The Basic
Process
The primary principle at work in a laser printer is static
electricity, the same energy that makes clothes in the dryer
stick together or makes a lightning bolt travel from a thundercloud
to the ground. Static electricity is simply an electrical
charge built up on an insulated object, such as a balloon
or your body. Since oppositely charged atoms are attracted
to each other, objects with opposite static-electricity fields
cling together.
The basic components of a laser printer
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A laser printer uses this phenomenon
as a sort of "temporary glue." The core component of this
system is the photoreceptor, typically a revolving
drum or cylinder. This drum assembly is made out of
highly photoconductive material that is discharged
by light photons.
Initially, the drum is given a
total positive charge by the charge corona wire,
a wire with an electrical current running through it. (Some
printers use a charged roller instead of a corona wire,
but the principle is the same). As the drum revolves, the
printer shines a tiny laser beam across the surface to discharge
certain points. In this way, the laser "draws" the letters
and images to be printed, as a pattern of electrical charges
-- an electrostatic image.
The laser "writes" on a photoconductive revolving
drum.
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After the pattern is set, the
printer coats the drum with positively charged toner
-- a fine, black powder. Since it has a positive charge, the
toner clings to the discharged areas of the drum, but not
to the positively charged "background." This is something
like writing on a soda can with glue and then rolling it over
some flour: The flour only sticks to the glue-coated part
of the can, so you end up with a message written in powder.
With the powder pattern affixed,
the drum rolls over a sheet of paper, which is moving along
a belt below. Before the paper rolls under the drum, it is
given a negative charge by the transfer corona wire
(charged roller). This charge is stronger than the negative
charge of the electrostatic image, so the paper can pull the
toner powder away. Since it is moving at the same speed as
the drum, the paper picks up the image pattern exactly. To
keep the paper from clinging to the drum, it is discharged
by the detac corona wire immediately after picking
up the toner.
Finally, the printer passes the
paper through the fuser, a pair of heated rollers.
As the paper passes through these rollers, the loose toner
powder melts, fusing with the fibers in the paper. The fuser
rolls the paper to the output tray, and you have your finished
page. The fuser also heats up the paper itself, of course,
which is why pages are always hot when they come out of a
laser printer or photocopier. So what keeps the paper from
burning up? Mainly, speed -- the paper passes through
the rollers so quickly that it doesn't get very hot.
After depositing toner on the
paper, the drum surface passes the discharge lamp.
This bright light exposes the entire photoreceptor surface,
erasing the electrical image. The drum surface then passes
the charge corona wire, which reapplies the positive charge.
Conceptually, this is all there
is to it. Of course, actually bringing everything together
is a lot more complex. In the following sections, we'll examine
the different components in greater detail to see how they
produce text and images so quickly, and so precisely.
The Controller
Before a laser printer can do anything else, it needs to receive
the page data and figure out how it's going to put everything
on the paper. This is the job of the printer controller.
The printer controller is the
laser printer's main onboard computer. It talks to the host
computer (for example, your desktop PC) through a communications
port, such as a parallel port or USB port. At the start of
the printing job, the laser printer establishes with the host
computer how they will exchange data. The controller may have
to start and stop the host computer periodically to process
the information it has received.
A typical laser printer has a few different types
of communications ports.
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In an office, a laser printer
will probably be connected to several separate host computers,
so multiple users can print documents from their machine.
The controller handles each one separately, but may be carrying
on many "conversations" concurrently. This ability to handle
several jobs at once is one of the reasons laser printers
are so popular.
For the printer controller and
the host computer to communicate, they need to speak the same
page-description language. In earlier printers, the
computer sent a special sort of text file and a simple code
giving the printer some basic formatting information. Since
these early printers had only a few fonts, this was a very
straightforward process.
These days, you might have hundreds
of different fonts to choose from, and you wouldn't think
twice about printing a complex graphic. To handle all of this
diverse information, the printer needs to speak a more advanced
language.
The primary printer languages
these days are Hewlett Packard's Printer Command Language
(PCL) and Adobe's Postscript. Both of these languages
describe the page in vector form -- that is, as mathematical
values of geometric shapes, rather than as a series of dots
(a bitmap image). The printer itself takes the vector
images and converts them into a bitmap page. With this system,
the printer can receive elaborate, complex pages, featuring
any sort of font or image. Also, since the printer creates
the bitmap image itself, it can use its maximum printer resolution.
Some printers use a graphical
device interface (GDI) format instead of a standard PCL.
In this system, the host computer creates the dot array itself,
so the controller doesn't have to process anything -- it just
sends the dot instructions on to the laser.
But in most laser printers, the
controller must organize all of the data it receives from
the host computer. This includes all of the commands that
tell the printer what to do -- what paper to use, how to format
the page, how to handle the font, etc. For the controller
to work with this data, it has to get it in the right order.
Once the data is structured, the
controller begins putting the page together. It sets the text
margins, arranges the words and places any graphics. When
the page is arranged, the raster image processor (RIP)
takes the page data, either as a whole or piece by piece,
and breaks it down into an array of tiny dots. As we'll see
in the next section, the printer needs the page in this form
so the laser can write it out on the photoreceptor drum.
In most laser printers, the controller
saves all print-job data in its own memory. This lets the
controller put different printing jobs into a queue,
so it can work through them one at a time. Additionally, it
saves time when printing multiple copies of a document, since
the host computer only has to send the data once.
