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The technology produces the same results as conventional laser printing and uses the same fundamental method of applying toner to the paper. A static charge is applied to a photo-receptive drum and, when the light from the LED hits it, the charge is reversed, creating a pattern of dots that corresponds to the image that will eventually appear on the page. After this, electrically charged dry toner is applied, which sticks to the areas of the drum that have had their charge reversed, and then applied to the paper as it passes past the drum on its way to the output tray. The difference between the two technologies lies in the method of light distribution.
LED printers function by means of an array of LEDs built into the cover of theprinter - usually more than 2,500 covering the entire width of the drum - which create an image when shining down at 90 degrees. A 600dpi LED printer will have 600 LEDs per inch, over the required page width. The advantage is that a row of LEDs is cheaper to make than a laser and mirror with lots of moving parts and, consequently, the technology presents a cheaper alternative to conventional laser printers. The LED system also has the benefit of being compact in relation to conventional lasers. Colour devices have four rows of LEDs - one each for cyan, magenta, yellow and black toners - allowing colour prints speeds the same as those for monochrome units.
The principal disadvantage of LED technology is that the horizontal resolution is absolutely fixed, and while some resolution enhancements can be applied, none of them will be as good as the possible resolution upgrades offered by true lasers. Moreover, an LED printer's drum performs at its best in terms of efficiency and speed when continuous, high-volume printing is called for. In much the same was as a light bulb will last less long the more it is switched on and off, so an LED printer's drum lifetime is shortened when used often for small print runs. LCD printers work on a similar principle, using a liquid crystal panel as a light source in place of a matrix of LEDs.
Lower resolution models can often vary the intensity of their laser/LED spots, but deliver coarser multi-level toner dots resulting in mixed "contone" and halftone printing, rather than continuous tone output. Rated print speeds vary between 3 and 5ppm in colour and 12 to 14ppm in monochrome. A key area of development, pioneered by Lexmark's 12ppm LED printer launched in the autumn of 1998, is to boost colour print speed up to the same level as mono with simultaneous processing of the four toners and one-pass printing.
The Lexmark Optra Colour 1200N achieves this by having completely separateprocesses for each colour. The compactness which results from use of LED arrays instead of the bulky focusing paraphernalia associated with a laser imaging unit allows the colour engine to be built with four complete printheads arranged. The CMY and K toner cartridges are laid out in-line down the paper path and each unit has its own photo-conductive drum. Above each unit in the printer's lid are four LED arrays - again, one for each colour. Data can be sent to all four heads simultaneously. The process starts with magenta and passes through cyan and yellow, with black laid down last.
Apart from their speed, one of the main advantages of colour lasers is thedurability of their output - a function of the chemically inert toners that are fused onto the paper's surface rather than absorbed into it, as with most inkjets. This allows colour lasers to print well on a variety of media, without the problems of smudging and fading that beset many inkjets. Furthermore, by controlling the amount of heat and pressure in the fusing process, output can be given a user-controllable "finish", from matte through to gloss.
If history is anything to go by, the future for laser and LED colour printing looks bright. Within four years of the first appearance of colour lasers in 1994 prices approximately halved. With the market continuing to be stimulated, both by falling prices and improved technology, it looks inevitable that the laser or LED colour laser will become as commonplace and as indispensable as the photocopier.
copyright 2002 PC Tech Guide