Xerography
From Wikipedia, the free encyclopedia
Xerography (or electrophotography) is a photocopying technique developed by Chester Carlson in 1938 and patented on October 6, 1942. He received U.S. Patent 2,297,691 for his invention. The name xerography came from the Greek radicals xeros (dry) and graphos (writing), because there are no liquid chemicals involved in the process, unlike earlier reproduction techniques like cyanotype.
Xerography is used in most photocopying machines and in laser and LED printers.
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[edit] The Xerographic Process
The most common realization is an office photocopier, but there are many digital printers which use the process. The steps are described here.
A metal cylinder is mounted to rotate about a horizontal axis. This is called the drum. The end to end dimension is the width of print to be produced plus a generous tolerance. The drum is manufactured with a surface coating of amorphous selenium (more recently ceramic or organic), applied by vacuum deposition. Amorphous selenium will hold an electrostatic charge in darkness and will conduct away such a charge under light.
The drum rotates at the speed of paper output. One revolution passes the drum surface through the steps described below. The drum may be a belt, and there are variants at every step.
Step 1. Charging An electrostatic charge is uniformly distributed over the surface of the drum by a corona discharge with output limited by a grid. There are several methods for depositing charge on the surface of a drum (or belt) photoreceptor; these include: bias charging rolls, corotron, scorotron ("screened" corotron), etc..
Step 2. Exposure The document to be copied is illuminated and passed over a lens, so that its image is projected onto the drum at exactly the same speed that the drum turns. In a laser or LED printer, a thin beam of light is modulated by the controller. Where there is text or image on the document, the light will not penetrate to the drum; the opposite is true where there is no image. The light that penetrates releases the charge on the drum, but only where there is no image. The resulting charge that remains on the drum is called the 'latent' image and is a positive of the original document.
Step 3. Development During development, the drum passes over a 'toner fountain' where toner particles are attracted to the electrostatic latent image on the drum surface. Thus, a visible toner image is created. Please note that toner is actually a particle of carbon having no electrostatic property of its own. In order to render a charge it must be mixed with a component known as carrier which is, essentially, iron filings. The mixture of toner and carrier together is called developer. The iron filings have an uneven surface to which the toner can adhere itself. The type of charge created when developer is mixed is known as a triboelectric charge. As is evident in any heavily charged atmosphere, there will be errant particles that will adhere themselves to the drum at undesired locations. This can result in poor copy quality or 'dirty' copies.
Step 4. Transfer Paper is passed between the drum and the transfer corona. Because oppositely charged particles attract, the toner image is transferred from the drum or belt surface to the paper once the transfer corona applies its opposing charge.
Step 5. Peeling Electric charges on the paper are partially neutralized by the detack saw. As a result, the paper is peeled off 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 or a radiant fusing technology to melt and bond the toner particles to the medium (usually paper) being printed on.
Step 7. Cleaning The drum is discharged and any remaining toner that did not transfer in Step 6 is removed from the drum surface by a rotating brush or a wiper blade under suction. In most cases, this 'waste' toner is routed into a special toner compartment for later disposal; however, in some systems, it is routed back into the main toner compartment for reuse. This process 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 fresh toner compartment, further reducing the overall efficiency of the toner in the system.
The development of xerography has led to new technologies that some predict will 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.
[edit] Xerography in animation
Ub Iwerks managed to adapt xerography to eliminate the hand-inking stage in the animation process by printing the animators drawings directly to the cels. At first only black lines were possible, but in the 80's lines in different colors were introduced and used in animated features like The Secret of NIMH.
[edit] External links
- Xerography - Design Dictionary Photocopying technique called xerography.
- Encyclopedia.com's Xerography Definition
- Xerography, The invention that no one ever wanted
[edit] Further reading
"Copies in Seconds: How a Lone Inventor and an Unknown Company Created the Biggest Communication Breakthrough Since Gutenberg - Chester Carlson and the Birth of the Xerox", by David Owen
L.B. Schein, Electrophotography and Development Physics, Springer Series in Electrophysics, Volume 14, (Springer-Verlag, Berlin 1988)
