page_title: Laser printing
section_title: Exposing
hierarchical_section_title: Laser printing / Printing process / Exposing
caption_attribution_description: English: Laser unit of a 2003 Dell P1500 laser printer. Visible are the rotating 6 sided mirror with the high speed BLDC motor on the lower left; the laser unit with feedback sensor on the right side; and the lenses and mirrors to guide the laser light to the photosensitive drum.
context_page_description: Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics by repeatedly passing a laser beam back and forth over a negatively charged cylinder called a "drum" to define a differentially charged image. The drum then selectively collects electrically charged powdered ink, and transfers the image to paper, which is then heated in order to permanently fuse the text, imagery, or both, to the paper. As with digital photocopiers, laser printers employ a xerographic printing process. Laser printing differs from traditional xerography as implemented in analog photocopiers in that in the latter, the image is formed by reflecting light off an existing document onto the exposed drum.
  Invented at Xerox PARC in the 1970s, laser printers were introduced for the office and then home markets in subsequent years by IBM, Canon, Xerox, Apple, Hewlett-Packard and many others. Over the decades, quality and speed have increased as the price has fallen, and the once cutting-edge printing devices are now ubiquitous.
context_section_description: A laser printer uses a laser because lasers are able to form highly focused, precise, and intense beams of light, especially over the short distances inside of a printer.  The laser is aimed at a rotating polygonal mirror which directs the light beam through a system of lenses and mirrors onto the photoreceptor drum, writing pixels at rates up to sixty-five million times per second. The drum continues to rotate during the sweep, and the angle of sweep is canted very slightly to compensate for this motion. The stream of rasterized data held in the printer's memory rapidly turns the laser on and off as it sweeps.
  The laser beam neutralizes (or reverses) the charge on the surface of the drum, leaving a static electric negative image on the drum's surface which will repel the negatively charged toner particles. The areas on the drum which were struck by the laser, however, momentarily have no charge, and the toner being pressed against the drum by the toner-coated developer roll in the next step moves from the roll's rubber surface to the charged portions of the surface of the drum.
  Some non-laser printers (LED printers) use an array of light-emitting diodes spanning the width of the page to generate an image, rather than using a laser. "Exposing" is also known as "writing" in some documentation.
caption: Laser unit from a Dell P1500. The white hexagon is the rotating scanner mirror.
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