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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 Electric charge cylinder called a "drum" to define a differentially charged image. The drum then selectively collects electrically charged powdered ink (toner), and transfers the image to paper, which is then heated to permanently fuse the text, imagery, or both to the paper. As with digital , laser computer printer employ a Xerography printing process. Laser printing differs from traditional xerography as implemented in analog photocopiers in that in the latter, the image is formed by reflecting light off an existing document onto the exposed drum.
The laser printer was 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 prices have decreased, and the once cutting-edge printing devices are now ubiquitous.
Soon after, in 1977, the Xerox 9700 was brought to market. Unlike the IBM 3800, the Xerox 9700 was not targeted to replace any particular existing printers; however, it did have limited support for the loading of Computer font. The Xerox 9700 excelled at printing high-value documents on cut-sheet paper with varying content (e.g., insurance policies). Inspired by the Xerox 9700's commercial success, Japanese camera and optics company Canon developed in 1979 the Canon LBP-10, a low-cost desktop laser printer. Canon then began work on a much-improved print engine, the Canon CX, resulting in the LBP-CX printer. Having no experience in selling to computer users, Canon sought partnerships with three Silicon Valley companies: Diablo Data Systems (who rejected the offer), Hewlett-Packard (HP), and Apple Computer.
In 1981, the first small personal computer designed for office use, the Xerox Star 8010, reached market. 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 () system, affordable only to a fraction of the businesses and institutions at which it was targeted. Later, in 1984, the first laser printer intended for mass-market sales, the HP LaserJet, was released; 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. A year later, Apple introduced the LaserWriter (also based on the Canon CX engine), but used the newly released PostScript page-description language (up until this point, each manufacturer used its own proprietary 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, developed by Aldus for the Macintosh and LaserWriter, was also released in 1985 and the combination became very popular for desktop publishing. Laser printers brought exceptionally fast and high-quality text printing in multiple fonts on a page, to the business and home markets. No other commonly available printer during this era could also offer this combination of features.
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 heat to instantly fuse the toner that represents the image onto the paper.
The laser is typically an aluminium gallium arsenide (AlGaAs) semiconductor laser, which emits red or infrared light.
The drum is coated with selenium, or more recently, with an organic compound photoconductor made of N-vinylcarbazole, an organic monomer.
There are typically seven steps involved in the process, detailed in the sections below.
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 printing 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 (still 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- or 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 margins, reducing the printable area to , 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 or A4-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 US dollars. The primitive microprocessors in early personal computers were so underpowered and insufficient for graphics work that attached laser printers usually had more onboard processing power. (2025). 9780321115645, Peachpit Press. ISBN 9780321115645 Memory prices later decreased significantly, while rapid improvements in the performance of PCs and peripheral cables (most importantly, SCSI) enabled the development of low-end laser printers which offload rasterization to the sending PC. For such printers, the operating system's print spooler renders the raw bitmap of each page into the PC's system memory at the target resolution, then sends that bitmap directly to the laser (at the expense of slowing down all other programs on the sending PC). The appearance of so-called "dumb" or "host-based" laser printers from NEC made it possible for the retail cost of low-end 300-dpi laser printers to decrease to as low as US$700 by early 1994 and US$600 by early 1995. In September 1997, HP introduced the host-based LaserJet 6L, which could print 600 dpi text at up to six pages per minute for only US$400.
1200 dpi printers have been widely available in the home market since 2008. 2400 dpi electrophotographic printing plate makers, essentially laser printers that print on plastic sheets, are also available.
An AC bias voltage is applied to the primary charge roller to remove any residual charges left by previous images. The roller will also apply a direct current 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.
The laser beam neutralizes (or reverses) the charge on the surface of the drum, leaving a static electric negative image on the drum's surface which will repel the negatively charged toner particles. The areas on the drum which were struck by the laser, however, momentarily have no charge, and the toner being pressed against the drum by the toner-coated developer roll in the next step moves from the roll's rubber surface to the charged portions of the surface of the drum. (2025). 9780596551513, O'Reilly Media. ISBN 9780596551513
Some non-laser printers () use an array of light-emitting diodes spanning the width of the page to generate an image, rather than using a laser. "Exposing" is also known as "writing" in some documentation.
