Computer printer

A computer printer, or more commonly just a printer, is a device that produces hard copy (permanent human-readable text and/or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies). Many printers are primarily used as computer peripherals, and are permanently attached to a computer which serves as a document source. Other printers, commonly known as network printer, 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 (10 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.

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).

Modern print technology

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

Toner-based printers

Toner-based printers work in the same fashion as many photocopiers, by adhering toner to the media. 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 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.

Liquid inkjet printers

Inkjet printers spray very small, precise amounts (usually a few picolitres) of ink onto the media. 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 inkjet printers

Solid ink-jet printers, also known as phase-change printers or crayon-jet printers, are a type of thermal transfer printer. They use solid sticks of CMYK colored ink (similar in consistancy 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 inkjet printers can produce excellent results, and are commonly found in office environments. Acquisition and operating costs are similar as for laser printers. Drawbacks of the technology include high power consumption, and the fact that the resulting prints are difficult to write on (the wax tends to repel inks from pens). In addition, this type of printer is only available from one manufacturer, Xerox. Previously, solid inkjet printers were manufactured by Tektronix, but Tek sold the printing business to Xerox in 2000.

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, printer paper or poster paper. 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.

Obsolete and special-purpose printing technologies

The following printing technologies are either obsolete, or limited to special applications (though several of them were, at one time, in widespread use).

Impact printers

Impact printers rely on a forcible impact to transfer ink to the media, similar to the action of a typewriter. Two main varieties are letter quality printers and dot matrix printers.

=Letter quality printers

Letter-quality printers operate in much the same fashion as a typewriter. An array of pre-formed hammers, each in the impression of a different letter, number, or symbol, are use to strike a ribbon of ink, depositing the ink on the page and thus printing a character. Two types include daisy wheel printers, where the type is moulded around the edge of a wheel, and so-called "golf ball" printers, where the type is distributed over the face of a globe-shaped printhead. These printers were referred to as letter-quality printers as during their heydey, they could produce text which was as clear and crisp as a typewriter (though they were nowhere near the quality of printing presses). Letter-quality printers, however, were slow, noisy, incapable of printing graphics or images, generally limited to monochrome, and limited to a fixed set (usually one) of typefaces, though certain font effects like underlining and boldface could be achieved by overstriking.

Letter-quality printers today are obsolete.

=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).

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, in example 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 rises and lowers the ribbons as needed. Color graphics are generally printed in 4 passes at standard resolution, thus slowing down printing considerably. As a result, color graphics can take up to 4 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. Two principle 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.

These 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 partly replaced by high-speed laser printers.

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

Thermal printers

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 fax machines.

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
Friden Flexowriter
IBM Selectric typewriter
Microsphere (special paper)
Spark printer (supplied for Sinclair ZX81)
Teletype
- barcode printer uses heat to print barcodes

Printing mode

The data received by a printer may be:

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 inkjet printers, 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 graduations of tone 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.

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.

Printer job classes

They are collections of printers. Print jobs sent to a class are forwarded to the first available printer in the class.

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. [http://dsc.discovery.com/news/briefs/20041011/printer.html

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 Electronic Frontier Foundation has investigated^*" target="_blank" >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 [http://www.eff.org/Privacy/printers/list.php additional printers.

External links

Mobile Imaging and Printing Consortium, MIPC.
Printer Forums Printer Help and Discussion Forums
Repair4Printer - A survey of do-it-yourself guides about repairing computer printers.

Computer printers

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