Xerography is a dry photocopying technique. Originally called electrophotography, it was renamed xerography—from the Greek roots Greek, Ancient (to 1453);: [[wikt:ξηρός|ξηρός]] Greek, Ancient (to 1453);: xeros, meaning "dry" and Greek, Ancient (to 1453);: [[wikt:-γραφία|-{{zwj Greek, Ancient (to 1453);: -{{zwj, meaning "writing"—to emphasize that unlike reproduction techniques then in use such as cyanotype, the process of xerography used no liquid chemicals.[1]
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.[2] 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.
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. Today this technology is used in photocopy machines, laser printers, and digital presses which are slowly replacing many traditional offset presses in the printing industry for shorter runs.
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. The same process is used in microform printers and computer output laser or LED printers. 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 or OPC), applied by vacuum deposition. Amorphous selenium will hold an electrostatic charge in darkness and will conduct away such a charge under light. 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. 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). Photoconductors using organic compounds are electrochemically charged vice versa to the preceding system in order to exploit their native properties in printing.[3] 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 build 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.[4]
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.
Whether in a scanning or a stationary optical system, combinations of lenses and mirrors are used to project the original image on the platen (scanning surface) onto the photoconductor. Additional lenses, with different focal lengths or zooming lenses are utilized to enlarge or reduce the image; the scanning speed must adapt to elements or reductions.[3]
A drum is inferior to a belt in the sense that although it is simpler than a belt, it must be buffered gradually in parts rolling on the curved drum, while the flat belt efficiently uses one exposure to make a direct passage.[3]
In a laser or LED printer, modulated light is projected onto the drum surface to create the latent image. The modulated light is used only to create the positive image, hence the term "blackwriting".
Where a negative image is required, as when printing from a microform negative, then the toner has the same polarity as the corona in step 1. Electrostatic lines of force drive the toner particles away from the latent image towards the uncharged area, which is the area exposed from the negative.
Early color copiers and printers used multiple copy cycles for each page output, using colored filters and toners. Modern units use only a single scan to four separate, miniature process units, operating simultaneously, each with its own coronas, drum and developer unit.
Some systems have abandoned the separate developer (carrier). These systems, known as monocomponent, operate as above, but use either a magnetic toner or fusible developer. There is no need to replace worn-out developer, as the user effectively replaces it along with the toner. An alternative developing system, developed by KIP from an abandoned line of research by Xerox, completely replaces magnetic toner manipulation and the cleaning system, with a series of computer-controlled, varying biases. The toner is printed directly onto the drum, by direct contact with a rubber developing roller which, by reversing the bias, removes all the unwanted toner and returns it to the developer unit for reuse.
The development of xerography has led to new technologies that have the potential to eventually eradicate traditional offset printing machines. These new machines that print in full CMYK color, such as Xeikon, use xerography but provide nearly the quality of traditional ink prints.
Xerographic documents (and the closely related laser printer printouts) can have excellent archival durability, depending on the quality of the paper used. If low-quality paper is used, it can yellow and degrade due to residual acid in the untreated pulp; in the worst case, old copies can literally crumble into small particles when handled. High-quality xerographic copies on acid-free paper can last as long as typewritten or handwritten documents on the same paper. However, xerographic copies are vulnerable to undesirable toner transfer if they are stored in direct contact or close proximity to plasticizers, which are present in looseleaf binders made with PVC. In extreme cases, the ink toner will stick directly to the binder cover, pulling away from the paper copy and rendering it illegible.
Ub Iwerks adapted xerography to eliminate the hand-inking stage in the animation process by printing the animator's drawings directly to the animation cels. The first animated feature film to use this process was One Hundred and One Dalmatians (1961), although the technique was already tested in Sleeping Beauty, released two years earlier. At first, only black lines were possible, but in 1977, gray lines were introduced and used in The Rescuers and in the 1980s, colored lines were introduced and used in animated features like The Secret of NIMH.[6]
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.[7] 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.[8]