Editing Laser printing (section)

==Printing process==
{{More citations needed section|date=January 2020}}
{{Main|Xerography}}
A laser beam projects an image of the page to be printed onto an electrically charged, [[Photoconductivity|photoconductive]], rotating, cylindrical drum.<ref name="Nagabhushana2010">{{cite book|author=S. Nagabhushana|title=Lasers and Optical Instrumentation|url=https://books.google.com/books?id=FUzjTAr0U4MC&pg=PA269|year=2010|publisher=I. K. International Pvt Ltd|isbn=978-93-80578-23-1|page=269}}</ref>   Photoconductivity conducts charged electrons away from the areas exposed to laser light.  Powdered ink ([[toner (printing)|toner]]) particles are then electrostatically attracted to remaining 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 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|organic]] [[photoconductor]] made of [[N-vinylcarbazole]], an organic [[monomer]].

There are typically seven steps involved in the process, detailed in the sections below.
[[File:Laser toner cartridge.svg|500px|thumb|left|Diagram of a laser printer]]
[[File: Laser printer, 2015-01-23.ogg|thumb|left|The sound of a laser printer]]
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===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]] (RIP) 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 scan|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 printing|inkjet]] can pause every few lines.<ref name="Ganeev">{{cite book |last1=Ganeev |first1=Rashid A. |title=Laser - Surface Interactions |date=2014 |publisher=Springer Science+Business Media |location=Dordrecht |isbn=978-94-007-7341-7 |page=56 |url=https://books.google.com/books?id=H8DEBAAAQBAJ&pg=PA56 |access-date=15 June 2020}}</ref> 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 (paper size)|letter]]- or [[A4 paper|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 {{convert|0.25|in|mm|adj=on}} margins, reducing the printable area to {{convert|8.0|x|10.5|in|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 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 computer]]s were so underpowered and insufficient for graphics work that attached laser printers usually had more onboard processing power.<ref name="Pfiffner_Page_53">{{cite book |last1=Pfiffner |first1=Pamela |title=Inside the Publishing Revolution: The Adobe Story |date=2003 |publisher=Peachpit Press |location=Berkeley |isbn=0-321-11564-3 |page=53}}</ref> 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).<ref name="Brownstein">{{cite journal |last1=Brownstein |first1=Mark |title=SCSI may solve printer data bottlenecks |journal=InfoWorld |date=November 18, 1991 |volume=13 |issue=46 |pages=25–28 |url=https://books.google.com/books?id=Bj4EAAAAMBAJ&pg=PA25 |access-date=July 8, 2023}}</ref> 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<ref name="Troast">{{cite news |last1=Troast |first1=Randy |title=Low-cost laser printers |url=https://books.google.com/books?id=DTsEAAAAMBAJ&pg=PA85 |work=InfoWorld |date=March 21, 1994 |pages=68–69, 84–85}}</ref> and US$600 by early 1995.<ref name="Grotta">{{cite news |last1=Grotta |first1=Daniel |last2=Grotta |first2=Sally Wiener |title=SuperScript 660: NEC's Dumb Printer Is a Smart Buy |url=https://books.google.com/books?id=eMKimy4DFaEC&pg=PA50 |work=PC Magazine |date=March 28, 1995 |page=50}}</ref>  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.<ref name="Mendelson">{{cite news |last1=Mendelson |first1=Edward |title=A New LaserJet Jewel: HP LaserJet 6L makes 6-ppm printing an affordable venture |url=https://books.google.com/books?id=eJVnzcZC5I0C&pg=PA68 |access-date=July 8, 2023 |work=PC Magazine |date=September 9, 1997 |page=68}}</ref>

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.

===Charging===
[[File:Corona charging.svg|thumb|Applying a negative charge to the photosensitive drum]]
In older printers, a [[Corona discharge|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 photoconductor 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 [[alternating current|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|DC]] bias on the drum surface to ensure a uniform negative potential.

Numerous patents{{Specify|date=January 2010}} 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|date=January 2010}} 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 [[Doping (semiconductor)|doped silicon]], notably silicon with [[oxygen]] or [[nitrogen]] which at sufficient concentration resembles machining [[silicon nitride]].

