Computer printer
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 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 (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 technologyPrinters 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.
According to the website of security expert Frank Abagnale 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 printersToner-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 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.
Liquid inkjet printersInkjet 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 Ink printersSolid 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, and are
commonly found in office environments. 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, however 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 2000.
Dye-sublimation printersA 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.
Thermal printersThermal 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.
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.
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 printersSeveral different
computer printers were simply computer-controlable 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 printersThe 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 printersDaisy-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 printersIn 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:
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 printersLine printers, as the name implies,
print an entire line of text at a time. Three 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.
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 plottersA 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 printersA 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)
barcode printer uses heat to print barcodes
Printing modeThe 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.
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 printersA monochrome
printer can only produce an image consisting of one color, usually
black. A monochrome printer may also be able to produce various hues 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 businessOften 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 speedThe 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 classesThey are collections of printers. Print jobs sent to a class are forwarded to the first available printer in the class.
Forensic identificationSimilar 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. [2] [3]
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 [4] [5]. The Electronic Frontier Foundation has
investigated[6] 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.
