laser printer or copier is operated for a long period of time in a small, poorly ventilated space, these gases can build up to levels at which the odor of ozone or irritation may be noticed. A potential for creating a health hazard is theoretically possible in extreme cases.^[19]

Respiratory health risks

According to a recent study conducted in Queensland, Australia, some printers emit sub-micrometre particles which some suspect may be associated with respiratory diseases.^[20] Of 63 printers evaluated in the Queensland University of Technology study, 17 of the strongest emitters were made by HP and one by Toshiba. The machine population studied, however, was only those machines already in place in the building and was thus biased toward specific manufacturers. The authors noted that particle emissions varied substantially even among the same model of machine. According to Professor Morawska of Queensland University, one printer emitted as many particles as a burning cigarette:^[21][22]

The health effects from inhaling ultrafine particles depend on particle composition, but the results can range from respiratory irritation to more severe illness such as cardiovascular problems or cancer.

— Queensland University of Technology

Muhle et al. (1991) reported that the responses to chronically inhaled copying toner, a plastic dust pigmented with carbon black, titanium dioxide and silica were also similar qualitatively to titanium dioxide and diesel exhaust.^[23]

In December 2011, the Australian government agency Safe Work Australia reviewed existing research and concluded that "no epidemiology studies directly associating laser printer emissions with adverse health outcomes were located" and that several assessments conclude that "risk of direct toxicity and health effects from exposure to laser printer emissions is negligible". The review also observes that, because the emissions have been shown to be volatile or semi-volatile organic compounds, "it would be logical to expect possible health effects to be more related to the chemical nature of the aerosol rather than the physical character of the ‘particulate’ since such emissions are unlikely to be or remain as ‘particulates’ after they come into contact with respiratory tissue."^[24]

Air transport ban

After the 2010 cargo plane bomb plot, in which shipments of laser printers with explosive-filled toner cartridges were discovered on separate cargo airplanes, the US Transportation Security Administration prohibited pass-through passengers from carrying toner or ink cartridges weighing over 1 pound (0.45 kg) on inbound flights, in both carry-on and checked luggage.^[25][26] PC Magazine noted that the ban would not impact most travelers, as the majority of cartridges do not exceed the proscribed weight.^[26]

See also

• Cardboard engineering
• Daisy wheel printer
• Document automation
• Dot matrix printer
• Dye-sublimation printer
• LED printer
• List of printer companies
• Managed Print Services
• Solid ink
• Steganography
• Thermal printer
• Winprinter

References

External links

Wikimedia Commons has media related to Laser printers.

• Howstuffworks "How Laser Printers Work"
• Detailed description, modelling and simulation of the electrophotographic print process (technical; 7.2 MB)
• How a Laser Printer Works Video
• The Evolution of Color Laser Printers
• Q. Why aren't there any CD or DVD laser printers?

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