Walk into almost any business office, and you'll probably
find a photocopier ("copier") with a line of people waiting to
use it. For most businesses, small or large, the copier has
become standard equipment, much like having a desk to work at
and a chair to sit in.
Photo courtesy Xerox A typical business photocopier from Xerox
What if you had to resort to making carbon copies of
important documents, as many people did before copiers came
along? Or worse, imagine how tedious it would be if you had to
recopy everything by hand! Most of us don't think about what's
going on inside a copier while we wait for copies to shoot
neatly out into the paper tray, but it's pretty amazing to
think that, in mere seconds, you can produce an exact replica
of what's on a sheet of paper! In this edition of HowStuffWorks,
we will explore what happens after you press "Start" on a
photocopier.
The Basics The human-end of making a copy
begins with a few basic steps:
Open the copier lid
Place the document to be photocopied face-down on the
glass
Select the options you want (number of pages,
enlargements, lighter/darker)
Press the Start button
What happens inside the
copier at this point is amazing! At its heart, a copier works
because of one basic physical principle: opposite charges
attract.
As a kid, you probably played with static
electricity and balloons. On a dry winter day, you can rub
a balloon on your sweater and create enough static electricity
in the balloon to create a noticeable force. For example, a
balloon charged with static electricity will attract small
bits of paper or particles of sugar very easily.
A copier uses a similar process.
Inside a copier there is a special drum. The drum
acts a lot like a balloon -- you can charge it with a form
of static electricity.
Inside the copier there is also a very fine black powder
known as toner. The drum, charged with static
electricity, can attract the toner particles.
There
are three things about the drum and the toner that let a
copier perform its magic:
The drum can be selectively charged, so that only
parts of it attract toner. In a copier, you make an "image"
-- in static electricity -- on the surface of the drum.
Where the original sheet of paper is black, you create
static electricity on the drum. Where it is white you do
not. What you want is for the white areas of the original
sheet of paper to NOT attract toner. The way this
selectivity is accomplished in a copier is with light --
this is why it's called a photocopier!
Somehow the toner has to get onto the drum and then onto
a sheet of paper. The drum selectively attracts toner. Then
the sheet of paper gets charged with static electricity and
it pulls the toner off the drum.
The toner is heat sensitive, so the loose toner
particles are attached (fused) to the paper with heat as
soon as they come off the drum.
The drum, or
belt, is made out of photoconductive material. Here are
the actual steps involved in making a photocopy:
The surface of the drum is charged.
An intense beam of light
moves across the paper that you have placed on the copier's
glass surface. Light is reflected from white areas of the
paper and strikes the drum below.
Wherever a photon of
light hits, electrons
are emitted from the photoconductive atoms in
the drum and neutralize the positive charges above. Dark
areas on the original (such as pictures or text) do not
reflect light onto the drum, leaving regions of positive
charges on the drum's surface.
Negatively charged, dry, black pigment called
toner is then spread over the surface of the drum,
and the pigment particles adhere to the positive charges
that remain.
A positively charged sheet of paper then passes over the
surface of the drum, attracting the beads of toner away from
it.
The paper is then heated and pressed to fuse the image
formed by the toner to the paper's surface.
This
diagram helps see the process:
When the copier illuminates the sheet of paper on
the glass surface of a copier, a pattern of the image is
projected onto the positively charged photoreceptive drum
below. Light reflected from blank areas on the page hits the
drum and causes the charged particles coating the drum's
surface to be neutralized. This leaves positive charges only
where there are dark areas on the paper that did not reflect
light. These positive charges attract negatively charged
toner. The toner is then transferred and fused to a positively
charged sheet of paper.
Inside a Photocopier If
you take a photocopier apart, you might be overwhelmed by how
many different parts there are. However, the actual
photocopying process relies on only a few, key pieces:
Photoreceptor drum (or belt)
Corona wires
Lamp and lenses
Toner
Fuser
In the following sections, you'll
learn about each of these parts.
