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offset lithography[′ȯf‚set li′thäg·rə·fē]
A system of printing that depends on the principle that the printing area accepts greasy ink while the nonprinting area is dampened with water and repels the ink; in practice, the image from a plate is offset onto the rubber blanket of an impression cylinder, and transferred to a sheet of paper.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
offset pressA printer that uses an intermediate rubber-coated cylinder known as a "blanket" to transfer the image onto the paper. Instead of transferring the image from a metallic drum onto paper as is done with most digital printers, the image is "offset" onto the blanket and then to paper. The blanket creates a smoother image on most types of paper and can print on rough or heavy stock as well as other media. Most offset presses use this lithographic method.
Sheet Fed and Web Fed
Sheet-fed offset printers use cut sheets of paper, whereas web-fed printers use rolls of paper that are cut and trimmed after printing. The web-based offset method is typically used for high-speed, high-volume applications such as printing newspapers and magazines.
Right Reading to Wrong Reading
Since offset presses use the intermediate blanket as the transfer mechanism, the original negative image on the drum is right-reading. It becomes wrong-reading on the blanket and then back to the original on the paper.
Combination Offset and Laser
Indigo USA, Woburn, MA (www.indigonet.com), which was acquired by HP in 2002, combines laser printing and offset printing. It makes a "digital offset color press," which is an electrophotographic printer that offsets to a blanket instead of imaging directly from the drum. It also uses a special liquid toner instead of dry toner, enabling it to print on polyester, PVC, films and other media as well as coated and uncoated paper. See lithography, digital printing, right-reading and DI press.
|Offset and Digital, Too|
|HP Indigo's "digital offset color presses" are digital printers that use a blanket to image the pages instead of directly from the drum. Combined with its Electroink dry toner, Indigo printers can print on a wide variety of substrates. (Image courtesy of Hewlett-Packard Company.)|
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