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An imagesetter is an ultra-high resolution large-format computer output device. It exposes rolls or sheets of either photographic film or bromide paper to a laser light source. Once the film or paper is developed, a very high quality black and white image is revealed. Development (processing) usually occurs in a unit separate to the imagesetter, as does raster image processing.
Imagesetter setter output ranges in width; usually between 12 and 44 inches. The resolution of an imagesetter is typically between 1200 and 4800 dpi.
The imagesetter has been largely superseded by the platesetter.
Imagesetter film is a silver halide-coated plastic film very similar to normal black & white photographic film, except the spectral sensitivity is reduced to a much narrower band around the output of the laser of the individual imagesetter. This allows the film to be handled under a (usually red) safelight, instead of in total darkness like most photographic film.
Output on bromide paper 
The use of bromide paper instead of film in an imagesetter allows it to produce paper output called bromides. Bromides are similar to ordinary laser prints but with four major differences.
- The output can be much bigger
- The output is of much higher resolution
- The black is darker, and is of much more consistent color
- The cost is approximately 100 times that of an equivalent laser print
The main use of bromides is for proofing documents before expensive lithography plates are made. Proofing on bromide paper is much easier than film, since it doesn't require a light table or other backlighting in order to read. Because of the uptake of platesetters, this function of imagesetters in modern printing houses has been reduced, although not completely eliminated, with proofs made on special proofing printers instead.
Bromide paper is very similar to black & white photographic paper, except the backing is much thinner, and like imagesetter film is of diminished spectral sensitivity.