Printing technology is another application that depends in an essential way on lasers. Laser printers have become relatively inexpensive and are used in many homes and businesses. They are the light of choice because they can be targeted with great precision and so produce a clean, sharply defined image.
As with the devices used to read UPCs, the laser of choice early in the development of laser printers was the HeNe laser. When cheap, reliable laser diodes became available, however, HeNe lasers gave way to laser diodes in this application as well. Now essentially all laser printers depend on laser diodes.
To understand how laser printers work, it is first necessary to know about a class of materials called photoconductors. Electrically speaking, most materials fall into one of two classes: They are either conductors—electrical charges flow easily through them—or they are insulators, in which case electrical charges do not flow easily through them. Copper is an example of a conductor. Electricity flows easily through copper, which is why underneath the plastic sheathing that covers most electrical wiring there is a copper core. By contrast, the plastic sheathing itself is an insulator. The electrical charges that enable lights to glow and computers to compute are constrained to flow along the copper core of the wires leading to these devices only because of the insulation that surrounds it. Without the insulation, the electrical charges would flow out of the wire in an uncontrolled way, creating a short circuit.
Under ordinary conditions, the electrical properties of the materials that make up the wire do not change: The copper core is always a conductor, and the plastic sheathing is always an insulator. These are intrinsic properties of these materials. The copper is a conductor even when there are no electrical charges flowing through it, and insulators can have electrically charged surfaces, but because they are insulators, the charges cannot flow through them or across their surfaces. However, there are other materials that fail to conform to such a simple classification scheme. In fact, there is one class of materials, called photoconductors, whose electrical properties depend on whether or not they are illuminated. Photoconductors are insulators when they are in the dark and conductors when they are in the light. Moreover, if one uses a narrow beam of light—a laser, for example—to illuminate a line on a photoconductor’s surface, only the illuminated region will conduct electricity. The rest of the surface will remain an insulator. It is the pairing of a laser with a cylinder to which a photoconductor has been applied that has made the laser printer possible. The procedure consists of five steps.
The first step occurs in the dark. The cylinder, often called a drum, on which a photoconducting material has been applied is rotated beneath a device that “charges the drum,” which is another way of saying that electrical charges are distributed evenly across the surface of the cylinder. Because this occurs in the dark, the cylinder’s surface is an insulator, and the charges maintain their position.
The second step involves the laser. The computer transfers to the printer information about the pattern to be reproduced. A tightly focused laser beam is directed across the spinning drum with the help of mirrors and with the goal of reproducing the desired image on the surface of the drum. Each point on the drum that is illuminated by the laser changes briefly from an insulator to a conductor, and the electrical charge that was residing there flows away. When the laser is finished with its work, what remains on the drum is a pattern of electrically charged and uncharged regions that—if one could see it—is an image of the pattern that will eventually emerge from the printer on a sheet of paper. (Notice that if the light is not tightly focused—if one used an incandescent light, for example—the stray rays would illuminate other parts of the drum, and the resulting pattern would bear little relationship to the input delivered by the computer.)
Third, toner, which is not ink but an extremely fine plastic powder, is given an electrical charge that is of the same type as that applied to the drum. The toner is then applied to the drum, but it sticks only to those regions that were illuminated by the laser. Why? Like charges repel, and so the toner cannot stick to those parts of the drum that still hold an electrical charge identical to its own.
Fourth, a piece of fresh paper is drawn from the paper tray and given an electrical charge that is opposite to that of the toner that is sticking to the previously illuminated parts of the drum. The result is that the toner is attracted from the drum to the paper, where it adheres, if only weakly, to the paper’s surface.
Finally, the paper is subjected to sufficient heat and pressure so that the toner particles are fused permanently to its surface. This last step explains why paper emerging from a laser printer is warm to the touch. (Notice that the role of the laser is to create a pattern on the drum not to supply the heat necessary to fuse the toner to the paper.) Laser printers are preferred over other designs for their precise copy and for their printing speed. Both these characteristics result from the use of the laser, which can produce extraordinarily detailed patterns at the speed of light.