Toner consists of fine particles of dry plastic powder mixed with carbon black or coloring agents. The toner particles are given a negative charge inside the toner cartridge, and as they emerge onto the developer drum they are electrostatically attracted to the photoreceptor's latent image (the areas on the surface of the drum which had been struck by the laser). Because negative charges repel each other, the negatively charged toner particles will not adhere to the drum where the negative charge (imparted previously by the charge roller) remains.
Some printers use a very thin flexible metal foil roller, so there is less thermal 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 very short contact time.
Different printers implement these steps in distinct ways. 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.
If the toner doctor blade does not ensure that a smooth, even layer of toner is applied to the developer roll, the resulting printout may have white streaks from this in places where the blade has scraped off too much toner. Alternatively, if the blade allows too much toner to remain on the developer roll, the toner particles might come loose as the roll turns, precipitate onto the paper below, and become bonded to the paper during the fusing process. This will result in a general darkening of the printed page in broad vertical stripes with very soft edges.
If the fuser roller does not reach a high enough temperature or if the ambient humidity is too high, the toner will not fuse well to the paper and may flake off after printing. If the fuser is too hot, the plastic component of the toner may smear, causing the printed text to look like it is wet or smudged, or may cause the melted toner to soak through the paper to the backside.
Different manufacturers claim that their toners are specifically developed for their printers and that other toner formulations may not match the original specifications in terms of either tendency to accept a negative charge, to move to the discharged areas of the photoreceptor drum from the developer roll, to fuse appropriately to the paper, or to come off the drum cleanly in each revolution.
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, albeit at a slower page-printing speed because of the longer paper path. 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.
In a commercial environment such as an office, it is becoming increasingly common for businesses to use external software that increases the performance and efficiency of laser printers in the workplace. The software can be used to set rules dictating how employees interact with printers, such as setting limits on how many pages can be printed per day, limiting usage of color ink, and flagging jobs that appear to be wasteful.
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.
Liquid electrophotography (LEP) is a similar process used in HP Indigo presses that uses electrostatically charged ink instead of toner, and using a heated transfer roller instead of a fuser, that melts the charged ink particles before applying them to the paper.
2-part Color laser transfers are part of a two-step process whereby the color laser printers use colored toner (dry ink), typically cyan, magenta, yellow, and black (CMYK); however, newer printers designed to print on dark T-shirts utilize a special white toner allowing them to make transfers for dark garments or dark business products.
The CMYK color printing process allows for millions of colors to be faithfully represented by the unique imaging process.
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 subtle gradations of color. Inkjet printers designed for printing photos can produce much higher quality color images. An in-depth comparison of inkjet and laser printers suggest that laser printers are the ideal choice for a high quality, volume printer, while inkjet printers tend to focus on large-format printers and household units. Laser printers offer more precise edging and in-depth monochromatic color. In addition, color laser printers are much faster than inkjet printers, although being generally larger and bulkier.
Digital-rights advocacy groups such as the Electronic Frontier Foundation are concerned about this erosion of the privacy and anonymity of those who print. In particular, the tracking dots were implicated as a tool that directly lead to the arrest and conviction of whistleblower Reality Winner.
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 specialized tools are called "ESD-safe" (Electrostatic Discharge-safe) or "toner vacuums".
However, some ozone escapes the filtering process in commercial printers, and ozone filters are not used at all in most smaller home 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 health hazard is theoretically possible in extreme cases.
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".
The German Social Accident Insurance has commissioned a human study project to examine the effects on health resulting from exposure to toner dusts and from photocopying and printing cycles. Volunteers (23 control persons, 15 exposed persons and 14 asthmatics) were exposed to laser printer emissions under defined conditions in an exposure chamber. The findings from the study based on a broad spectrum of processes and subjects fail to confirm that exposure to high laser printer emissions initiates a verifiable pathological process resulting in the reported illnesses.
A much-discussed proposal for reducing emissions from laser printers is to retrofit them with filters. These are fixed with adhesive tape to the printer's fan vents to reduce particle emissions. However, all printers have a paper output tray, which is an outlet for particle emissions. Paper output trays cannot be provided with filters, so it is impossible to reduce their contribution to overall emissions with retrofit filters.
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