from the group: Electrophotographic
Electrophotography and Digital Press
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Also referred to as laser print, photocopy, digital offset press, xerox, indigo print Common use: 1960-Present Process of the Technology Electrophotography is a process that manipulates electrical charges and light to fuse toner particles onto a substrate in the form of an image or text via a photoreceptor drum, or a metal roller coated in material that conducts light. The most common electrophotographic machines are copiers, laser printers and digital offset presses. Electrophotographic copiers can be subdivided into dry toner and liquid toner systems. Dry toner systems utilize a light-sensitive electronic chip or computer to first expose an image onto the photoreceptor drum, which attracts or repels toner particles as it rolls. Simultaneously, the surface of paper passing below receives the toner, followed by a pass through heated pressure rollers that fix the toner in place. Dry toner is mostly composed of dusty pigments and resin. Liquid toner copiers, known today as digital offset presses, are systems that merge traditional offset lithography with the dry toner technology described above. Digital presses proceed as if they were dry toner systems until the toner, which is liquid instead of dry, is offset to another roller where it is heated until the toner solvent evaporates and the resinous colorants melt. Finally, this layer is adhered to the paper surface in the form of an image or text. Image Structure Image structure in electrophotographic prints is dependent on two variables, printer design (analog or digital) and toner type (dry or liquid). Prints produced by an analog machine have no inherent dot pattern, however, a weak pattern from a copied original may be present. Mid-tone shadow areas created during the copying process, such as those formed by the gutter of a book make for the best examination areas, as they contain no remnant screen patterns. Conversely, digitally produced prints, such as those created by laser printers, have a clearly visible dot pattern. Machines using dry toner will produce images made up of visible particulates, typically with a speckle appearance in white areas. These features that will be absent from those prints produced with liquid toner. |
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Analog dry toner (click to enlarge) |
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Digital dry toner (click to enlarge) |
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Digital dry toner 2010 (click to enlarge) |
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Digital liquid toner photo book 2006 (click to enlarge) |
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Halftone Screen
The placement of toner reflects a pattern of halftone dots in the shape of a rosette. Other patterns that resemble a linear or herringbone effect are possible. Irregular dot placement that reflects no distinguishable order or sequence is also common.
Differential Gloss
This effect is a difference in gloss between the printed and non-printed areas of an image. Less differential gloss is observed in liquid toner prints due to the more consistent surface texture and low gloss of the toner.
Powdery/Dusty Appearance
This effect applies to dry toner electrophotographic prints because of the particulate nature of the dry colorants as opposed to the smoother resinous appearance of the liquid toner material.
Image Relief
When viewed in specular light, an electrophotographic print may exhibit relief in areas of density due to the build up of toner on the surface.