A photocopier (or copier) is a machine that 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 (using xerography)

main: Xerography

1. Charging: The surface of a cylindrical drum is electrostatically charged by either a high voltage wire called a corona wire or a charge roller. The drum has a coating of a photoconductive material. A photoconductor is a semiconductor that becomes conductive when exposed to light1.
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. (In digital machines, the original document is scanned and digitized and a laser is employed to discharge the drum in a similar fashion)
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 heat and pressure rollers.
6. Cleaning: The drum is wiped clean with a rubber blade and completely discharged by light.