Photocopiers

A photocopier is an office or home machine which makes paper copies of documents and other visual images quickly and cheaply. Most current photocopiers use a technology called xerography, a dry process using heat. (Copiers can also use other output technologies such as ink jet, but xerography is standard for office copying.)

Xerographic office photocopying was introduced by Xerox in the 1960s, and over the following 20 years it gradually replaced copies made by Verifax, Photostat, carbon paper, mimeograph machines, and other duplicating machines. The prevalence of its use is one of the factors that prevented the development of the paperless office heralded early in the digital revolution.

Photocopying is widely used in business, education, and government. There have been many predictions that photocopiers will eventually become obsolete as information workers continue to increase their digital document creation and distribution and rely less on distributing actual pieces of paper. However, photocopiers are undeniably more convenient than computers for the very common task of creating a copy of a piece of paper.

How a photocopier works

1. Charging: The surface of a cylindrical drum is given an electrostatic charge by either a high voltage wire called a corona wire or a charge roller. The drum is coated with a photoconductive material, such as selenium. A photoconductor is a semiconductor that becomes conductive when exposed to light.
2. Exposure: A bright lamp illuminates the original document, and the white areas of the original document reflect the light onto the surface of the photoconductive drum. The areas of the drum that are exposed to light (those areas that correspond to white areas of the original document) become conductive and therefore discharge to ground. The area of the drum not exposed to light (those areas that correspond to black portions of the original document) remain negatively charged. The result is a latent electrical image on the surface of the drum.
3. Developing: The toner is positively charged. When it is applied to the drum to develop the image, it is attracted and sticks to the areas that are negatively charged (black areas), just as paper sticks to a toy balloon with a static charge.
4. Transfer: The resulting toner image on the surface of the drum is transferred from the drum onto a piece of paper with a higher negative charge than the drum.
5. Fusing: The toner is melted and bonded to the paper by high-heat and high-pressure rollers.
6. Cleaning: The drum is wiped clean with a rubber blade and completely discharged by light before beginning the process again.
   

This example is of a negatively charged drum and paper, and positively charged toner. Some copiers employ the opposite: a positively charged drum and paper, and negatively charged toner.


Digital technology
In recent years, all new photocopiers have adopted digital technology, replacing the older analog technology. With digital copying, the copier effectively consists of an integrated scanner and laser printer. This design has several advantages, such as automatic image quality enhancement and the ability to "build jobs" or scan page images independently of the process of printing them. Some digital copiers can function as high-speed scanners; such models typically have the ability to send documents via email or make them available on a local area network.

The greatest advantage of a digital copier is "automatic digital collation." When copying a set of twenty pages twenty times, for example, a digital copier scans each page only once, then uses the stored information to produce twenty sets. In an analog copier, either each page is scanned twenty times (a total of 400 scans), making one set at a time, or twenty separate output trays are used for the twenty sets.

Low-end copiers also use digital technology, but they tend to consist of a standard PC scanner coupled to an inkjet or low-end laser printer, both of which are far slower than their counterparts in high-end copiers. However, low-end scanner inkjets can provide color copying at a far lower cost than a traditional color copier. The cost of electronics is such that combined scanner-printers sometimes have built-in fax machines.

Copyright issues
Photocopying material which is subject to copyright (such as books or scientific papers) is subject to restrictions in most countries. It is common practice, especially by students, as the cost of purchasing a book for the sake of one article or a few pages may be excessive. The principle of fair use (in the United States) or fair dealing (in other Berne Convention countries) allows this type of copying for research purposes.

In some countries, such as Canada, some universities pay royalties from each photocopy made at university copy machines and copy centers to copyright collectives out of the revenues from the photocopying and these collectives distribute these funds to various scholarly publishers. In the United States, photocopied compilations of articles, handouts, graphics, and other information called readers are often required texts for college classes. Either the instructor or the copy center is responsible for clearing copyright for every article in the reader and attribution information is included in the front of the reader.