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Laser Printer Technology Explained
DocumentID: 647595
Revision Date: 29-Feb-96 8:19:22 PM

The information in this document applies to:
WordPerfect® 5.1 for DOS

Problem

Solutions: This memo is going to answer these questions: How do page printers work and how are the technologies different?

A generic page printer engine works almost the same as a desktop copier. In the center is a photosensitive drum, which the drum corona wire covers with an even charge. When the drum is exposed to light, the charge on its surface changes, creating an exposure area in which the image is "painted" on the drum by laser or other means.

The developer unit is filled with captive plastic beads that impart a charge to the toner particles and carry them to the drum, where they are attracted to the spots on the drum surface that are to be printed. Meanwhile, the paper travels past the transfer corona wire, which gives it a charge that will allow it to attract the toner.

As the paper passes the drum, the toner is pulled onto the paper. Excess toner is then wiped from the drum and is "erased" by the drum corona wire to make it ready for the next image. The paper then travels through the two fusing rollers, which use heat and pressure to melt the plastic toner powder into the paper. Finally, the paper transport system carries the page the rest of the way to the output tray.

There are three basic technologies that are currently being used by page printers. Each of the three has its proponents, who argue that their technology results in more-accurate images or a more reliable design.

Most popular of all is the laser, in large part because of the incredible success of the small Canon laser print engines. A diode laser generates a beam that is reflected off a spinning, many-sided mirror, causing it to sweep across the width of the drum. By rapidly turning the laser on and off, the individual dots may be placed to create the image. The entire page is processed in memory prior to being imaged on the page.

Instead of one light source, there are thousands of tiny points in a LED array. Each pixel that is present in the 300-dot-per-inch print line has its own tiny light that blinks on and off as the drum turns, exposing a tiny strip of drum to create its part of the image.

The liquid crystal shutter engine operates with a process that falls somewhere between the two that were already described. Like a laser, it has a single light source. In this case, it is often a halogen light. AS the LED array engine, every pixel is controlled individually. A vast array of liquid crystal shutters wink open and shut in order to let light through and expose the turning drum.

The results of these different approaches are nearly identical. Each design produces 300-dpi output. All three are capable of at least 8-page-per-minute speed ratings and all three methods have been engineered to fit in remarkable small and lightweight packages.

The different constructions have minor implications in terms of longevity. The laser has the fewest components, so presumably there is less chance of something breaking down. On the other hand, the beam must be reflected from one end of the drum to the other, and maintaining accurate control of the beam can be an exceedingly difficult task. The LED and LCS engines do not depend on a moving part like the rotating mirror in the laser engine, so they could be more reliable than the laser. Still, there are thousands of tiny electrical connections that are involved in each of the systems, and one LED or liquid crystal shutter failure will permanently disable a pixel.

As tested in PC Labs, printers using these three technologies didn't produce output that differentiates them from one another. The variations tended to be more pronounced from engine to engine than from category to category. But even so, nonlaser page printers deserve some respect, even if they don't get a new name.

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