A photoplotter is an electro-mechanical-optical machine that produces a latent image on a media, usually high-contrast monochromatic (black-and-white) photographic film, using a light source under computer control.
Photoplots are used primarily in the electronics industry for the production of printed circuit boards. Other application areas include chemical milling, and some specialized graphic arts.
The first photoplotter was introduced by the Gerber Scientific Company (now Barco ETS) in the 1960s. Early machines used a xenon flash lamp, and projected an image mounted in a rotating aperture wheel onto the photosensitive surface of the film or glass plate. The imaging head assembly traversed over the surface of the media (without touching it) to produce draws and flashes. Draws are vectors or arcs created by using effectively continuous illumination as the imaging head moves, and flashes are like rubber stamping a simple graphic in a location.
Mostly any conceivable image can be formed by combining draws and flashes of simple round and square shapes of various sizes.
Modern photoplotters are generally raster-scan devices that use a laser beam focussed to one or more spots, and modulated at multi-megahertz rates, to form the image. In years past, green argon-ion lasers were frequently used, along with red helium-neon and blue helium-cadmium lasers. More recent models utilize red laser diodes. One company uses red LEDs (light-emitting diodes). In the next few years, models using violet laser diode light sources are expected.
Photoplotters are closely related to imagesetters. Some photoplotters differ from their imagesetting counterparts in only a few details, primarily the type of controller used to produce the image, and attention to the resolution and absolute accuracy of the image, with photoplotters designed to meet more stringent specifications than imagesetters.
Once the imaging step is complete, the media is processed in a film processor using a developer solution, along with fixing, washing, and drying.
The most recent development related to photoplotting is LDI (Laser Direct Imaging) which utilizes a high-power laser to directly expose photoresist on a coated substrate instead of exposing photographic film. This eliminates the handling of photographic film. LDI machines currently sell for prices in the one-half million U.S. dollar range.
Photoplotting is the first step of making photolithography masks for printed circuit boards. In the PCB industry, these masks are called photoplots and are generally limited to features of 20 µm or more. Integrated circuits are made in a similar fashion utilizing photomasks with sub-micrometer feature sizes. The phototmasks were traditionally made by photoreducing photoplotter output.
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