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Computer Printer

A computer printer, or more commonly a printer, produces a hard copy (permanent human-readable text and/or graphics) of documents stored in electronic form, usually on physical print media such as paper transparencies. Many printers are primarily used as computer peripherals, and are attached by a printer cable to a computer which serves as a document source. Other printers, commonly known as network printers, have built-in network interfaces (typically wireless or Ethernet), and can serve as a hardcopy device for any user on the network.

In addition, many modern printers can directly interface to electronic media such as memory sticks or memory cards, or to image capture devices such as digital cameras, scanners; some printers are combined with a scanners and/or fax machines in a single unit.

A printer which is combined with a scanner can essentially function as a photocopier.

Printers are designed for low-volume, short-turnaround print jobs; requiring virtually no setup time to achieve a hard copy of a given document. However, printers are generally slow devices (30 pages per minute is considered fast; and many consumer printers are far slower than that), and the cost-per-page is relatively high.

In contrast, the printing press (which serves much the same function), is designed and optimized for high-volume print jobs such as newspaper print runs--printing presses are capable of hundreds of pages per minute or more, and have an incremental cost-per-page which is a fraction of that of printers.

The printing press remains the machine of choice for high-volume, professional publishing. However, as printers have improved in quality and performance, many jobs which used to be done by professional print shops are now done by users on local printers; see desktop publishing.

The world's first computer printer was a 19th century mechanically driven apparatus invented by Charles Babbage for his Difference Engine.

In 2007, a study revealed that toner-based printers produced pollution as harmful as that from cigarettes. [1]

Printing technology

Printers are routinely classified by the underlying print technology they employ; numerous such technologies have been developed over the years.

The choice of print engine has a substantial effect on what jobs a printer is suitable for, as different technologies are capable of different levels of image/text quality, print speed, low cost, noise; in addition, some technologies are inappropriate for certain types of physical media (such as carbon paper or transparencies).

Another aspect of printer technology that is often forgotten is resistance to alteration: liquid ink such as from an inkjet head or fabric ribbon becomes absorbed by the paper fibers, so documents printed with liquid ink are more difficult to alter than documents printed with toner or solid inks, which do not penetrate below the paper surface.

Checks should either be printed with liquid ink or on special "check paper with toner anchorage".[1] For similar reasons carbon film ribbons for IBM Selectric typewriters bore labels warning against using them to type negotiable instruments such as checks.


Modern print technology

The following printing technologies are routinely found in modern printers, as of April 2006:


Toner-based printers

Toner-based printers work using the Xerographic principle that is at work in most photocopiers: by adhering toner to a light-sensitive print drum, then using static electricity to transfer the toner to the printing medium to which it is fused with heat and pressure.

The most common type of toner-based printer is the laser printer, which uses precision lasers to cause adherence. Laser printers are known for high quality prints, good print speed, and a low (Black and White) cost-per-copy; they are the most common printer for many general-purpose office applications. They are far less commonly used as consumer printers due to a high initial cost.

Laser printers are available in both color and monochrome varieties.

Another toner based printer is the LED printer which uses an array of LEDs instead of a laser to cause toner adhesion to the print drum.

Recent research has also indicated that Laser printers emit potentially dangerous ultrafine particles, possibly causing health problems associated with respiration. [2] The degree of particle emissions varies with age, model and design of each printer but is generally proportional to the amount of toner required. Furthermore, a well ventilated workspace would allow such ultrafine particles to disperse thus reducing the health side effects.


Liquid inkjet printers

Inkjet printers spray very small, precise amounts (usually a few picolitres) of ink onto the media. These droplets of ink will carry a slight electrical charge. The placement of the ink on the page is then determined by the charge of a cathode and electrode between which the ink moves towards the paper. Inkjet printing (and the related bubble-jet technology) are the most common consumer print technology; as high-quality inkjet printers are inexpensive to produce.

Virtually all modern inkjet printers are color devices; some, known as photo printers, include extra pigments to better reproduce the color gamut needed for high-quality photographic prints (and are additionally capable of printing on photographic card stock, as opposed to plain office paper).

