Xerography

Xerography (or electrophotography) is a photocopying technique invented by Jean-Jacques Trillat in 1935 but independently rediscovered by Chester Carlson in 1938 and patented on October 6, 1942. Chester received U.S. patent 2,297,691 for his invention. Although dry electrostatic printing processes had been invented as far back as 1778 by Georg Christoph Lichtenberg, Carlson's innovation combined electrostatic printing with photography. The original 1942 process was cumbersome and used manual processing steps with flat plates. It was almost 18 years before a fully automated process was developed, the key breakthrough being use of a drum coated with selenium instead of a flat plate, resulting in the first commercial automatic copier, released by Haloid/Xerox in 1960.

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

The Xerographic Process

The first commercial use was hand processing of a flat photosensor 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 such as Xerox iGen3 and Xeikon 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.

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.

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 photo conductor or OPC), applied by vacuum deposition. Amorphous selenium will hold an electrostatic charge in darkness and will conduct away such a charge under light.

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, as well as a surface layer of silicon doped with oxygen or nitrogen, silicon nitride is a scuff resistant material

The drum rotates at the speed of paper output. One revolution passes the drum surface through the steps described below.

In place of the drum there may be a belt.

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. This can also be achieved with the use of a contact roller with a charge applied to it. The polarity is chosen to suit the photosensor; amorphous selenium requires a positive charge while others require a negative charge.

Step 2. Exposure The document or microform to be copied is illuminated and passed over a lens, so that its image is projected onto the drum moving, exactly with the turning drum surface. 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 positive of the original document.

In a laser or LED printer, modulated light is projected onto the drum surface to create a latent image.

Step 3. Development The drum is presented with a slowly turbulent mixture of toner particles and larger, metallic, carrier particles. The mix is manipulated with a magnet to present to the surface a brush of toner. By contact with the carrier each toner particle has an electric charge of polarity opposite to the charge of the latent image on the drum. The charge attracts toner to form a visible image on the drum.

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.

Color copiers and printers provide multiple copy cycles for each page output, using colored toners.

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 by electrostatic attraction, from the drum to the paper.

Step 5. Separation Electric charges on the paper are partially neutralized by the detack saw. As a result, the paper 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 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 waste 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, 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.

Xerography 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 feature animated film to use this process was One Hundred and One Dalmatians (1961). At first only black lines were possible, but in the 1980s, colored lines were introduced and used in animated features like The Secret of NIMH.

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)

External links

• Encyclopedia.com's Xerography Definition