The Laser
Since it actually draws the page, the printer's laser system
-- or laser scanning assembly -- must be incredibly
precise. The traditional laser scanning assembly includes:
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a laser
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a movable mirror
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a lens
The laser receives the page data
-- the tiny dots that make up the text and images -- one horizontal
line at a time. As the beam moves across the drum, the laser
emits a pulse of light for every dot to be printed, and no
pulse for every dot of empty space.
The laser doesn't actually move
the beam itself. It bounces the beam off a movable mirror
instead. As the mirror moves, it shines the beam through a
series of lenses. This system compensates for the image
distortion caused by the varying distance between the mirror
and points along the drum.
The laser assembly moves in only
one plane, horizontally. After each horizontal scan, the printer
moves the photoreceptor drum up a notch so the laser assembly
can draw the next line. A small print-engine computer
synchronizes all this perfectly, even at dizzying speeds.
Some newer laser printers use
a strip of light-emitting diodes (LEDs) to write the
page image instead of a single laser. Each dot position has
its own dedicated light, which means the printer has one set
print resolution. These systems cost less to manufacture than
true laser assemblies, but they produce inferior results.
Typically, you'll only find them in less expensive printers.
Toner
One of the most distinctive things about a laser printer (or
photocopier) is the toner. It's such a strange concept for
the paper to grab the "ink," rather than the printer applying
it. And it's even stranger that the "ink" isn't really ink
at all.
So what is toner? The short
answer is: It's an electrically-charged powder with two main
ingredients: pigment and plastic.
The role of the pigment is fairly
obvious -- it provides the coloring (black, in a monochrome
printer) that fills in the text and images. This pigment is
blended into plastic particles, so the toner will melt when
it passes through the heat of the fuser. This quality gives
toner a number of advantages over liquid ink. Chiefly, it
firmly binds to the fibers in almost any type of paper, which
means the text won't smudge or bleed easily.
A developer bead coated with small toner particles
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So how does the printer apply
this toner to the electrostatic image on the drum? The powder
is stored in the toner hopper, a small container built
into a removable casing. The printer gathers the toner from
the hopper with the developer unit. The "developer"
is actually a collection of small, negatively-charged magnetic
beads. These beads are attached to a rotating metal roller,
which moves them through the toner in the toner hopper.
Because they are negatively charged,
the developer beads collect the positive toner particles as
they pass through. The roller then brushes the beads past
the drum assembly. The electrostatic image has a stronger
negative charge than the developer beads, so the drum pulls
the toner particles away.
In a lot of printers, the toner hopper, developer
and drum assembly are combined in one replaceable cartridge.
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The drum then moves over the paper,
which has an even stronger charge and so grabs the toner.
After collecting the toner, the paper is immediately discharged
by the detac corona wire. At this point, the only thing keeping
the toner on the page is gravity -- if you were to blow on
the page, you would completely lose the image. The page must
pass through the fuser to affix the toner. The fuser rollers
are heated by internal quartz tube lamps, so the plastic
in the toner melts as it passes through.
But what keeps the toner from
collecting on the fuser rolls, rather than sticking to the
page? To keep this from happening, the fuser rolls must be
coated with Teflon, the same non-stick material that
keeps your breakfast from sticking to the bottom of the frying
pan.
Color Printers
Initially, most commercial
laser printers were limited to monochrome printing (black
writing on white paper). But in the last few years,
more and more color laser printers have hit the market.
Essentially, color printers work the
same way as monochrome printers, except they go through
the entire printing process four times -- one pass each
for cyan (blue), magenta (red), yellow and black. By
combining these four colors of toner in varying proportions,
you can generate the full spectrum of color.
Inside a color laser printer
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There are several different ways of doing
this. Some models have four toner and developer units
on a rotating wheel. The printer lays down the electrostatic
image for one color and puts that toner unit into position.
It then applies this color to the paper and goes through
the process again for the next color. Some printers
add all four colors to a plate before placing the image
on paper.
Some more expensive printers actually
have a complete printer unit -- a laser assembly, a
drum and a toner system -- for each color. The paper
simply moves past the different drum heads, collecting
all the colors in a sort of assembly line.
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Advantages of a Laser
So why get a laser printer rather than a cheaper inkjet printer?
The main advantages of laser printers are speed, precision
and economy. A laser can move very quickly, so it can "write"
with much greater speed than an ink jet. And because the laser
beam has an unvarying diameter, it can draw more precisely,
without spilling any excess ink.
Laser printers tend to be more
expensive than inkjet printers, but it doesn't cost as much
to keep them running -- toner powder is cheap and lasts a
long time, while you can use up expensive ink cartridges very
quickly. This is why offices typically use a laser printer
as their "work horse," their machine for printing long text
documents. In most models, this mechanical efficiency is complemented
by advanced processing efficiency. A typical laser-printer
controller can serve everybody in a small office.
When they were first introduced,
laser printers were too expensive to use as a personal printer.
Since that time, however, laser printers have gotten much
more affordable. Now you can pick up a basic model for just
a little bit more than a nice inkjet printer.
As technology advances, laser-printer
prices should continue to drop, while performance improves.
We'll also see a number of innovative design variations, and
possibly brand-new applications of electrostatic printing.
Many inventors believe we've only scratched the surface of
what we can do with simple static electricity!