Photocopying - Xerography, Addition Of Toner And Fusing, Color Copying, Electrostatic Copying, Thermography, Diazo Copying
carlson invention process
Photocopying is the process by which light is used to make copies of book pages and other paper documents. Today the most widely used form of photocopying is xerography ("dry writing"), invented by New York patent attorney Chester Carlson in the 1930s. Indeed, the name of the company founded to develop Carlson's invention, Xerox Corporation, has become synonymous with the process of photocopying. However, a number of other forms of photocopying pre-dated the Carlson invention and are still used for special applications. Among these other forms of photocopying are thermography, diazo processes, and electrostatic copying.
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Many different models of xerographic copying machines are available today, but they all operate on some common principles. The core of such machines is a photoconducting surface to which is added a negative charge of about 600 volts. The surface could be a selenium-coated drum or an endless moving belt mounted on rollers, for example. The charged placed on the photo-conducting surface is usually o…
The next step in copying involves the addition of a toner to the photoconducting surface. A toner is a positively charged material that is added to the photoconducting surface. Since it carries an electrical charge opposite that of the negatively-charged photoconducting surface, the toner sticks to the surface. The photoconducting surface now carries toner on its surface that matches regions of ne…
A process somewhat similar to that used in xerography is electrostatic copying. The major difference between these two processes is that in electrostatic copying, the endless photoconducting surface is omitted from the machine and the copy paper is specially treated to pick up the toner. The paper used in electrostatic copying is treated with a material consisting of zinc oxide combined with a the…
Thermography ("heat writing") is a method of copying that is based on the fact that dark regions of a document absorb heat more readily than do light spaces. If heat is applied to this page, for example, it will be absorbed more readily by the letters on the page than by the white spaces between the letters. As with electrostatic copying, themographic copying requires the use of spec…
Diazo copying gets its name from the fact that it makes use of copy paper that has been treated with a type of chemical known as diazonium compounds. As with the thermographic process described above, diazonium compounds change color when exposed to heat. In diazo copying, the original document and the diazotreated copy paper are placed in contact with each other in a light box and then exposed to…
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