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How Your Laser Printer Prints

In 1969, a researcher named Gary Starkweather invented the laser printer. He modified a xerographic copier to make a working prototype by 1972. The first printers easily filled a room. Technology improved over the years, along with pricing and size.

Now it is easy to get a cheap machine that can print pretty much anything. Regarding size, companies making desktop printers constantly for smaller footprints – there are even mobile printers. The seven-step xerographic process summarized below – though incredibly streamlined and featuring an growing array of innovation and material upgrades – has, however, remained fundamentally the same.

Raster Image Processing: The Raster Image Processor (RIP) is the brain of your printer, turning the image you want printed into a bitmap. It does this by using a page description language to encode the image in a way that the machine can ‘understand’. A page is made up of horizontal lines of dots, called scan lines or raster lines. This data is stored as a bitmap in the RIP’s memory. A color image requires four bitmaps, corresponding to each of the CMYK (cyan, magenta, yellow and black) toner layers.

Step Two – Charging: A negative electrostatic charge is projected across the surface of a photoreceptor. The photoreceptor is a revolving drum or belt that holds this charge while in darkness. In other words, once charged, the drum is like a roll of film waiting to be exposed to light – in this case, a laser beam.

Step Three – Exposing: The laser is aimed at a system of mirrors and lenses that bounces it precisely onto the revolving drum. The rasterized data in memory switches the beam on and off to form the dots of each scan line. The beam affects the negative charge that has been applied to the drum, neutralizing or reversing the polarity of the charge wherever it touches, i. E., it ‘cuts out’ the ‘to-be-printed’ parts. In this way, the bitmap is applied to the drum as a static electric negative image. Lasers are used because of the uniformly narrow beam they generate. Many printers that are incorrectly termed ‘laser’ expose using page-width spanning LEDs rather than a laser.

Developing: This static electric ‘cut out’ is then exposed to toner – carbon black (or other colorant) mixed with fine particles of dry wax- or plastic powder. The toner’s particles are negatively charged. As such, they ‘stick’ to the positive/neutral bits created by the laser and do not adhere to the negatively-charged bits.

Transferring: The drum or belt is pressed or rolled onto a page, which transfers the toner to the surface of the paper.

Fusing (Step 6): The sheet goes into a fuser assembly, where pressure and heat bonds the toner to the page.

Step Seven – Cleaning: Once the print is done, a soft plastic blade (electrically neutral) cleans any remaining toner from the drum into the waste reservoir and a discharge lamp removes any remaining charge from the drum.

These steps take place in such rapid succession that the process can occur before the drum completes one revolution – this alone is amazing. Color printers use several laser scanner assemblies – a true marvel of modern innovation. The next time you print out a document or picture, take a minute to appreciate the marvelous complexity of your laser printer.

In his spare time, Giles Cunningham likes to blog about technology. Read his latest posts about a laser printer and a colour laser printer. Read his articles at Oyyy.co.uk.