Inkjet printers consist of nozzles that produce very small ink bubbles that turn into tiny droplets of ink. The dots formed are the size of tiny pixels. Ink-jet printers can print high quality text and graphics. They are also almost silent in operation. Inkjet printers have a much lower initial cost than do laser printers, but have a much higher cost-per-copy, as the ink needs to be frequently replaced.

In addition, consumer printer manufacturers have adapted a business model similar to that employed by manufacturers of razors; the printers themselves are frequently sold below cost, and the ink is then sold at a high markup.

Various legal and technological means are employed to try and force users to only purchase ink from the manufacturer (thus leading to vendor lock-in); however there is a thriving aftermarket for such things as third-party ink cartridges (new or refurbished) and refill kits.

Inkjet printers are also far slower than laser printers. Inkjet printers also have the disadvantage that pages must be allowed to dry before being aggressively handled; premature handling can cause the inks (which are adhered to the page in liquid form) to run.


Solid ink printers

Solid Ink printers, also known as phase-change printers, are a type of thermal transfer printer. They use solid sticks of CMYK colored ink (similar in consistency to candle wax), which are melted and fed into a piezo crystal operated print-head. The printhead sprays the ink on a rotating, oil coated drum. The paper then passes over the print drum, at which time the image is transferred, or transfixed, to the page.

Solid ink printers are most commonly used as color office printers, and are excellent at printing on transparencies and other non-porous media. Solid ink printers can produce excellent results. Acquisition and operating costs are similar to laser printers. Drawbacks of the technology include high power consumption and long warm-up times from a cold state.

Also, some users complain that the resulting prints are difficult to write on (the wax tends to repel inks from pens), and are difficult to feed through Automatic Document Feeders, but these traits have been significantly reduced in later models. In addition, this type of printer is only available from one manufacturer, Xerox, manufactured as part of their Xerox Phaser office printer line. Previously, solid ink printers were manufactured by Tektronix, but Tek sold the printing business to Xerox in 2001


Dye-sublimation printers

A dye-sublimation printer (or dye-sub printer) is a printer which employs a printing process that uses heat to transfer dye to a medium such as a plastic card, paper or canvas. The process is usually to lay one color at a time using a ribbon that has color panels. Dye-sub printers are intended primarily for high-quality color applications, including color photography; and are less well-suited for text. While once the province of high-end print shops, dye-sublimation printers are now increasingly used as dedicated consumer photo printers.


Inkless printers

Inkless printers use paper with colorless dye crystals embedded between the two outer layers of the paper. When the printer is turned on, heat from the drum causes the crystals to colorize at different rates and become visible. The technology was worked on by Zink Imaging and is now available (2007). Because of the way it prints, the printer can be as small as a business card, the images are waterproof, and in fact, one product slated for release by Zink Imaging is a digital camera with a printer built into it. Xerox is also working on an inkless printer which will use a special reusable paper coated with a few micrometres of UV light sensitive chemicals. The printer will use a special UV light bar which will be able to write and erase the paper. As of early 2007 this technology is still in development and the text on the printed pages can only last between 16-24 hours before fading [3].


Obsolete and special-purpose printing technologies

The following technologies are either obsolete, or limited to special applications though most were, at one time, in widespread use. Among these types are impact printers and pen-based plotters.

Thermal printers work by selectively heating regions of special heat-sensitive paper. These printers are limited to special-purpose applications such as cash registers and the printers in ATMs and gasoline dispensers. They are also used in some older inexpensive fax machines.

Impact printers rely on a forcible impact to transfer ink to the media, similar to the action of a typewriter. All but the dot matrix printer rely on the use of formed characters, letterforms that represent each of the characters that the printer was capable of printing. In addition, most of these printers were limited to monochrome printing in a single typeface at one time, although bolding and underlining of text could be done by overstriking, that is, printing two or more impressions in the same character position. Impact printers varieties include, Typewriter-derived printers, Teletypewriter-derived printers, Daisy wheel printers, Dot matrix printers and Line printers.

