How does a photocopier work?
The office photocopier has been around for some time now and has seen many changes over the years and is now a clear example of
modern technology.
In years gone by you would resort to making carbon copies of important documents, 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 article, we will explore what happens after you press "Start" on a modern
copier.
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 photocopier uses a similar process.
Inside a photocopier 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 photocopier 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.
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.
What’s 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.
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.
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.
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 unit
The fuser unit 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 fuse
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.
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.
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.
There’s a lot more to the office copier than the mechanics these days, take a look at the multifunctional device and see what today’s copier can offer!
The Photocopier and it's history
The inventor of the photocopier was Chester Carlson
In 1937, a process called Xerography was invented by an American law student Chester Carlson. Chester had invented a new copying
process based on electrostatic energy. Xerography became available in 1950 by the Xerox Corporation. Xerography comes from the Greek
for "dry writing".
The inventor Chester was born on February 8, 1906 in Seattle. His father was a barber who ended up settling the
family in San Bernardino, California. Unfortunately, his father developed crippling arthritis. Then, to make things
even worse, both mom and dad contracted tuberculosis. By the time Chester was fourteen years old, he was the main source
of income for the Carlson household. His mother died when Chester was seventeen years old.
Despite all of Chester's hardships, he
managed to enroll himself in a junior college at Riverside, California. He then moved on to earn his Bachelor of Science
degree in Physics from the California Institute of Technology in 1930. This left Carlson $1,400 in debt during the Depression.
Finding a job to pay off this debt was not easy. Chester sent out letters to 82 different companies. He only received two replies,
and no job offers.
Carlson ended up working as a research engineer for Bell Laboratories in New York City for just $35
per week. This job didn't last very long. As the depression deepened, Bell was forced to lay Carlson off.
Realizing
that he probably could not find a job in his desired field, Carlson settled for a job at the electronics firm of P.
R. Mallory, which was famous for its batteries. He was eventually promoted to manager of Mallory's patent department.
At night, he went to law school to become a patent lawyer.
The job at Mallory ended up leading Carlson to the invention
that would change the world. He found that there were never enough copies of patents around. There were only two choices
at the time to get more copies: either send the patents out to be photographed or laboriously write new ones. Both methods
proved to be very expensive and time consuming. To make matters worse, Carlson was nearsighted and started to suffer from crippling
arthritis.
Carlson knew there had to be a better way to make copies. The only problem was that no one knew how. He was going to find
out.
Carlson would have liked to have immediately found a solution
to this copy problem, but it wasn't that easy.
Chester's first step was to head straight to the library - the New York
Public Library to be specific. He spent many months pouring through tons of scientific articles. Articles related to
the field of photography were immediately ruled out. This field was loaded with tons of corporate researchers that had
extensively analyzed every nook-and-cranny of the process. Besides, photography was a wet and messy process. No, the
answer to quick copies had to lie elsewhere.
Carlson turned his attention to the field of photoconductivity. This was
a relatively new field that was discovered by the Hungarian physicist Paul Selenyi. It seems that when light strikes
the surface of certain materials, it's conductivity (flow of electrons) increases.
Carlson, being a physicist, had that
flash of inspiration that all inventors talk so much about. Perhaps you just made the same realization. (You're about
seventy years too late.) Carlson realized that if the image of an original photograph or document were projected onto
a photoconductive surface, current would only flow in the areas that light hit upon. The print areas would be dark and
not allow any current to flow.
But, as all inventors know, inspiration doesn't make for an invention, it's the perspiration.
You know, thirty seconds to think of the solution, sometimes an entire lifetime to actually get it to work.
Carlson
set up his lab in every inventor's favorite workplace, the kitchen. It was in the kitchen of his Jackson Heights, Queens
apartment that the basic principles of what he termed "electrophotography" were
put down. His first patent was applied for in October of 1937.
Unfortunately, his wife was getting sick of these endless
experiments and demanded that he get out of her kitchen. (She eventually walked out of his life for good. I bet she
regretted that decision after he made his millions.) The laboratory was moved to a room in the back of a beauty salon
owned by his mother-in-law in Astoria, Queens. Since he was suffering from arthritis and had little patience for the
endless experiments, Chester hired an unemployed German physicist named Otto Kornei to help him out.
You may remember
from your schooldays that sulfur is a yellow mineral that does not conduct electricity. This is true, but when exposed
to light, it will conduct a small amount of charge. So, one day Otto took a zinc plate and covered it with a coating
of freshly prepared batch of sulfur. He then wrote the words "10-22-38
Astoria" on
to a microscope slide in India ink. The room was darkened. The sulfur was rubbed with a handkerchief to give it a charge.
