Xerography

Xerography or electrophotography is a dryphotocopying technique. Its fundamental principlewas invented by Hungarian physicist Pál Selényi andbased on Selényi’s publications Chester Carlson appliedfor and was awarded U.S. Patent 2,297,691 on October6, 1942. The technique was originally called electropho-tography. It was later renamed xerography—from theGreek roots ξηρός xeros, “dry” and -γραφία -graphia,“writing”—to emphasize that, unlike reproductiontechniques then in use such as cyanotype, this processused no liquid chemicals.[1]

Carlson’s innovation combined electrostatic printing withphotography, unlike the dry electrostatic printing pro-cess invented by Georg Christoph Lichtenberg in 1778.[2]Carlson’s original process was cumbersome, requiringseveral manual processing steps with flat plates. It wasalmost 18 years before a fully automated process was de-veloped, the key breakthrough being use of a cylindricaldrum coated with selenium instead of a flat plate. This re-sulted in the first commercial automatic copier, the Xerox914, being released by Haloid/Xerox in 1960. Beforethat year, Carlson had proposed his idea to more than adozen companies, but none were interested. Xerographyis now used in most photocopying machines and in laserand LED printers.

1 Xerographic process

The first commercial use was hand processing of a flatphotosensor (an electronic component that detects thepresence of visible light) with a copy camera and a sepa-rate processing unit to produce offset lithographic plates.Today this technology is used in photocopy machines,laser printers, and digital presses which are slowly replac-ing many traditional offset presses in the printing industryfor shorter runs.By using a cylinder to carry the photosensor, auto-matic processing was enabled. In 1960, the automaticphotocopier was created and many millions have beenbuilt since. The same process is used in microform print-ers and computer output laser or LED printers. A metalcylinder called the drum is mounted to rotate about a hor-izontal axis. The drum rotates at the speed of paper out-put. One revolution passes the drum surface through thesteps described below.The end-to-end dimension is the width of print to beproduced plus a generous tolerance. The drums in

15 kV

Cylindrical drumphotoconductor

1

Light

2

Toner

3

Paper

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Schematic overview of the xerographic photocopying process

the copiers originally developed by Xerox Corporationwere manufactured with a surface coating of amorphousselenium (more recently ceramic or organic photocon-ductor or OPC), applied by vacuum deposition. Amor-phous selenium will hold an electrostatic charge in dark-ness and will conduct away such a charge under light.In the 1970s, IBM Corporation sought to avoid Xerox’spatents for selenium drums by developing organic pho-toconductors as an alternative to the selenium drum. Inthe original system, photocopiers that rely on silicon orselenium (and its alloys) are charged positively in use(hence work with negatively-charged “toner” powder).

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2 1 XEROGRAPHIC PROCESS

Photoconductors using organic compounds (like zinc ox-ide or cadmium sulfide) are electrochemically chargedvice-versa to the preceding system in order to exploit theirnative properties in printing.[3] Organic photoconductorsare now preferred because they can be deposited on a flex-ible, oval or triangular, belt instead of a round drum, fa-cilitating significantly smaller device build size.Laser printer photo drums are made with a doped silicondiode sandwich structure with a hydrogen-doped siliconlight-chargeable layer, a boron nitride rectifying (diode-causing) layer that minimizes current leakage, and a sur-face layer of silicon doped with oxygen or nitrogen; sili-con nitride is a scuff-resistant material.The steps of the process are described below as appliedon a cylinder, as in a photocopier. Some variants are de-scribed within the text. Every step of the process hasdesign variants. The physics of the xerographic processare discussed at length in a book.[4]

Step 1. Charging

An electrostatic charge of −600 volts is uniformly dis-tributed over the surface of the drum by a corona dis-charge from a corona unit (Corotron), with output lim-ited by a control grid or screen. This effect can also beachieved with the use of a contact roller with a chargeapplied to it. Essentially, a corona discharge is generatedby a narrow wire 1⁄4 to 1⁄2 inch (6.35 to 12.7 mm) apartfrom the photoconductor. A negative charge is placed onthe wire, which will ionize the space between the wireand conductor, so electrons will be repelled and pushedaway onto the conductor. The conductor is set on top ofa conducting surface, kept at ground potential.[5]

The polarity is chosen to suit the positive or negative pro-cess. Positive process is used for producing black onwhite copies. Negative process is used for producingblack on white from negative originals (mainly microfilm)and all digital printing and copying. This is to economizeon the use of laser light by the “blackwriting” or “write toblack” exposure method.

