Laser and LED printers use a fairly simple principle to produce printed pages. They all involve a drum coated in a photo-conductive medium â€“ which can be charged up to a high electrostatic voltage â€“ a source of intense light and a heater to fuse the toner to the paper.
When you print a page on either of these types of printer, the page image is built up within the printerâ€™s memory. At the same time the printer's drum rotates and is charged up. The laser beam or array of high-intensity LEDs then plays on the charged drum and because of its photo-conductive nature, the charge is removed wherever the light touches it.
The toner, a very fine powder, where each particle is made up of plastic and black pigment (three primary colour pigments for colour lasers) is attracted to the areas of the drum which still retain their electrostatic charge. This coats the drum with an image of the page being printed.
The paper is then fed through very close to the drum and itself receives a high electrostatic charge, higher than that on the drum. The toner transfers from the drum to the paper, reproducing the image where itâ€™s wanted. At the moment, though, the toner is just sitting on the paper â€“ you may have experienced this if you've had to clear a paper jam and you find the toner comes off on your fingers. The page hasn't yet been through the fuser.
The fuser briefly heats the toner and paper to a high temperature. It's long enough to melt the plastic toner, so it bonds with the nap of the paper. This is why pages coming straight out of a laser printer are warm to the touch.
The final stage in the process is to clean the excess toner from the drum so that the whole process can repeat. Modern laser printers don't have drums big enough to hold an entire page image, so they build up sections of the image continuously, as the drum rotates. Laser printers have fast microprocessors inside to handle the data transfer and page creation which has to be very quick when theyâ€™re printing at up to 20 pages per minute.
LED printers, pioneered by OKI, use a long strip array of high-intensity LEDs to replace the laser beam. The array has the same function in the print process in reducing the static charge on the drum in areas where the final image requires no toner. While the mechanics of an LED printer are much simpler than for a laser, which requires mirrors to direct the light across the drum, the potential for an individual element to fail in an array which has 600 LEDs per inch is that much greater.
Like all good technologies, laser and LED printers have been refined over the years, so the engines they use can be made cheaply, while still increasing their resolution and the quality of the printed images they produce.