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The imagesetter has been largely superseded by the platesetter

Platesetter

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....

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Imagesetter film is a silver halide

Silver halide

A silver halide is one of the compounds formed between silver and one of the halogens — silver bromide , chloride , iodide , and three forms of silver fluorides. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX...

-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

Safelight

A safelight is a light source suitable for use in a photographic darkroom. It provides illumination only from parts of the visible spectrum to which the photographic material in use is nearly or completely insensitive.- Design :...

, 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

Artist's proof

An artist's proof is, at least in theory, an impression of a print taken in the printmaking process to see the current printing state of a plate while the plate is being worked on by the artist...

 documents before expensive lithography plates are made. Proofing on bromide paper is much easier than film, since it doesn't require a light table

Light table

A light table is a viewing device that is used to review photographic film or artwork placed on top of it. It provides even illumination of the subject from below through a translucent cover and fluorescent lights that emit little heat. They can also be found mounted on the walls of hospitals and...

 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.