How does a laser printer work?

The history of the process

First, a quick lesson in laser-printer history. The laser printer relies on the principles of electrophotography – a process developed by an American patent lawyer, Chester Carlson, in 1938. Carlson discovered that you could create a copy of a page of text by reflecting light from the white areas of the paper on to a charged drum.

The light neutralised the charge on the drum, so that when oppositely charged particles of fine, dry, coloured powder were applied to the unexposed areas it would stick. This “toner” could then be rolled from the drum on to a sheet of paper, where heat and pressure would fuse it in place. Carlson’s invention led both to the first photocopiers, and to the creation of a company that became synonymous with photocopying – Xerox.

In 1969, a Xerox researcher names Gary Starkweather took electrophotography one stage further. Instead of using a photographic process to create the image on the drum, he thought, why not use a laser to draw a digital image? Eight years later, Xerox released its 9700 Electronic Printing System: an early laser printer.

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Xerox’s technology worked, but it wasn’t ready for the mass market. That took a unique partnership of innovators. In the mid-1970s, Canon, having developed a prototype laser printer, asked HP whether it would be interested in helping bring the technology to a commercial business printer. This led to the development of HP’s first laser printer – the HP 2680A (see the charming promotional photo, above). From there came the first mass-market laser, the original 1985 HP LaserJet.

Inside the laser printer

While LaserJet technology has changed dramatically over 30 years, the basic process is still fundamentally the same. Instructions are sent from a PC to the printer in the form of a printer command language (PCL), which tells the printer what text to print, where to print it and how to style it, while also breaking down any graphic elements in to PCL code. A raster image processor (RIP) on the printer then converts these instructions in to the image to be printed on the finished page.

But how does this image make it there? First, a negative charge is applied to a cylindrical drum by a primary charge roller or corona wire. Then a laser, working through an arrangement of lenses and mirrors, etches the image created by the RIP on to the drum surface one line at a time. The areas struck by the laser have a more positive charge. This means that, when negatively charged toner is transferred to the surface of the drum, it sticks to the areas marked by the laser and falls off the areas that remain negatively charged.

The toner is then transferred from the surface of the drum to the paper by a transfer roller, which applies a positive charge to the underside of the paper, attracting the negatively charged toner from the drum. With the toner held in place by static electricity, it’s passed through to a fusing unit, which uses a combination of heat and pressure to fix the toner permanently in place.

Black-and-white and colour lasers both use the same basic process – but with one key difference. In a mono laser printer, there’s just one drum and one toner cartridge, but in a colour laser printer you’ll find four cartridges – cyan, magenta, yellow and black – each with their own drum, toner and primary charge rollers and associated mechanisms.

In fact, the majority of the technology in a colour laser actually resides within the toner cartridges, while the formulation of the toner is crucial for print quality, performance and reliability. When manufacturers urge users to stick with original toner cartridges, it’s because they know these cartridges are mechanically solid and reliable, and that the toner being used is the one designed for that line of printers.

Laser-printer advantages and limitations

As a system, the laser-printer process is incredibly effective. Refinements have seen speeds increase from less than one page per minute (ppm) to over 50ppm, while resolutions have more than quadrupled. What’s more, laser printers have historically enjoyed several advantages over rival print technologies, like the inkjet or solid ink printers.

Lasers produce crisp text and bright, full-colour graphics, even on plain paper, and there’s little difference in print speeds between colour and black-and-white. Laser printing is also a reliable technology, enabling laser printers to handle monthly workloads of anywhere between 4,000 and 15,000 pages. That’s why laser printers still make up the majority of business-ready workgroup printers, although the latest inkjets now provide stiff competition.

All the same, some limitations hold the laser back. First, the toner particles in a cartridge spend a lot of time cycling through the mechanisms, meaning that they have a tendency to degrade over time. This makes it impossible to use all the toner in the cartridge, so some goes to waste. Secondly, the laser printer’s fuser unit needs a lot of heat to fuse the toner to the paper, which adds to energy bills and has a negative impact on the environment.

HP is leading the way in this area, with new toner formulations and a new line of printers – the LaserJet M series – that are smaller, faster and more energy-efficient. Through ongoing research, development and innovation, the laser printer will only keep getting stronger.

What is toner?

The early laser printers used a blend of resins, pigments and various additives, blended while hot to form a paste, then cooled and pulverised into a dry powder. Toner works best when the particles are of as even a size and shape as possible, so these toners were sifted to get rid of the smallest and largest particles. Pulverised toners are still used in many laser printers today, although the size of the particles is now a fraction of what it once was.

In 1997, however, HP began using a chemical process to “grow” the cyan, magenta and yellow toner particles for use in its flagship laser printers, with each small, spherical particle grown from a core to the exact size and shape required. This resulted in a toner that was more controllable and flowed better through the cartridge and the print engine, enhancing print speeds, resolution and the number of pages that could be printed by each cartridge. Now, all HP’s ColorSphere and ColorSphere 3 toner is produced in this way, with the latest version wrapping a soft core of ink inside a durable outer shell, both ensuring higher page yields and allowing the toner to fuse at a lower melting point.

Images are copyright HP and HP Computer Museum.