At present, laser printers are among the most advanced. As early as 1938 Chester Carlson invented the printing method called electrography. Nobody saw a dot-matrix printer at that time.
In 1969 an employee of the XEROX company realised that the idea of copying devices may well be applied to printers. Thanks to that idea the first laser printer was produced already in 1971 but not in mass production. Only in 1977 the company has released the Xerox 9700 Electronic Printing System. This was occasionally challenged by IBM claiming that the laser printer IBM 3800 created in 1975 produced pages with the text following exactly the technology of laser printers.
Laser printer technology
The technology is as follows. A negative charge is put on the drum with photosensitive coating. Then a laser beam removes a portion of the charge where you need something printed by passing through the photoconductor drum. Toner is the consumable material of a laser printer. Toner consists of a mixture of different polymers, metal shavings, resins, coal dust and other chemicals. Toner itself also has a negative charge and thus it sticks to the point where the laser passes to remove the charge because opposite charges always attract each other. While continuing to rotate the drum, onto which the toner image has been applied, touches the paper. On the reverse side the same paper touches the transfer roller which carries a positive charge. The negative toner particles are attracted to the paper, and the image shows up. Then the paper goes into the fuser assembly, the toner is melted under the influence of the heating roller and baked to paper.
Cleaning the drum is the next step. A certain amount of toner is still not transferred to the paper but remains on the drum because the drum needs to be cleaned. A special blade directly clears away all the remaining toner on the drum and sends it in a tray. Erasing the image is the last stage. The charging roller causes an evenly negative charge on the surface of the drum and restores the charge in those places where this charge was dropped under the influence of the laser.
Colour printing
Colour and black and white laser printers are almost identical. The difference lies in the fact that, for colour printing uses four types of toner, black, cyan, magenta and yellow according to the CMYK colour model. Each colour contributes to the final image applied on a sheet of paper. In some models of colour laser printers, a sheet of paper passes successively through all colour and black cartridges where each colour has its own laser, a drum and a toner cartridge (single-pass printing). In the less expensive printers an intermediate carrier (transfer belt) is used, which has consistently applied the image of all four colours, and only then it is transferred onto paper and into the fuse assembly for fixing toner to the paper (multi-pass printing).
The idea behind colour laser printing is as follows. In the beginning of printing the rendering engine will process a digital document once or several times thus providing a bitmap image. At the second stage a laser or a group of LEDs makes a charge on the surface of the rotating photoconductor drum corresponding to the received image. The toner fine particles charged by the laser consist of the colouring pigment, resins and polymers attracted to the drum surface. Then paper is rolled through the drum, and toner is transferred onto it. Most colour laser printers use four separate passes, corresponding to different colours. Then the paper passes through the fuse assembly where the resin and polymers are melted in toner and fixed on paper which results in the final image.
Lasers can focus very precisely resulting in incredibly thin beams which charge the photoconductor drum parts. Thanks to the contemporary laser printers, both colour and black-and-white support a very high enough resolution. As a rule, the resolution in black and white ranges from 600 x 600 up to 1200 x 1200, and the colour resolution up to 9600 x 1200.
Compared to inkjet printers, the laser ones have many advantages. First of all, they have a higher rate, because the laser beam can move much faster than the print head with dozens and even hundreds of nozzles spraying microscopic ink droplets at the time of printing with certain intervals. Laser beams are also more accurate and it is possible to obtain higher resolution thanks to more compact focusing. Laser printers are more economical than the jet ones because the toner cartridges are usually enough for over one thousand pages but the ink cartridges run out quickly and need to be replaced more frequently. A disadvantage of laser printers is the fact that they are more expensive, and toner cartridges usually cost between $ 60 and $ 100 each. However if there is a need to print a large number of sheets, then laser printers will cope with this task faster than inkjets, and the cost of printing a page becomes lower.
Pros and cons of black-and-white laser printers
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ideally suitable for text
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high-speed printing
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designed for high-load printing
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low cost per print
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not suitable for images
Pros and cons of colour laser printers
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possible printing of documents and pages which contain images and colour diagrams
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high print speed compared to a colour inkjet printer
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high price
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large size
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