Step 2. Exposure

The document or microform to be copied is illuminatedby flash lamps on the platen and either passed over a lensor is scanned by a moving light and lens, such that itsimage is projected onto and synchronized with the mov-ing drum surface. Alternatively, the image may be ex-posed using a xenon strobe, onto the surface of the mov-ing drum or belt, fast enough to render a perfect latentimage. Where there is text or image on the document, thecorresponding area of the drum will remain unlit. Wherethere is no image the drum will be illuminated and thecharge will be dissipated. The charge that remains on thedrum after this exposure is a 'latent' image and is a nega-tive of the original document.[5]

Whether in a scanning or a stationary optical system,combinations of lenses andmirrors are used to project theoriginal image on the platen (scanning surface) onto thephotoconductor. Additional lenses, with different focallengths or zooming lenses are utilized to enlarge or reducethe image. The scanning system, though, must change itsscanner speed to adapt to elements or reductions.[3]

A drum is inferior to a belt in the sense that although it issimpler than a belt, it must be buffered gradually in partsrolling on the drum. As a result, the belt is more efficientto use one exposure to make a direct passage.[3]

In a laser or LED printer, modulated light is projectedonto the drum surface to create the latent image. Themodulated 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 aslowly turbulent mixture of toner particles and larger,iron, reusable carrier particles. Toner is a powder; itsearly form was carbon powder, then melt-mixed with apolymer. The carrier particles have a coating which, dur-ing agitation, generates a triboelectric charge (a form ofstatic electricity), which attracts a coating of toner par-ticles. In addition, the mix is manipulated with a mag-netic roller to present to the surface of the drum or belta brush of toner. By contact with the carrier each neutraltoner particle has an electric charge of polarity oppositeto the charge of the latent image on the drum. The chargeattracts toner to form a visible image on the drum. Tocontrol the amount of toner transferred, a bias voltage isapplied to the developer roller to counteract the attractionbetween toner and latent image.Where a negative image is required, as when printingfrom a microform negative, then the toner has the samepolarity as the corona in step 1. Electrostatic lines offorce drive the toner particles away from the latent imagetowards the uncharged area, which is the area exposedfrom the negative.Early color copiers and printers used multiple copy cy-cles for each page output, using colored filters and ton-ers. Modern units use only a single scan to four separate,miniature process units, operating simultaneously, eachwith 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 onthe toner. The toner image is transferred from the drumto the paper by a combination of pressure and electrostaticattraction. On many color and high-speed machines, it iscommon to replace the transfer corona with one or morecharged bias transfer rollers, which apply greater pressureand produce a higher quality image.

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Step 5. Separation or detack

Electric charges on the paper are partially neutralized byAC from a second corona, usually constructed in tandemwith the transfer corona and immediately after it. As aresult, the paper, complete with most (but not all) of thetoner image, is separated from the drum or belt surface.

Step 6. Fixing or fusing

The toner image is permanently fixed to the paper us-ing either a heat and pressure mechanism (hot roll fuser)or a radiant fusing technology (oven fuser) to melt andbond 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 gauzesprinkled with a volatile liquid, such as ether. When thetransferred image was brought into proximity with the va-por from the evaporating liquid, the result was a perfectlyfixed copy without any of the distortion or toner migrationwhich can occur with the other methods. This method isno longer used due to emissions of fumes.

Step 7. Cleaning

The drum, having already been partially discharged dur-ing detack, is further discharged by light. Any remain-ing toner, that did not transfer in step 6, is removed fromthe drum surface by a rotating brush under suction, ora squeegee known as the cleaning blade. This 'waste'toner usually is routed into a waste toner compartment forlater disposal; however, in some systems, it is routed backinto the developer unit for reuse. This process, known astoner reclaim, is much more economical, but can possi-bly lead to a reduced overall toner efficiency through aprocess known as 'toner polluting' whereby concentrationlevels of toner/developer having poor electrostatic prop-erties are permitted to build up in the developer unit, re-ducing the overall efficiency of the toner in the system.Some systems have abandoned the separate developer(carrier). These systems, known as monocomponent, op-erate as above, but use either a magnetic toner or fusibledeveloper. There is no need to replace worn-out de-veloper, as the user effectively replaces it along withthe toner. An alternative developing system, developedby KIP from an abandoned line of research by Xerox,completely replaces magnetic toner manipulation and thecleaning system, with a series of computer-controlled,varying biases. The toner is printed directly onto thedrum, by direct contact with a rubber developing rollerwhich, by reversing the bias, removes all the unwantedtoner and returns it to the developer unit for reuse.The development of xerography has led to new technolo-gies that has the potential to eventually eradicate tradi-tional offset printing machines. These new machines thatprint in full CMYK color, such as Xeikon, use xerographybut provide nearly the quality of traditional ink prints.

