Xerography
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Xerography is a dry
History
Xerography was invented by American physicist Chester Carlson, based significantly on contributions by Hungarian physicist Pál Selényi. Carlson applied for and was awarded U.S. patent 2,297,691 on October 6, 1942.
Carlson's innovation combined electrostatic printing with
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
Xerography is now used in most
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. Today this technology is used in
By using a cylinder to carry the photosensor, automatic processing was enabled. In 1960, the automatic
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
Laser printer photo drums are made with a
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.[5]
- Step 1. Charging
An
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.[6]
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.[4]
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.[4]
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".
- Step 3. Development
In high-volume copiers, the drum is presented with a slowly turbulent mixture of toner particles and larger, iron, reusable carrier particles. Toner is a powder; its early form was carbon powder, then melt-mixed with a polymer. The carrier particles have a coating which, during agitation, generates a
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.
- Step 4. Transfer
Paper is passed between the drum and the transfer corona, which has a polarity that is the opposite of the charge on the toner. The toner image is transferred from the drum to the paper by a combination of pressure and electrostatic attraction. On many color and high-speed machines, it is common to replace the transfer corona with one or more charged bias transfer rollers, which apply greater pressure and produce a higher quality image.
- Step 5. Separation or detack
Electric charges on the paper are partially neutralized by AC from a second corona, usually constructed in tandem with the transfer corona and immediately after it. As a result, the paper, complete with most (but not all) of the toner image, is separated from the drum or belt surface.
- Step 6. Fixing or fusing
The toner image is permanently fixed to the paper using either a heat and pressure mechanism (hot roll fuser) or a radiant fusing technology (oven fuser) to melt and bond the toner particles into the medium (usually paper) being printed. There also used to be available "offline" vapor fusers. These were trays covered in cotton gauze sprinkled with a volatile liquid, such as ether. When the transferred image was brought into proximity with the vapor from the evaporating liquid, the result was a perfectly fixed copy without any of the distortion or toner migration which can occur with the other methods. This method is no longer used due to emissions of fumes.
- Step 7. Cleaning
The drum, having already been partially discharged during detack, is further discharged by light. Any remaining toner, that did not transfer in step 6, is removed from the drum surface by a rotating brush under suction, or a squeegee known as the cleaning blade. This 'waste' toner usually is routed into a waste toner compartment for later disposal; however, in some systems, it is routed back into the developer unit for reuse. This process, known as toner reclaim, is much more economical, but can possibly lead to a reduced overall toner efficiency through a process known as 'toner polluting' whereby concentration levels of toner/developer having poor electrostatic properties are permitted to build up in the developer unit, reducing the overall efficiency of the toner in the system.
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
Durability
Xerographic documents (and the closely related laser printer printouts) can have excellent
Uses in animation
Ub Iwerks adapted xerography to eliminate the hand-inking stage in the animation process by printing the animator's drawings directly to the 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.[7]
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.[8] 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.[9]
References
- ISBN 978-3527306732.
- ^ "Definition of Xerography". Merriam-Webster.
- ISBN 0-520-23802-8.
electrophorus volta.
- ^ a b c "Photocopying processes". McGraw-Hill Encyclopedia of Science and Technology vol. 13, p. 395, 10th edition, 2007
- ^ The Physics and Technology of Xerographic Processes, Edgar M. Williams, John Wiley and Sons (Wiley-Interscience), New York, 1984.
- ^ a b Photocopying processes". McGraw-Hill Encyclopedia of Science and Technology vol. 13, p. 394, 10th edition, 2007
- ^ "Xerography and animated films". Archived from the original on 2019-01-29.
- ISBN 2-89314-094-7.)
{{cite book}}: CS1 maint: others (link - ^ Proctor, Roy (April 14, 1980). "1708 provides a showcase for photocopier art". Richmond, Virginia: The Richmond News Leader. p. A-44.
Baudelaire thought machines would be the death of art," New York artist Louise Neaderland said this week during a chat at 1708 East Main [Gallery]. "On the other hand, if Leonardo da Vinci had had a photocopier, I think he would have used it.
Further reading
- ISBN 0-7432-5117-2.
- Schein, L. B. (1988). Electrophotography and Development Physics. Springer Series in Electrophysics. Vol. 14. Berlin: Springer-Verlag.
- Eichhorn, Kate (2016). Adjusted Margin: Xerography, Art, and Activism in the Late Twentieth Century. Cambridge: The MIT Press. ISBN 978-0262033961
External links
- "Static Pops Pictures On Paper" detailed 1949 Popular Science article on the history and technology of xerography
