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A device used for the digital imaging of offset printing plates. The internal drum platesetter resembles the letter "C" because of its open concave design. The plate is mounted on the inside of the drum and is held in place with a vacuum that draws the plate tight against the curved surface. A single laser beam writes the digital image onto the plate. The laser beam is projected onto a spinning mirror that rotates at a very high speed. The spinning mirror deflects the laser beam onto the plate at a 90° angle. As the mirror travels down the axis of the drum, one scanned line per revolution of the mirror is written onto the plate. The concave drum remains stationary. In order to change the resolution of the image on the plate, the diameter of the laser beam is changed. When imaging thermal plates on an internal drum platesetter, there may be limits on how fast the mirror can spin when deflecting the laser light onto the plate. This is because the slower emulsion of the thermal plates requires high energy and a long imaging duration, which means that the production rate will be slower compared to that of other types of plates.
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