Pen-based plotters were an alternate printing technology once common in engineering and architectural firms. Pen-based plotters rely on contact with the paper (but not impact, per se), and special purpose pens that are mechanically run over the paper to create text and images.

Only plotters, dot matrix printers, and certain line printers were capable of printing graphics.


Typewriter-derived printers

Several different computer printers were simply computer-controllable versions of existing electric typewriters. The Friden Flexowriter and IBM Selectric typewriter were the most-common examples. The Flexowriter printed with a conventional typebar mechanism while the Selectric used IBM's well-known "golf ball" printing mechanism. In either case, the letter form then struck a ribbon which was pressed against the paper, printing one character at a time. The maximum speed of the Selectric printer (the faster of the two) was 15.5 characters per second.


Teletypewriter-derived printers

The common teleprinter could easily be interfaced to the computer and became very popular except for those computers manufactured by IBM. Some models used a "typebox" that was positioned (in the X- and Y-axes) by a mechanism and the selected letter from was struck by a hammer. Others used a type cylinder in a similar way as the Selectric typewriters used their type ball. In either case, the letter form then struck a ribbon to print the letterform. Most teleprinters operated at ten characters per second although a few achieved 15 CPS.


Daisy wheel printers

Daisy-wheel printers operate in much the same fashion as a typewriter. A hammer strikes a wheel with petals (the daisy wheel), each petal containing a letter form at its tip. The letter form strikes a ribbon of ink, depositing the ink on the page and thus printing a character. By rotating the daisy wheel, different characters are selected for printing.

These printers were also referred to as letter-quality printers because, during their heyday, they could produce text which was as clear and crisp as a typewriter (though they were nowhere near the quality of printing presses). The fastest letter-quality printers printed at 30 characters per second.


Dot-matrix printers

In the general sense many printers rely on a matrix of pixels, or dots, that together form the larger image. However, the term dot matrix printer is specifically used for impact printers that use a matrix of small pins to create precise dots. The advantage of dot-matrix over other impact printers is that they can produce graphical images in addition to text; however the text is generally of poorer quality than impact printers that use letterforms (type).

A Tandy 1000 HX with a Tandy DMP-133 dot-matrix printer.
A Tandy 1000 HX with a Tandy DMP-133 dot-matrix printer.

Dot-matrix printers can be broadly divided into two major classes:

  • Ballistic wire printers (discussed in the dot matrix printers article)
  • Stored energy printers

Dot matrix printers can either be character-based or line-based (that is, a single horizontal series of pixels across the page), referring to the configuration of the print head.

At one time, dot matrix printers were one of the more common types of printers used for general use - such as for home and small office use. Such printers would have either 9 or 24 pins on the print head. 24-pin print heads were able to print at a higher quality. Once the price of inkjet printers dropped to the point where they were competitive with dot matrix printers, dot matrix printers began to fall out of favor for general use.

Some dot matrix printers, such as the NEC P6300, can be upgraded to print in color. This is achieved through the use of a four-color ribbon mounted on a mechanism (provided in an upgrade kit that replaces the standard black ribbon mechanism after installation) that raises and lowers the ribbons as needed. Color graphics are generally printed in four passes at standard resolution, thus slowing down printing considerably. As a result, color graphics can take up to four times longer to print than standard monochrome graphics, or up to 8-16 times as long at high resolution mode.

Dot matrix printers are still commonly used in low-cost, low-quality applications like cash registers, or in demanding, very high volume applications like invoice printing. The fact that they use an impact printing method allows them to be used to print multi-part documents using carbonless copy paper (like sales invoices and credit card receipts), whereas other printing methods are unusable with paper of this type. Dot-matrix printers are now (as of 2005) rapidly being superseded even as receipt printers.


Line printers

Line printers, as the name implies, print an entire line of text at a time. Three principal designs existed. In drum printers, a drum carries the entire character set of the printer repeated in each column that is to be printed. In chain printers (also known as train printers), the character set is arranged multiple times around a chain that travels horizontally past the print line. In either case, to print a line, precisely timed hammers strike against the back of the paper at the exact moment that the correct character to be printed is passing in front of the paper. The paper presses forward against a ribbon which then presses against the character form and the impression of the character form is printed onto the paper.

