Diagram of a laser printer The sound of a laser printer A laser beam typically, an aluminium gallium arsenide AlGaAs semiconductor laser projects an image of the page to be printed onto an electrically charged, selenium -coated, rotating, cylindrical drum  or, more commonly in subsequent versions, a drum called an organic photoconductor made of N-vinylcarbazolean organic monomer. Photoconductivity allows the charged electrons to fall away from the areas exposed to light. Powdered ink toner particles are then electrostatically attracted to the charged areas of the drum that have not been laser-beamed.
Diagram of a laser printer The sound of a laser printer A laser beam typically, an aluminium gallium arsenide AlGaAs semiconductor laser projects an image of the page to be printed onto an electrically charged, selenium -coated, rotating, cylindrical drum  or, more commonly in subsequent versions, a drum called an organic photoconductor made of N-vinylcarbazolean organic monomer.
Photoconductivity allows the charged electrons to fall away from the areas exposed to light. Powdered ink toner particles are then electrostatically attracted to the charged areas of the drum that have not been laser-beamed.
The drum then transfers the image onto paper which is passed through the machine by direct contact. There are typically seven steps involved in the process: Each horizontal strip of dots across the page is known as a raster line or scan line.
Laser printing differs from other printing technologies in that each page is always rendered in a single continuous process without any pausing in the middle, while other technologies like inkjet can pause every few lines.
To avoid a buffer underrun where the laser reaches a point on the page before it has the dots to draw therea laser printer typically needs enough raster memory to hold the bitmap image of an entire page. Memory requirements increase with the square of the dots per inchso dpi requires a minimum of 4 megabytes for monochrome, and 16 megabytes for color still at dpi.
At dpi, there are 90, dots per square inch dots per linear inch. In a color printer, each of the four CMYK toner layers is stored as a separate bitmap, and all four layers are typically preprocessed before printing begins, so a minimum of 4 megabytes is needed for a full-color letter-size page at dpi.
During the s, memory chips were still very expensive, which is why entry-level laser printers in that era always came with four-digit suggested retail prices in US dollars. Memory prices later plunged, and dpi printers have been widely available in the consumer market since Charging[ edit ] Applying a negative charge to the photosensitive drum In older printers, a corona wire positioned parallel to the drum or, in more recent printers, a primary charge roller, projects an electrostatic charge onto the photoreceptor otherwise named the photo conductor unita revolving photosensitive drum or belt, which is capable of holding an electrostatic charge on its surface while it is in the dark.
An AC bias voltage is applied to the primary charge roller to remove any residual charges left by previous images.
The roller will also apply a DC bias on the drum surface to ensure a uniform negative potential. Numerous patents[ specify ] describe the photosensitive drum coating as a silicon sandwich with a photocharging layer, a charge leakage barrier layer, as well as a surface layer.
One version[ specify ] uses amorphous silicon containing hydrogen as the light receiving layer, Boron nitride as a charge leakage barrier layer, as well as a surface layer of doped siliconnotably silicon with oxygen or nitrogen which at sufficient concentration resembles machining silicon nitride.
Laser light selectively neutralizes the negative charge on the photoreceptive drum, to form an electrostatic image Laser unit from a Dell P The white hexagon is the rotating scanner mirror.
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.
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. Developing[ edit ] The surface with the latent image is exposed to toner which has been applied in a micron-thick layer to the developer roll.
Toner consists of fine particles of dry plastic powder mixed with carbon black or coloring agents. Because negative charges repel each other, the negatively charged toner particles will not adhere to the drum where the negative charge imparted previously by the charge roller remains.
Transferring[ edit ] A sheet of paper is then rolled under the photoreceptor drum, which has been coated with a pattern of toner particles in the exact places where the laser struck it moments before.
Some machines also use a positively charged "transfer roller" on the back side of the paper to help pull the negatively charged toner from the photoreceptor drum to the paper. One roller is usually a hollow tube heat roller and the other is a rubber backed roller pressure roller. A radiant heat lamp is suspended in the centre of the hollow tube, and its infrared energy uniformly heats the roller from the inside.
For proper bonding of the toner, the fuser roller must be uniformly hot. Some printers use a very thin flexible metal foil roller, so there is less thermal mass to be heated and the fuser can more quickly reach operating temperature.
If paper moves through the fuser more slowly, there is more roller contact time for the toner to melt, and the fuser can operate at a lower temperature. Smaller, inexpensive laser printers typically print slowly, due to this energy-saving design, compared to large high speed printers where paper moves more rapidly through a high-temperature fuser with a very short contact time.
Cleaning and recharging[ edit ] Magnification of color laser printer output, showing individual toner particles comprising 4 dots of an image with a bluish background As the drum completes a revolution, it is exposed to an electrically neutral soft plastic blade which cleans any remaining toner from the photoreceptor drum and deposits it into a waste reservoir.
A charge roller then re-establishes a uniform negative charge on the surface of the now clean drum, readying it to be struck again by the laser. Continuous printing[ edit ] Once the raster image generation is complete, all steps of the printing process can occur one after the other in rapid succession.
This permits the use of a very small and compact unit, where the photoreceptor is charged, rotates a few degrees and is scanned, rotates a few more degrees and is developed, and so forth.
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