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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.
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