thumb|Schematic drawing of the xerographic photocopying process

Xerography (from the Greek roots , meaning "dry" and , meaning "writing") is a technique of printing and photocopying. Originally called electrophotography, it was renamed to emphasize that it uses no liquid chemicals, unlike reproduction techniques then in use such as cyanotype.

History

Xerography was invented by American physicist Chester Carlson, based significantly on contributions by Hungarian physicist Pál Selényi. Carlson applied for and was awarded on October 6, 1942.

Carlson's innovation combined electrostatic printing with photography, unlike the dry electrostatic printing process invented by Georg Christoph Lichtenberg in 1778. Carlson's original process was cumbersome, requiring several manual processing steps with flat plates.

In 1946, Carlson signed an agreement with Haloid Photographic Company to develop it as a commercial product. Before that year, Carlson had proposed his idea to more than a dozen companies, but none was interested. Haloid's president, Joseph C. Wilson, saw the promise of Carlson's invention, and saw to it that Haloid diligently worked to produce a working commercial product.

It was almost 18 years before a fully automated process was developed, the key breakthrough being the use of a cylindrical drum coated with selenium instead of a flat plate. This resulted in the first commercial automatic copier, the Xerox 914, being released by Haloid/Xerox in 1960.

Xerography is now used in most photocopying machines and in laser and LED printers.

Process

The first commercial use was hand processing of a flat photosensor (an electrostatic component that detects the presence of visible light) with a copy camera and a separate processing unit to produce offset lithographic plates.

By using a cylinder to carry the photosensor, automatic processing was enabled. In 1960, the automatic photocopier was created and many millions have been built since. Today this technology is used in photocopiers, computer laser printers and LED printers, microform printers and even digital presses which are slowly replacing many traditional offset presses in the printing industry for shorter runs.

A metal cylinder called the drum is mounted to rotate about a horizontal axis. The drum rotates at the speed of paper output. One revolution passes the drum surface through the steps described below.

The end-to-end dimension is the width of print to be produced plus a generous tolerance. The drums in the copiers originally developed by Xerox Corporation were manufactured with a surface coating of amorphous selenium (more recently ceramic or organic photoconductor), applied by vacuum deposition. Amorphous selenium will hold an electrostatic charge in darkness and will conduct away such a charge under light. In the original system, photocopiers that rely on silicon or selenium (and its alloys) are charged positively in use (hence work with negatively charged "toner" powder). In the 1970s, IBM Corporation sought to avoid Xerox's patents for selenium drums by developing organic photoconductors as an alternative to the selenium drum. Photoconductors using organic compounds are electrochemically charged vice versa to the preceding system in order to exploit their native properties in printing. Organic photoconductors are now preferred because they can be deposited on a flexible, oval or triangular, belt instead of a round drum, facilitating significantly smaller device size.

Laser printer photo drums are made with a doped silicon diode sandwich structure with a hydrogen-doped silicon light-chargeable layer, a boron nitride rectifying (diode-causing) layer that minimizes current leakage, and a surface layer of silicon doped with oxygen or nitrogen; silicon nitride is a scuff-resistant material.

The steps of the process are described below as applied on a cylinder, as in a photocopier. Some variants are described within the text. Every step of the process has design variants. The physics of the xerographic process are discussed at length in a book.

Step 1: charging

An electrostatic charge of −600 volts is uniformly distributed over the surface of the drum by a corona discharge from a corona unit (Corotron), with output limited by a control grid or screen. This effect can also be achieved by using a contact roller with a charge applied to it. Essentially, a corona discharge is generated by a very thin

wire to  inch (6 to 13 mm) away from the photoconductor. A negative charge is placed on the wire, which will ionize the space between the wire and conductor, so electrons will be repelled and pushed away onto the conductor. The conductor is set on top of a conducting surface, kept at ground potential.

The polarity is chosen to suit the positive or negative process. Positive process is used for producing black on white copies. Negative process is used for producing black on white from negative originals (mainly microfilm) and all digital printing and copying. This is to economize on the use of laser light by the "blackwriting" or "write to black" exposure method.

Step 2: exposure

The document or microform to be copied is illuminated by flash lamps on the platen and either passed over a lens or scanned by a moving light and lens, such that its image is projected onto and synchronized with the moving drum surface. Alternatively, the image may be exposed using a xenon strobe illuminating the surface of the moving drum or belt, fast enough to render a perfect latent image. Where there is text or image on the document, the corresponding area of the drum will remain unlit. Where there is no image the drum will be illuminated and the charge will be dissipated. The charge that remains on the drum after this exposure is a 'latent' image and is a negative of the original document.

Uses in art

Xerography has been used by photographers internationally as a direct imaging photographic process, by book artists for publishing one-of-a-kind books or multiples, and by collaborating artists in portfolios such as those produced by the International Society of Copier Artists founded by American printmaker and book artist, Louise Odes Neaderland. Art critic Roy Proctor said of artist/curator Louise Neaderland during her residency for the exhibition Art ex Machina at 1708 Gallery in Richmond, Virginia, "She's living proof that, when a new technology begins to be mass-produced, artists will be curious enough—and imaginative enough—to explore its creative uses.

References

Further reading

  • "Static Pops Pictures On Paper" detailed 1949 Popular Science article on the history and technology of xerography