LED (light-emitting diode) page printing – invented by Casio, championed by Oki and also used by Lexmark – was touted as the next big thing in laser printing in the mid-1990s. However, five years on – notwithstanding its environmental friendliness – the technology had yet to make a significant impact in the market.
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 the printer – 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.