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|History of printing|
Laser printing is an electrostatic digital printing process that rapidly produces high quality text and graphics by passing a laser beam over a charged drum to define a differentially charged image. The drum then selectively collects charged toner and transfers the image to paper, which is then heated to permanently fix the image. As with digital photocopiers and multifunction printers (MFPs), laser printers employ a xerographic printing process, but differ from analog photocopiers in that the image is produced by the direct scanning of the medium across the printer's photoreceptor. Hence, it proves to be a much faster process compared to the latter.
- 1 History
- 2 Printing process
- 3 Performance
- 4 Color laser printers
- 5 Safety hazards, health risks, and precautions
- 6 See also
- 7 References
- 8 External links
In the 1960s, the American Xerox Corporation held a dominant position in the copier market. In 1969, Gary Starkweather, who worked at Xerox in product development, had the idea of using a laser to draw an image 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). The following year, he 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.
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 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 were sold.
The Xerox 9700 was brought to market in 1977. Unlike the 3800, it was not targeted to replace any particular existing printers, but it did have limited support for the loading of fonts. It excelled at printing high-value documents with varying content such as insurance policies.
The first laser printer designed for use in an office setting was released with the Xerox Star 8010 in 1981. The system used a desktop metaphor that was not surpassed in a commercially successful product until the Apple Macintosh. Although it was innovative, the Star workstation was an expensive (US$17,000) system that was purchased by only a few businesses and institutions. After personal computers became more widespread, the first laser printer intended for a mass market was the HP LaserJet, released in 1984, using a Canon engine controlled by HP software. The HP LaserJet printer was quickly followed by laser printers from Brother Industries, IBM and others. First-generation machines had large photosensitive drums, of circumference greater than the paper length. Once faster-recovery coatings were developed, the drums could touch the paper multiple times in a pass, and could therefore be smaller in diameter.
In 1985, Apple Computer introduced the LaserWriter which used the newly released PostScript page description language. Up until this point, each manufacturer used their own page description language, making the supporting software complex and expensive. PostScript allows the use of text, fonts, graphics, images, and color largely independent of the brand of the printer or its resolution. PageMaker, written by Aldus for the Macintosh and LaserWriter, was also released in 1985 and the combination became very popular for desktop publishing.:364:13/23
Laser printers brought fast, high quality text printing with multiple fonts on a page to the business and consumer markets. No other commonly available printer could offer this combination of features.
A laser beam, typically from an aluminium gallium arsenide semiconductor laser, projects an image of the page to be printed onto an electrically charged rotating drum coated with selenium or, more common in modern printers, organic photoconductors. Photoconductivity allows charge to leak away from the areas exposed to light. Powdered ink (toner) particles are then electrostatically picked up by the drum's charged areas, which have not been exposed to the laser beam. The drum then prints the image onto paper by direct contact and heat, which fuses the ink to 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 in the raster memory.
Each horizontal strip of dots across the page is known as a raster line or scan line. 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 (203 × 267 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. 1200 DPI printers have been commonly available since 2008. 2400 DPI electrophotographic printing plate makers, essentially laser printers that print on plastic sheets, are also available.
In a color printer, each of the four CYMK 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.
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[specify] 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[specify] 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.
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).
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.
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.
The paper passes through rollers in the fuser assembly where heat of up to 200 °C (392 °F) and pressure bond the plastic powder to the paper.
One roller is usually a hollow tube (heat roller) and the other is a rubber backing roller (pressure roller). A radiant heat lamp is suspended in the center of the hollow tube, and its infrared 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.
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.
As with most electronic devices, the cost of laser printers has fallen markedly over the years. In 1984, the HP LaserJet sold for $3500, had trouble with even small, low resolution graphics, and weighed 32 kg (71 lb). As of 2008[update], 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.
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
While monochrome printers only use one laser scanner assembly, color printers often have two or more scanner assemblies.
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.
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.
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.
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.
Safety hazards, health risks, and precautions
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.
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.
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. Of 63 printers evaluated in the Queensland University of Technology study, 17 of the strongest emitters were made by Hewlett-Packard 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:
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.
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."
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 prohibited pass-through passengers from carrying certain printer cartridges on flights. The US Transportation Security Administration said it would ban toner and ink cartridges weighing over 1 pound (0.45 kg) from all passenger flights. U.S. Homeland Security Secretary Janet Napolitano said the ban would apply to both carry-on bags and checked bags on domestic and international flights inbound to the U.S. PC Magazine opined that the ban would not affect average travelers, whose ink cartridges are generally lighter, but would affect the importing of laser printer supplies, as many laser toner cartridges weigh well in excess of a pound.
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