Photoreceptor Drum The photoreceptor
drum (or, in some photocopiers, belt) is the heart of the
system. A drum is basically a metal roller covered by a layer
of photoconductive material. This layer is made out of
a semiconductor
such as selenium, germanium or silicon. What makes elements
like selenium so cool is that they can conduct electricity in
some cases, but not in others. In the dark, the
photoconductive layer on the drum acts as an insulator,
resisting the flow of electrons from one atom to
another. But when the layer is hit by light, the
energy of the photons liberates electrons and allows current
to pass through! These newly freed electrons are what
neutralizes the positive charge coating the drum to form the
latent image.
Various photocopier
drums
It's easy to imagine how you might project a copy of an
image on a photoreceptive belt that has roughly the same
dimensions as the sheet of paper containing the image. A
problem emerges when you think about doing the same thing on a
thin, cylindrical drum. How can the surface area of the drum
possibly match the real estate on a sheet of paper? The
solution is to simply rotate the drum while you're making a
copy. If you rotate the drum in lockstep with the movement of
the light beam across the original document, you can build the
image strip by strip. After one strip of light is focused onto
a corresponding swath of the drum, the drum rotates to expose
a fresh area of the photoconductor. Meanwhile, the previously
exposed region of the drum swings into contact with the toner,
and then with the paper.
Because the length of a standard printed page is a lot
larger than the circumference of the drum in a modern
photocopier, one full rotation of the drum will only replicate
a small piece of the page. The drum actually has to be
cleaned, recharged with ions, exposed to photons, and
sprinkled with toner multiple times in order to duplicate the
entire original. To the casual observer, the process appears
continuous, because it's all seamlessly coordinated inside the
photocopier as the drum rotates.
Corona Wires For a photocopier to work, a
field of positive charges must be generated on the surface of
both the drum and the copy paper. These tasks are accomplished
by the corona wires. These wires are subjected to a
high voltage, which they subsequently transfer to the drum and
paper in the form of static electricity.
Photo courtesy Xerox The corona wire uses static electricity to
coat both the photoreceptive drum and the copy paper
with a layer of positively charged
ions.
One of these wires is stretched parallel to the drum
surface and charges the photoconductive surface with positive
ions, and the other wire is positioned to coat the paper's
surface as the paper shoots by on its way to the drum.
Lamp and Lenses Making a photocopy requires
a light source with enough energy to boot electrons out
of the photoconductive atoms. What wavelengths of light can do
this? It turns out that most of the visible
spectrum of light contains enough energy to drive the
process, especially the green and blue end of the spectrum.
Anything lower than the red portion of the visible spectrum
doesn't have enough gusto to activate the photoconductor. And,
although UV light has more than enough firepower to make a
photocopy, it can be very damaging to our eyes and skin. This
is why photocopiers use a plain old incandescent
or fluorescent
bulb to flash light onto the original document.
A strong lamp illuminates
the sheet of paper to be
copied.
When the lamp in the copier is turned on, it moves across
the inside of the copier, illuminating one strip of the paper
at a time. A mirror attached to the lamp assembly directs
reflected light through a lens onto the rotating drum below.
The lens works just like the one on your camera. It allows you
to focus a copy of the image in a specific place. Although you
can't really focus the image on a photocopier to make the
final product more or less blurry, you can change the distance
between the lens and the original or between the lens and drum
to either reduce or magnify the size of the
original image on your copy.
Toner Toner is sometimes referred to as
dry ink, but toner isn't actually ink at all! Ink is a
pigmented liquid. Toner is a fine, negatively charged,
plastic-based powder. The black color in photocopier toner
comes from pigments blended into the plastic particles while
they are being made.
Photo courtesy Xerox A small bead coated with particles of
toner
In your photocopier, toner is stuck on larger, positively
charged beads and stored inside a toner cartridge. When
toner-coated beads are rolled over the drum, the toner
particles find the positively charged ions on the unexposed
areas on the drum's surface much more attractive than the
weakly charged bead. The same particles are subsequently even
more drawn to the electrostatically charged paper. The plastic
in the toner lets you keep it from jumping ship once you've
finally got it on the paper; all you have to do is apply heat
to the toner, and the plastic particles melt and fuse the
pigment to the paper.
