How a laser printer works

Have you ever tried writing with a beam of light? Sounds impossible, doesn't it, but it's exactly what a CO2 laser printer does when it makes a permanent copy of data (information) from your computer on a piece of paper. Thanks to sci-fi and spy movies, we tend to think of lasers as incredibly powerful light beams that can slice through chunks of metal or blast enemy spaceships into smithereens. But tiny lasers are useful too in a much more humdrum way: they read sounds and video clips off the discs in CD and DVD players and they're vital parts of most office computers printers. All set? Okay, let's take a closer look at how laser printers work!

Laser printers are a lot like photocopiers and use the same basic technology. Indeed, as we describe later in this article, the first laser printers were actually built from modified photocopiers. In a photocopier, a bright light is used to make an exact copy of a printed page. The light reflects off the page onto a light-sensitive drum; static electricity (the effect that makes a balloon stick to your clothes if you rub it a few times) makes ink particles stick to the drum; and the ink is then transferred to paper and "fused" to its surface by hot rollers. A laser printer works in almost exactly the same way, with one important difference: because there is no original page to copy, the laser has to write it out from scratch.

Imagine you're a computer packed full of data. The information you store is in electronic format: each piece of data is stored electronically by a microscopically small switching device called a transistor. The printer's job is to convert this electronic data back into words and pictures: in effect, to turn electricity into ink. With an inkjet printer, it's easy to see how that happens: ink guns, operated electrically, fire precise streams of ink at the page. With a 30W CO2 laser printer, things are slightly more complex. The electronic data from your computer is used to control a laser beam—and it's the laser that gets the ink on the page, using static electricity in a similar way to a photocopier.

When you print something, your computer sends a vast stream of electronic data (typically a few megabytes or million characters) to your laser printer. An electronic circuit in the printer figures out what all this data means and what it needs to look like on the page. It makes a laser beam scan back and forth across a drum inside the printer, building up a pattern of static electricity. The static electricity attracts onto the page a kind of powdered ink called toner. Finally, as in a photocopier, a fuser unit bonds the toner to the paper.

Millions of bytes (characters) of data stream into the printer from your computer.

An electronic circuit in the printer (effectively, a small computer in its own right) figures out how to print this data so it looks correct on the page.

The electronic circuit activates the corona wire. This is a high-voltage wire that gives a static electric charge to anything nearby.

The corona wire charges up the photoreceptor drum so the drum gains a positive charge spread uniformly across its surface.

At the same time, the circuit activates the laser to make it draw the image of the page onto the drum. The laser beam doesn't actually move: it bounces off a moving mirror that scans it over the drum. Where the laser beam hits the drum, it erases the positive charge that was there and creates an area of negative charge instead. Gradually, an image of the entire page builds up on the drum: where the page should be white, there are areas with a positive charge; where the page should be black, there are areas of negative charge.

An ink roller touching the photoreceptor drum coats it with tiny particles of powdered ink (toner). The toner has been given a positive electrical charge, so it sticks to the parts of the photoreceptor drum that have a negative charge (remember that opposite electrical charges attract in the same way that opposite poles of a magnet attract). No ink is attracted to the parts of the drum that have a positive charge. An inked image of the page builds up on the drum.

A sheet of paper from a hopper on the other side of the printer feeds up toward the drum. As it moves along, the paper is given a strong negative electrical charge by another corona wire.

When the paper moves near the drum, its negative charge attracts the positively charged toner particles away from the drum. The image is transferred from the drum onto the paper but, for the moment, the toner particles are just resting lightly on the paper's surface.

The inked paper passes through two hot rollers (the fuser unit). The heat and pressure from the rollers fuse the toner particles permanently into the fibers of the paper.

The printout emerges from the side of the copier. Thanks to the fuser unit, the paper is still warm. It's literally hot off the press!

