A computer-driven, plain-paper output device used to generate proofs, masters, or general on-demand output. Most laser printers can output text, photographs, halftones, or other images. Laser printers are increasingly the standard output device attached to home and office computers, having replaced in most cases the old dot-matrix printer, especially as laser printers have come down in price and increased in quality.
A laser printer works generally on the same principle as a photocopier, imaging on paper by means of electrophotography. A laser printer contains an electrically-charged, light-sensitive metal drum or cylinder. Controlled by information received from the printer driver installed in the computer, the laser exposes the charged drum in those regions corresponding to image areas, forming an image out of many tiny, virtually microscopic dots (more correctly called spots, especially to distinguish them from halftone dots). The areas of drum exposed by the laser become oppositely-charged from the rest of the drum, and will accept oppositely-charged particles of toner, a fine, black or colored powder. The toner adheres to the drum in those regions exposed by the laser, and is transferred to the substrate passing beneath it. Heat and pressure are then used to fuse or "fix" the toner to the substrate, preventing it from being easily rubbed off.
One of the drawbacks of most laser printers (as of this writing) is that their maximum resolution is 600 dpi, which is adequate for most text and line-art output, but inadequate for halftone output, which usually need—depending upon the screen count—around 1200:1500 dpi. Color laser printers are also increasing in popularity and coming down in price. Laser printers are primarily used to generate galleys, proofs, and general output, while high-end imagesetters are typically used to generate high-quality camera-ready output or film.