Comb printers represent the third major design. These printers were a hybrid of dot matrix printing and line printing. In these printers, a comb of hammers printed a portion of a row of pixels at one time (for example, every eighth pixel). By shifting the comb back and forth slightly, the entire pixel row could be printed (continuing the example, in just eight cycles). The paper then advanced and the next pixel row was printed. Because far less motion was involved than in a conventional dot matrix printer, these printers were very fast compared to dot matrix printers and were competitive in speed with formed-character line printers while also being able to print dot-matrix graphics.

Line printers were the fastest of all impact printers and were used for bulk printing in large computer centres. They were virtually never used with personal computers and have now been replaced by high-speed laser printers.

The legacy of line printers lives on in many computer operating systems, which use the abbreviations "lp", "lpr", or "LPT" to refer to printers.


Pen-based plotters

A plotter is a vector graphics printing device which operates by moving a pen over the surface of paper. Plotters have been (and still are) used in applications such as computer-aided design, though they are being replaced with wide-format conventional printers (which nowadays have sufficient resolution to render high-quality vector graphics using a rasterized print engine). It is commonplace to refer to such wide-format printers as "plotters", even though such usage is technically incorrect.


Other printers

A number of other sorts of printers are important for historical reasons, or for special purpose uses:

  • Digital minilab (photographic paper)
  • Electrolytic printers
  • Microsphere (printer) (special paper)
  • Spark printer (supplied for Sinclair ZX81)
  • brand printer uses heat to print barcodes


Printing mode

The data received by a printer may be:

  1. a string of characters
  2. a bitmapped image
  3. a vector image

Some printers can process all three types of data, others not.

  • Daisy wheel printers can handle only plain text data or rather simple point plots.
  • Plotters typically process vector images.
  • Modern printing technology, such as laser printers and [brands printet]]s, can adequately reproduce all three. This is especially true of printers equipped with support for PostScript and/or PCL; which includes the vast majority of printers produced today.

Today it is common to print everything (even plain text) by sending ready bitmapped images to the printer, because it allows better control over formatting.  Many printer drivers do not use the text mode at all, even if the printer is capable of it.


Monochrome, color and photo printers

A monochrome printer can only produce an image consisting of one color, usually black. A monochrome printer may also be able to produce various tones of that color, such as a grey-scale.

A color printer can produce images of multiple colors.

A photo printer is a color printer that can produce images that mimic the color range (gamut) and resolution of photographic methods of printing. Many can be used autonomously (without a computer), with a memory card or USB connector.


The printer manufacturing business

Often the razor and blades business model is applied. That is, a company may sell a printer at cost, and make profits on the ink cartridge, paper, or some other replacement part. This has caused legal disputes regarding the right of companies other than the printer manufacturer to sell compatible ink cartridges.


Printing speed

The speed of early printers was measured in units of characters per second. More modern printers are measured in pages per minute. These measures are used primarily as a marketing tool, and are not well standardised. Usually pages per minute refers to sparse monochrome office documents, rather than dense pictures which usually print much more slowly. PPM are most of the time referring to A4 paper in Europe and letter paper in the US, resulting in a 5-10% difference.


Forensic identification

Similar to forensic identification of typewriters, computer printers and copiers can be traced down by imperfections in their output. The mechanical tolerances of the toner and paper feed mechanisms cause banding, which contain information about the individual device's mechanical properties. It is sometimes possible to identify the manufacturer and brand, but in some cases the individual printer can be identified from a set of known ones by comparing their outputs. [4] [5]

Some high-quality color printers and copiers steganographically embed their identification code into the printed pages, as fine and almost invisible patterns of yellow dots. The sources identify Xerox and Canon as companies doing this [6] [7]. The Electronic Frontier Foundation (EFF) has investigated[8] this issue and documented how the Xerox DocuColor printer's serial number, as well as the date and time of the printout, are encoded in a repeating 8×15 dot pattern in the yellow channel. EFF is working to reverse engineer additional printers.

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