The slide was then placed on top of the sulfur and placed under a bright light for a few seconds. The slide was then
removed and the sulfur surface was covered with lycopodium powder (the waxy spores from clubmoss).
With one giant breath
of air, the lycopodium was blown off of the sulfur surface. And there it was - an almost exact mirror image that said
- you guessed it - "10-22-38
Astoria".
The real trick was in preserving the image. Carlson took wax paper and heated it over the remaining powder. The wax
cooled around the spores and was then peeled away. Yes, the first photocopy (if you consider the spores of a fungus
to be a copy) had been made. Needless to say, this product was not quite ready for the office. A tremendous amount of
work still needed to be done, but Carlson's theory was confirmed. But, research takes money, and Carlson didn't have
any. Kornei couldn't see where this was all leading and quit. He went to work for IBM and was later rewarded for his
efforts with stock from Carlson.
With such a great product, one would think that the companies would be banging at his
door throwing large wads of cash into his lap. This was not the case. Between 1939 and 1944, Carlson was turned down
by more than twenty of the large corporations, including IBM, Kodak, General Electric, RCA, and the like.
During this
time, Carlson continued his work at P. R. Mallory, which occasionally took him to the Battelle Memorial Institute, a
nonprofit organization that invested in technological research. During one visit in 1944, Carlson casually mentioned
that he held several patents on a new reproduction process. As a result of this encounter, Battelle officials expressed
interest and signed a royalty-sharing deal with Carlson, giving Carlson a 40% share in the proceeds. Battelle was well
aware of the amount of research that needed to be done, but went to work to solve the many problems.
Battelle assigned
the project over to a man named Roland M. Schaffert, a research physicist and a former printer. Schaffert worked on
the project all by his lonesome self for nearly a year. (After all, this was during World War II and our nation's research
energy was focused elsewhere.) When the war ended, Battelle provided Schaffert a small group of assistants to improve
on the process.
The first step that the Battelle staff took was to develop a new photoconductive plate. Carlson's sulfur
plate just didn't do it.
Instead, Battelle developed a new plate that was covered with Selenium, which was a much better photoconductor. Next,
they spent nearly a year developing the corona wire to serve a dual role: to apply the electrostatic charge to the plate
and to transfer the powder from the plate to the paper.
One of the most important developments was the invention of
a better dry ink. Carlson's use of lycopodium powder and other materials produced a somewhat blurry image. Battelle
researchers substituted a fine iron powder for dry ink and mixed in ammonium chloride salt and a plastic material. The
ammonium chloride was included to clean up the image. It had the same charge as the metal plate, so in the areas where
there is low charge or no image, the iron particles stuck to the salt and not to the plate. The plastic material was
designed to melt when heated and fuses the iron particles to the paper. They called this material toner, since one could
very simply use different tones of developer to produce any color desired. (Three superimposed colors could be used to produce
full color copies.)
On January 2, 1947, Battelle signed a licensing agreement with a small Rochester company known as
Haloid. Haloid manufactured photographic products at the time and was looking for new technology to develop. Haloid’s
investment in electro-photography was a big gamble, since the company had only earned $101,000 on sales of $6,750,000
in 1946. The research would cost Haloid a minimum investment of $25,000 per year.
Battelle and Haloid demonstrated electro-photography
to the world on October 22, 1948, ten years to the day after Carlson's first successful experiment. The first photocopiers
were introduced in 1949. The whole process was inefficient and was not practical when making a dozen or more copies.
It took fourteen different steps by the user and some forty-five seconds to produce a single copy. These flat plate
(as opposed to the rotating drums currently used) machines were rejected for being too complicated.
Back to the drawing
board.
Haloid came up with a better name for the process. Somehow the name electro-photography was not very catchy.
An Ohio State professor suggested xerography from the Greek words xeros for "dry" and graphos for "writing". Haloid named its first
photocopier the XeroX Model A, the last X being added to make the name similar to that of Kodak, another Rochester corporation.
In 1958, Haloid officially changed their name to Haloid Xerox, and finally to just plain old Xerox in 1961.
Success
didn't really come to Haloid until 1959 when they introduced the Model 914, the first fully automated photocopier. It
was called the 914 because it could handle paper up to 9 x 14" in
size (legal size). This machine was so popular that by the end of 1961 Xerox had nearly $60,000,000 in revenue. By 1965,
revenues leaped to over $500,000,000.
Of course, all good things must come to an end. Chester Carlson, finally enjoying
the profits from his years of hard work, collapsed and died on September 19, 1968 while walking down 57th Street in
New York City. He had been attending a conference and was on his way to see a movie during some spare time. Of the estimated
$150 million dollars he had earned from Xerox, he had generously given about $100 million to charity.
A very nice deed
on the part of the man who changed our lives forever. And to think that nobody wanted his invention... |