Laser Printing - What is laser printing?
A method of printing; this process creates an image by using static electricity to attract toner to specific areas of an imaging drum before it is transferred and bonded onto a substrate. Laser printing is a form of xerographic printing; a latent image is created first using a photoconductive drum (the image carrier or imaging device) and static electricity, but where xerography uses light to create a copy of a document on an image drum, laser printing uses a laser to project a digital image onto the image drum, meaning it is also a form of digital printing.
There are typically seven steps involved; image processing, charging, exposure, development, transfer, fusing, and cleaning.
While laser printers may be used to print original documents or copies of documents, it must always start with a digital image. A Raster Image Processor converts the document or image being printed into a raster image or "bitmap"; the document or image is represented by a grid of pixels (points of colour). This bitmap is then sent to the printer for output.
Next the photoconductive drum (image drum) is given a negative (or positive) charge by the primary charge corona roller or wire.
Exposure involves the creation of a latent image (an “invisible” image that is created by exposing a photosensitive material to light); in laser printing, a laser scans across the image drum, turning on and off according to the bitmap to indicate if an area should show the "non-content" (blank, white areas) or "content" of the image. When the light is on, the charge on that area of the drum is reversed or discharged; this creates a pattern of dots that are positively (or negatively) or neutrally charged and forms the latent image, which is a negative of the original.
The latent image is then developed as the drum rotates past a toner cartridge; the toner particles have a negative (or positive) charge, meaning that they will either be attracted to the positively charged areas of the drum or repelled by the negatively charged areas of the drum and fall into the neutral areas.
At this point, the substrate passes between the image drum and the transfer corona; the transfer corona gives the substrate a negative (or positive) charge that is stronger than that of the image drum. The toner is attracted to the paper and, because the paper has a stronger charge than the drum, is transferred onto the substrate.
The substrate then passes through a fuser unit, which uses heat and pressure to bond the toner in place.
In the final stage, light discharges the remaining charge on the drum, and any excess toner is brushed away in order to clean the drum before the next sheet is printed.
Laser printers can use one colour of toner to create monochromatic images or use four toners to create full colour images through four colour process printing (each of the process colours has its own colour and its own bitmap; laser printers with one image drum will repeat the expose, develop, and transfer steps for each colour, while other laser printers will have one image drum per colour and a "transfer belt", which passes between each toner cartridge and image drum in succession before transferring the four layers of toner onto the substrate in a single step).
Laser printers are able to produce high print resolution, at a quiet operating volume, and often at high speed.Go Back to Glossary