Laser Printers -- A Brief Tour

While a typical inkjet printer houses a small set of circuits which receive information from the computer and set the print job in motion, a laser printer contains a much more sophisticated onboard computer, called the printer controller. The printer controller communicates with the host computer (or computers) using a language such as PCL (HP’s Printer Command Language) or Adobe’s Postscript.

Breaking down the page

The printer controller typically takes the planned layout of a page and uses a raster image processor (RIP) to break down the page into millions of tiny dots. It then saves all of this data in its own memory, so that the host computer only has to send one copy of the document, even when printing multiple copies.

Charging the drum assembly

At the core of a laser printer are the laser scanning assembly and the drum assembly. The drum assembly consists of a rotating cylinder made of a material that is very sensitive to light. This drum is electrically charged at the beginning of each print job. As the drum revolves, a laser beam emits a pulse of light for every dot that will appear on the page, and no light for every dot of blank space. This light, which then travels off a mirror and through several lenses on its way to the drum, reverses the electrical charge at just the spots along the drum which will translate into dots on the page.

Coating the drum with toner

With the page’s layout set as a pattern of positive and negative electrical charges on the printer’s drum, the drum then is coated with toner, a fine, electrically-charged powder made of pigmented plastic particles. This toner clings only to the appropriately charged areas of the drum, forming the characters and graphics of your document.

Printing the page

The sheet of printer paper, which is also given an electrical charge at the start of the print job, pulls the toner powder off of the drum and onto its surface. It then passes through a pair of Teflon-coated hot rollers (called the fuser), in order to melt the toner permanently onto the page’s surface.

Printing in color

In recent years, color laser printers have become more and more widely available. These machines work in very much the same way as monochrome laser printers, but make use of four different colored toners (cyan, magenta, yellow, and black), combined in various proportions, to create all of the necessary colors. There are many different ways in which color laser printers get the four separate toners onto the page: some models apply each toner to the page in sequence, fusing one color to the page before applying the next color; other models apply all four toners to a single plate before transferring the final image to paper; still others have four separate drum and toner assemblies, so the paper simply passes by each drum head in sequence.