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A computer printer is a computer peripheral device that produces a hard copy (permanent human-readable text and/or graphics, usually on paper) from data stored in a computer connected to it.
Printing mode
The data received by a printer may be:
- a string of characters
- a bitmapped image
- 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.
PostScript and PCL printers can combine all three types of data.
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.
Methods of image creation
The medium for most printers is paper, so they are usually classified according to the method of image creation:
Toner-based printers
Laser printers refer to the method used to adhere toner to the media. The advent of cost-effective, precision lasers has made them the dominant toner-based monochrome printer type for home and office applications. Another toner based printer is the LED printer which uses an array of LEDs instead of a laser to cause toner adhesion.
Inkjet printers
- Main article: Inkjet printer
Inkjet printers spray very small, precise amounts (usually a few picolitres) of ink onto the media. For color applications including photo printing, ink jet methods are dominant.
Impact printers
Impact printers rely on a forcible impact to transfer ink to the media, similar to typewriters, that are typically limited to reproducing text. A daisy wheel printer is a specific type of impact printer where the type is moulded around the edge of a wheel. A golf ball typewriter is similar to the daisy wheel type but has the characters distributed over the face of the globe shape.
Dot-matrix printers
- Main article: Dot matrix printer
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.
Dot-matrix printers can be broadly divided into two major classes:
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.
Dot matrix printers are still commonly used in low-cost, low-quality applications like cash registers. 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.
Other printers
A number of other sorts of printers are important for historical reasons, or for special purpose uses:
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 usually 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. [1] [2]
Some high-quality color printers and copiers also reportedly 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. [3] [4] The Electronic Frontier Foundationhas done some preliminary analysis of and is currently investigating this issue.
See also
External links
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