I remember sitting through long dreary physics lessons many years ago (1975) trying to get my head round the concepts of electrostatics, optics, refraction semiconductors and the behaviour of light. What kind of lame geek could find any possible use for this stuff in the real world?
Back then in the dim mists of time no one had even heard of a laser printer and Mr Hewlett and Mr. Packard were probably having a well deserved beer in the sunshine after slaving away in that shed for years. I`ve been slaving in mine for years and never come up with anything, let alone something as brilliant and as revolutionary as the laser printer and apparently they didnt either. (see below). Now, we take laser printers for granted and some models have become so cheap that they are throwaway items. So how do these devices work? First we have to go back to the physics lesson.

At the heart of every laser printer lies a device called a photoconductive drum, or photoconductor. Its a metal cylinder, usually aluminium which is coated in a special material.
This material has the property of being to store an electrical charge on its surface in darkness. In order to charge the drum, it is rotated against an insulated roller which has a voltage of 6000 volts applied to it. This coats the drum surface in a uniform electrostatic charge. Ok, hold that in your head for a while.

The next item we have to look at is the item which gives the printer its name - the laser. Come to think of it, the printer could just as easily have been called an electrostatic or photoconductor printer but laser sounds so much more cool and hi-tech, there really isn`t any contest.
Natural light is a mixture of light of different wavelengths. However, a laser beam consists of light at one wavelength, and the waves are all in phase (the peaks and troughs in the waves all coincide).
As the waves are all in phase, the light is very intense (if peaks and troughs do not coincide, they tend to cancel each other out, reducing the power of the beam). Natural light can be focused, but it cannot be focused to so fine a point as laser light can.
Lasers used to be massive devices - several metres long, requiring huge amounts of power, fabulously expensive and the sole preserve of universities, research foundations and evil masterminds in James Bond films. All that changed in 1962 when Robert Hall invented the semiconductor laser which was small, compact and required little power to operate. It looked like a transistor with a window on top and thats essentially what it was. However without this little device we wouldnt have cd players, DVD`s, CDROM drives, digital copiers, supermarket checkouts, corrective eye surgery (or super accurate bombs)- ah well, I guess you just gotta take the rough with the smooth.

Back to our charged drum, waiting patiently in the dark inside the printer. The drum is coated with a special material called a photoconductor which retains the charge in darkness and dissipates it in light. If we fire the needle sharp focused laser beam in a swath across the rotating drum we can lay a very precise image on the drum surface. This is done by firing the beam in pulses at a rotating hexagonal mirror which reflects the laser light onto the drum. This wipes out charge on particular areas of the drum and leaves the charge on other places. In this way we can write an image composed of electrical charge on the drum surface.

In order to make the image visible, it has to be developed on the drum surface. The drum rotates against a development roller which carries a black powder called toner. This powder has an electrical charge which is opposite to the charge on the image areas on the drum surface. The black powder is attracted to the image areas, making them visible.

The process of getting the image off the drum and onto the paper is called transfer. A sheet of paper is fed up so that it contacts the drum surface just at the point where the image starts. A roller beneath the paper applies electrical charge to the underside of the sheet and this attracts the powder image off the drum and onto the paper.

Footnote:

Many people wrongly believe that Hewlett-Packard invented the laser printer. When President George Bush visited the HP printer division in Boise, Idaho, he gave a rousing speech in which he congratulated HP engineers for “inventing the laser printer”. He told them they were setting an excellent example, keeping America competitive..

In fact, the first true desktop laser was brought out by Canon in 1982. They needed a partner to market this revolutionary machine and went to Xerox. In a move which must have had them crying in their pretzels for years to come Xerox spurned Canon in favour of their own machine, the Xerox 4045 which went on to become as much use as a one legged man at an ass kicking contest. Dealers couldnt shift with a forklift. Canon shrugged their shoulders and went off to talk to HP, who knew a good thing when they saw it. They have used the Japanese print engine in every printer they have marketed since.