The Fuser The fuser
provides the finishing touches that make the toner image on a
sheet of paper permanent. The fuser has to do two things:
Melt and press the toner image into the paper
Prevent the melted toner and/or the paper from sticking
to the fuser
All that's required to accomplish these tasks is quartz
tube lamps and Teflon-coated rollers. The sheet of
paper is sent between two of the rollers. Then, the rollers
gently press down on the page to embed the toner in the paper
fiber. Meanwhile, inside the rollers, the lamps are on,
generating enough heat to melt the toner. Why doesn't the
toner melt onto the rollers instead? Just like non-stick
coating prevents your dinner from becoming glued to the bottom
of your frying pan, the Teflon coating the rollers keeps the
toner and paper from sticking to them.
Putting It All Together In a photocopier,
the light-induced conductivity of the drum is exploited
to create a latent image in the form of electrical
charges on the surface of the drum. This image is made visible
and transferred to paper using a special, charged toner.
Here's how it all comes together to make a copy:
For the photocopier to work its magic, the surface of
the photoconductive material must first be coated with a
layer of positively charged ions by the corona wire.
Before you press start, the photoconductive
selenium, germanium, or silicon surface of the drum is
already blanketed with positive
charge.
When you hit the Start button, a strong lamp moves
across the inside of the copier and casts light onto the
paper you're copying, and the drum starts to rotate. As
light reflects off of blank areas of the paper, mirrors
direct it through onto the drum surface. Like dark clothing
on a hot sunny day, the dark areas of the original absorb
the light, and the corresponding areas on the drum's surface
are not illuminated.
In the places that light strikes the rotating drum, the
energy of the photons
kicks electrons away from the photoconductive atoms.
Opposites attract -- the positively charged ions coating
the photoconductive layer attract the freed electrons. The
marriage of one ion and one electron produces a neutral
particle. Charged particles remain only in places where
light didn't hit the drum because it wasn't reflected from
the original -- the dark spaces taken up by text and
pictures on the page!
This part of the process loosely resembles how a camera
takes a picture. If you've read How
Photographic Film Works, you know that when film is
exposed to light, the energy of the photons causes chemical
changes in the silver halide grains coating the film.
This creates a negative image of what you see through
the viewfinder. With a photocopier, however, you end up with
a real image created from a pattern of positive
charges left after exposure to light. And while you have to
develop film using special chemical processes and print it
on light-sensitive photographic paper, the photocopier
produces a visible image with only dry ink, heat and regular
paper.
Voltage is applied to the aluminum core of the
drum. Since light renders selenium conductive, current can
flow through the photoconductive layer while the drum is
being illuminated, and the electrons released by the atoms
are quickly replaced by the electrons that form the current
flowing through the drum.
The exposed areas of the drum rotate past rollers
encrusted with beads of toner. Tiny particles of toner are
pressed against the drum's surface. The plastic-based toner
particles have a negative charge and are attracted to areas
of positive charges that remain on the drum's surface.
The corona wire passes over a sheet of paper so
that the paper's surface becomes electrically charged.
The area of the drum freshly coated with toner spins
into contact with a positively charged sheet of paper. The
electric field surrounding the paper exerts a stronger pull
than the ions coating the drum's surface, and the toner
particles stick to the paper as the drum passes by.
Once the entire original has been recreated on toner in
the page, the paper proceeds on through the copier to the
fuser. The weak attraction between the toner
particles and the surface of the sheet of paper can easily
be disrupted. To fix the toner image in place on the paper's
surface, the entire sheet is shunted through the fuser's
heated rollers. The heat melts the plastic material in
the toner and fuses the pigment to the page.
By the time you reach for your copy in the collection tray,
the photocopier has already prepared for the next go-round by
again cleaning off the drum's surface and applying a fresh
coat of positively charged ions to it.
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