Laser printing
Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics by repeatedly passing a laser beam back and forth over a negatively charged cylinder called a "drum" to define a differentially charged image. The drum then selectively collects electrically charged powdered ink, and transfers the image to paper, which is then heated to permanently fuse the text, imagery, or both to the paper. As with digital photocopiers, laser printers employ a xerographic 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 photoconductor 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.
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 in 1971, Starkweather adapted a Xerox 7000 copier to make SLOT. In 1972, Starkweather worked with Butler Lampson and Ronald Rider to add a control system and character generator, resulting in a printer called EARS —which later became the Xerox 9700 laser printer. In 1976, the first commercial implementation of a laser printer, the IBM 3800, was released. 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, at a resolution of 240 dots per inch. Over 8,000 of these printers were sold.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 fonts. The Xerox 9700 excelled at printing high-value documents on cut-sheet paper with varying content. 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, Hewlett-Packard, 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, but used the newly released PostScript page-description language. 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.
Printing process
A laser beam projects an image of the page to be printed onto an electrically charged, photoconductive, rotating, cylindrical drum. Photoconductivity conducts charged electrons away from the areas exposed to laser light. Powdered ink 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 semiconductor laser, which emits red or infrared light.
The drum is coated with selenium, or more recently, with an organic photoconductor made of N-vinylcarbazole, an organic monomer.
There are typically seven steps involved in the process, detailed in the sections below.
Raster image processing
The document to be printed is encoded in a page description language such as PostScript, Printer Command Language, or Open XML Paper Specification. 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, 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. 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. 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. Memory prices later decreased significantly, while rapid improvements in the performance of PCs and peripheral cables 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. 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.
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, 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 voltage 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
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 polygonal mirror which directs the light beam through a system of lenses and mirrors onto the photoreceptor drum, writing pixels at rates up to sixty-five million times per second. 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 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.
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.