Kodak Magnus 800 Platesetter, at printing press from Iropa in Saint-√Čtienne-du-Rouvray.

A platesetter is a machine which receives a raster image from a raster image processor and in turn, creates a lithographic plate suitable for use on an offset press.

For the most part, there are three types of Computer to plate: Capstan, External Drum and Internal Drum. Capstan type platesetters feed a roll of plate material through the imaging area at a set, continuous speed. After exposure, the material is cut and the plate is ready for the press. Capstan devices are generally considered the most affordable, but least accurate method of producing plates. External drum platesetters work by having the plate affixed to the outside of a rotating drum. As the drum rotates, the imaging unit moves parallel along the length of the drum, imaging as it moves. An Internal drum platesetter works by having a plate set inside of the drum. The imaging unit, or a spinning mirror, traverses the center of the drum, imaging as it moves.

Plates are often hole punched by the platesetter. External drum units typically punch a plate then load it onto the drum, using the punches to ensure registration on the drum. Internal drum machines will load the plate, punch and image the plate, then release the punches, also to ensure registration. Capstan devices lack the punching mechanism so are considered less accurate in positioning the image on the plate. Capstan devices are most often used for 1 or 2 color printing where their positional inaccuracies are less important, or in newspaper work where their higher speed is an asset.

The imaging of substrate on the platesetter is accomplished by laser. Of the many types of lasers employed in this process they come into two categories. Thermal Laser - the thermal laser images the plate by the heat generated by the laser on the plate. Visible Light Laser - the visible laser images the plate photographically by exposing the plate with light generated by the laser. Of these two types, thermal is the better and more widely used type of imaging for modern platesetters.

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