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Know It All: Laser printers -- Pretty in Prints

Published in Geare magazine, Issue #31, 2005

When you call something a laser printer, you expect it to have a lot to do with lasers. Correct? But the principle used mostly in 'laser' printers is electrostatics. Let's explore this further.

'Laser' stands for Light Amplification by the Stimulated Emission of Radiation. Most light is random. Different frequencies of light, shooting in different directions, largely out of phase. That's white light.

But a laser is different. Another term for laser is 'coherent light'. A laser beam consists of just a single frequency of light (that is, one precise colour), all going in the same direction. That makes it tremendously focused (as you can see from the common red laser pointers, which maintain a tight dot across a large room).

A wonderful invention indeed, but it performs only one task in a laser printer: to selectively discharge an electrostatic charge.

What's the main job to be done in a laser printer? Take a fine black powder (toner) from a container, arrange it neatly in the form of letters and shapes on a piece of paper, and make it stick. The laser does the job of defining the letters and shapes. But it is the manipulation of electrostatic charges that gets everything around the place.

So let's take all this in sequence. A laser printer is a black and white device. By that, I mean it has no grey scales. All it can do is put a tiny dot of black onto a spot on the paper. Or not. The only way it can make grey is by only covering part of the paper with black dots, leaving white bits between them, and relying on the human eye to merge the two into grey.

So the data that forms, say, the letter 'A' will consist of a 'raster' pattern: row after row of 1s and 0s, indicating to the printer where the black dots are to be, and where they are to be left off. The raster pattern can be generated in either the printer or the computer feeding it the data. Generally the raster patterns to form text are created by the printer's engine, and those for graphics are created by the computer. But printing languages such as PostScript allow the printer to do the whole job.

Once the raster pattern is created, this is used to control the laser in the printer. This just sweeps across, from side to side, flashing on and off in accordance with the raster pattern, onto a drum rotating before it. Some printers skip the laser completely and use a row of very small light emitting diodes (LEDs) for this part of the job and, so, really shouldn't be called laser printers at all.

The surface of the drum is constructed of a photoconductive material. That is, a material that conducts electricity better in the presence of light than in its absence. Before the laser (or the light from the LEDs) hits it, the surface of the drum is given a positive charge via a stretched corona wire. As the light hits a dot, it increases the dot's conductivity, allowing electrons to flow into the dot, eliminating the positive charge. Meanwhile, the fine toner powder is also positively charged. This is placed onto the drum. The bits of the drum that have been discharged allow the toner to stick, while the positive charge on the rest of the drum rejects the toner.

Remember, with both electical and magnetic fields, like rejects like. Positively charged objects repel other positively charged objects.

Now comes the paper. A corona wire wipes over this, but this time it creates a negative charge. The roller, with little letters and pictures held onto its surface by an electrostatic charge, is rolled over the negatively charged paper. This attracts the toner which, you will recall, was charged positively at the outset (opposites attract). So the toner transfers from roller to paper. Another corona wire dispels the charge of the paper and the toner, and then the paper is squeezed out between a pair of hot rollers, which softens the toner powder and fuses it onto the surface of the paper.

And that's all there is to it. Sure, there's a laser in there (except for LED printers), but all the heavy lifting is done by the same electrostatic forces that make your hair stick out on a dry day.

© 2002-2009, Stephen Dawson