The Basics:
Drum
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 system can also
work with the charges reversed -- that is, a positive electrostatic image on
a negative background.
|
The laser "writes" on a photoconductive revolving
drum.
|
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 negative 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.
|
The basic components of a laser printer
|
The Basics: Fuser
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.
|
The basic components of a laser printer
|
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 precisely.
The Controller: The
Conversation
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 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.
|
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 why laser printers are so popular.
The Controller: The
Language
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.
The Controller: Setting
up the Page
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. It also saves time when
printing multiple copies of a document, since the host computer only has to
send the data once.
The Laser Assembly
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:
 | A laser |
| A movable mirror |
| 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.
Writing the Page
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 of this perfectly, even at dizzying speeds.
Some 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.
|
Photocopiers
Laser printers work the same basic way as
photocopiers, with a few significant differences. The most obvious
difference is the source of the image: A photocopier scans an image by
reflecting a bright light off of it, while a laser printer receives the
image in digital form.
Another major difference is how the electrostatic
image is created. When a photocopier bounces light off a piece of paper,
the light reflects back onto the photoreceptor from the white areas but
is absorbed by the dark areas. In this process, the "background" is
discharged, while the electrostatic image retains a positive charge.
This method is called "write-white."
In most laser printers, the process is reversed: The
laser discharges the lines of the electrostatic image and leaves the
background positively charged. In a printer, this "write-black" system
is easier to implement than a "write-white" system, and it generally
produces better results. |
Toner Basics
One of the most distinctive things about a laser printer 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.
|
Photo courtesy
Xerox
A developer bead coated with small toner particles
|
Applying Toner
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.
|
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 now, there are lots of color
laser printers on 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
|
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.
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!
need other links go to SITE MAP
home B & I SNOWDEN-Find a Wealth of Products and Services
|