A mnemonic sentence that is helpful to remember thesteps sequence in the xerographic process is: Can I EricDo The Funky Chicken Dance? (Charging, Imaging, Ex-posing, Developing, Transferring, Fusing, Cleaning andDischarging)..

2 Durability

Xerographic documents (and the closely related laserprinter printouts) can have excellent archival durability,depending on the quality of the paper used. If low-qualitypaper is used, it can yellow and degrade due to residualacid in the untreated pulp; in the worst case, old copiescan literally crumble into small particles when handled.High-quality xerographic copies on acid-free paper canlast as long as typewritten or handwritten documents onthe same paper. However, xerographic copies are vul-nerable to undesirable toner transfer if they are stored indirect contact or close proximity to plasticizers, which arepresent in looseleaf binders made with PVC. In extremecases, the ink toner will stick directly to the binder cover,pulling away from the paper copy and rendering it illegi-ble.

3 Uses in animation

Ub Iwerks adapted xerography to eliminate the hand-inking stage in the animation process by printing the an-imator’s drawings directly to the cels. The first animatedfeature film to use this process wasOne Hundred and OneDalmatians (1961), although the technique was alreadytested in Sleeping Beauty (for the animation of the rocksfalling and turning into bubbles), released two years ear-lier. At first, only black lines were possible, but in the1980s, colored lines were introduced and used in ani-mated features like The Secret of NIMH.

3.1 Examples of feature films using the xe-rography process

• One Hundred and One Dalmatians (1961)

• The Sword in the Stone (1963)

• Mary Poppins (1964)

• The Jungle Book (1967)

• The Aristocats (1970)

• Bedknobs and Broomsticks (1971)

• Robin Hood (1973)

• The Twelve Tasks of Asterix (1976)

• The Many Adventures of Winnie the Pooh (1977)

4 5 EXTERNAL LINKS

• The Rescuers (1977)

• Pete’s Dragon (1977)

• The Ballade of the Daltons (1978)

• The Fox and the Hound (1981)

• The Secret of NIMH (1982)

• The Black Cauldron (1985)

• The Great Mouse Detective (1986)

• An American Tail (1986)

• Who Framed Roger Rabbit (1988)

• The Land Before Time (1988)

• Oliver & Company (1988)

• The Little Mermaid (1989)

• All Dogs Go to Heaven (1989)

4 References[1] http://www.merriam-webster.com/dictionary/

xerography

[2] Schiffer, Michael B.; Hollenback, Kacy L.; Bell, CarrieL. (2003). Draw the Lightning Down: Benjamin Franklinand Electrical Technology in the Age of Enlightenment.Berkeley: University of California Press. pp. 242–44.ISBN 0-520-23802-8.

[3] “Photocopying processes”. McGraw-Hill Encyclopedia ofScience and Technology vol. 13, p. 395, 10th edition,2007

[4] The Physics and Technology of Xerographic Processes,Edgar M. Williams, John Wiley and Sons (Wiley-Interscience), New York, 1984.

[5] Photocopying processes”. McGraw-Hill Encyclopedia ofScience and Technology vol. 13, p. 394, 10th edition,2007

• Owen, David (2004). Copies in Seconds: Howa Lone Inventor and an Unknown Company Cre-ated the Biggest Communication Breakthrough SinceGutenberg. New York: Simon & Schuster. ISBN0-7432-5117-2.

• Schein, L. B. (1988). Electrophotography and De-velopment Physics. Springer Series in Electrophysics14. Berlin: Springer-Verlag.

5 External links• “Static Pops Pictures On Paper” detailed 1949Popular Science article on the history and technol-ogy of xerography

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6 Text and image sources, contributors, and licenses

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