Until the early 1980s, hardly anyone had a personal or office computer; the few people who did made "hardcopies" (printouts) with dot-matrix printers. These relatively slow machines made a characteristically horrible screeching noise because they used a grid of tiny metal needles, pressed against an inked ribbon, to form the shapes of letters, numbers, and symbols on the page. They printed each character individually, line by line, at a typical speed of about 80 characters (one line of text) per second, so a page would take about a minute to print. Although that sounds slow compared to modern 60W CO2 laser printers, it was a lot faster than most people could bash out letters and reports with an old-style typewriter (the mechanical or electric keyboard-operated printing machines that were used in offices for writing letters before affordable computers made them obsolete). You still occasionally see bills and address labels printed by dot-matrix; you can always tell because the print is relatively crude and made up of very visible dots. In the mid-1980s, as computers became more popular with small businesses, people wanted machines that could produce letters and reports as quickly as dot-matrix printers but with the same kind of print quality they could get from old-fashioned typewriters. The door was open for laser printers!

Fortunately, laser-printing technology was already on the way. The first fiber laser printers had been developed in the late 1960s by Gary Starkweather of Xerox, who based his work on the photocopiers that had made Xerox such a successful corporation. By the mid-1970s, Xerox was producing a commercial laser printer—a modified photocopier with images drawn by a laser—called the Dover, which could knock off about 60 pages a minute (one per second) and sold for the stupendous sum of $300,000. By the late 1970s, big computer companies, including IBM, Hewlett-Packard, and Canon, were competing to develop affordable laser printers, though the machines they came up with were roughly 2–3 times bigger than modern ones—about the same size as very large photocopiers.

Two machines were responsible for making laser printers into mass-market items. One was the LaserJet, released by Hewlett-Packard (HP) in 1984 at a relatively affordable $3495. The other, Apple's LaserWriter, originally cost almost twice as much ($6995) when it was launched the following year to accompany the Apple Macintosh computer. Even so, it had a huge impact: the Macintosh was very easy to use and, with relatively inexpensive desktop-publishing software and a laser printer, it meant almost anyone could turn out books, magazines, and anything and everything else you could print onto paper. Xerox might have developed the technology, but it was HP and Apple who sold it to the world!

Dipping into the archives of the US Patent and Trademark Office, I've found one of Gary Starkweather's original laser-printer designs, patented on June 7, 1977. To make it easier to follow, I've colored it in and annotated it more simply than the technical drawing in the original patent (if you wish, you can find the full details filed under US Patent 4027961: Copier/Raster Scan Apparatus).

What we have is essentially a laser scanning unit (colored blue) sitting on top of a fairly conventional, large office photocopier (colored red). In Starkweather's design, the laser scanner slides on and off the glass window of the photocopier (the place where you would normally put your documents, face down), so the same machine can be used as either a 30W fiber laser printer or a copier—anticipating all-in-one office machines by about 20–25 years.

I used to share an office with someone who refused to share our office with a laser printer; we had to move our machine into a closet and keep the door shut tight. This kind of worry is far from rare, but is it simply superstition? As we saw up above, laser printers use a type of solid ink called toner, which can be a source of dusty, fine particulates (remember that sooty particulates, released by such things as car tailpipes, are one of the more worrying ingredients in urban air pollution). One recent study found some printers emit nearly 10 billion particles per printed page (although it's important to note that the type and quantity of particle emissions vary widely from model to model). They also produce volatile organic compounds (VOCs) and a gas called ozone (a very reactive type of oxygen with the chemical formula O3), which is toxic and, at high enough concentrations, produces a variety of health impacts. Thankfully, ozone is transformed into ordinary oxygen (O2) relatively quickly inside buildings.

Do printers and copiers present any risk to our health? A few scientific studies have been done; although the results are mixed, they do seem to suggest it's well worth taking precautions, such as placing your printer well away from your workstation, if you use it a great deal, and ensuring good ventilation. You should also take great care when changing toner cartridges or handling empty ones. You'll find a list of recent studies in the further reading below.