===Exposing===
[[File:Laser printer-Writing.svg|thumb|Laser light selectively neutralizes the negative charge on the photoreceptive drum, to form an electrostatic image.]]
[[File: Laser unit dell p1500 print.jpg|thumb|Laser unit from a Dell P1500. The white hexagon is the rotating scanner mirror.]]
A laser printer uses a laser because lasers are able to form highly focused, precise, and intense beams of light, especially over the short distances inside of a printer.  The laser is aimed at a rotating [[polygon]]al mirror which directs the light beam through a system of lenses and mirrors onto the photoreceptor drum, writing [[pixel]]s at rates up to sixty-five million times per second.<ref>{{cite web|title=how Laser Process Technology animation (''sic'')|url=https://www.youtube.com/watch?v=MDLM5fMFyA4| archive-url=https://web.archive.org/web/20131221054838/http://www.youtube.com/watch?v=MDLM5fMFyA4&gl=US&hl=en| archive-date=2013-12-21 |publisher=Lexmark|date=14 July 2012}}</ref> The drum continues to rotate during the sweep, and the angle of sweep is canted very slightly to compensate for this motion. The stream of rasterized data held in the printer's memory rapidly turns the laser on and off as it sweeps.

The laser beam neutralizes (or reverses) the charge on the surface of the drum, leaving a [[static electricity|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.<ref>{{cite web
|title=CompTIA A+ Rapid Review: Printers |website=MicrosoftPressStore.com
|url=https://www.microsoftpressstore.com/articles/article.aspx?p=2201308
|quote=Laser printers .. complex imaging process ... charge neutralizes ... the drum}}</ref><ref>{{cite book |last1=Bhardwaj |first1=Pawan K. |title=A+, Network+, Security+ Exams in a Nutshell: A Desktop Quick Reference |date=2007 |publisher=O'Reilly Media |isbn=978-0-596-55151-3 |quote=in most laser printers. ... the surface of the drum. }}{{pn|date=February 2024}}</ref>

Some non-laser printers ([[LED printer]]s) use an array of [[light-emitting diode]]s spanning the width of the page to generate an image, rather than using a laser. "Exposing" is also known as "writing" in some documentation.

===Developing===
As the drums rotate, [[toner (printing)|toner]] is continuously applied in a 15-[[micron]]-thick layer to the ''developer roll''. The surface of the photoreceptor with the latent image is exposed to the toner-covered developer roll.

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.

===Transferring===
A sheet of paper is then rolled under the photoreceptor drum, which has been coated with a pattern of toner particles in the exact places where the laser struck it moments before.  The toner particles have a very weak attraction to both the drum and the paper, but the bond to the drum is weaker and the particles transfer once again, this time from the drum's surface to the paper's surface. Some machines also use a positively charged "transfer roller" on the backside of the paper to help pull the negatively charged toner from the photoreceptor drum to the paper.

===Fusing===
[[File:Laser printer fusing.svg|thumb|left|Toner is fused onto paper with heat and pressure.]]
The paper passes through rollers in the fuser assembly, where temperatures up to {{convert|427|C}} and pressure are used to permanently bond the toner 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 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 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.

===Cleaning and recharging===
[[File:Color Laser Printer Magnified.jpg|thumb|Magnification of color laser printer output, showing individual toner particles comprising 4 dots of an image with a bluish background]]
As the drum completes a revolution, it is exposed to an electrically neutral soft plastic blade that cleans any remaining toner from the photoreceptor drum and deposits it into a waste reservoir.  A charge roller then re-establishes a uniform negative charge on the surface of the now-clean drum, readying it to be struck again by the laser.

===Continuous printing===
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 printer]]s use a linear array of [[light-emitting diode]]s 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.

===Malfunctions===
The mechanism inside a laser printer is somewhat delicate and, once damaged, often impossible to repair.  The drum, in particular, is a critical component: it must not be left exposed to ambient light for more than a few hours, as light is what causes it to lose its charge and will eventually wear it out.  Anything that interferes with the operation of the laser such as a scrap of torn paper may prevent the laser from discharging some portion of the drum, causing those areas to appear as white vertical streaks.  If the neutral wiper blade fails to remove residual toner from the drum's surface, that toner may circulate on the drum a second time, causing smears on the printed page with each revolution.  If the charge roller becomes damaged or does not have enough power, it may fail to adequately negatively charge the surface of the drum, allowing the drum to pick up excessive toner on the next revolution from the developer roll and causing a repeated but fainter image from the previous revolution to appear down the page.

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.{{citation needed|date=February 2016}}