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Colour Photocopiers

Colour photocopiersphotocopiers of history

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Although coloured toner became available in the 1950s, full-colour photocopiers were not commercially available until 3M released the colour-in-colour photocopier in 1968, using what is known as a dye-sublimation process rather than conventional electrostatic technology. It was Canon that released the first electrostatic colour photocopier in 1973. -- More on History of The Photocopier

Colour photocopying has always been a concern to governments, as right from the start it became a possibility to counterfeit currency. In many countries anti-counterfeiting technologies have been employed with their currency in order to make it more difficult to accomplish counterfeiting using a colour photocopier. These technologies include watermarks, microprinting, holograms, tiny security strips made of plastic and-or foil, and ink that seemingly changes colour as the currency is viewed from an angle. Most high quality colour photocopiers have special software hard-coded in the machine that prevents photocopying currency that contains a special pattern.

Digital Technology in Colour Photocopiers

how the scanner in a digital photocopier scans and records the imageAll today's colour photocopiers use digital technology in contrast to the older, now obsolete, analogue technology. With digital photocopying, the photocopier effectively consists of an integrated scanner and laser printer. This new design is able to employ the latest in reprographic technologies making the potential for higher quality prints and colour photocopying. Automatic image quality enhancement, utilising digital lens, high resolution scanning and interpolation, and the ability to "build jobs" - that is, to scan page images independently of the process of printing them - go towards making colour photocopiers a much more highly efficient office utility with the utilisation of today's digital technologies.

Using these digital technologies colour photocopiers have become much more than their name suggests, with the trend moving towards referring to them as Multifunctionals, MFPs (MultiFunctional Printers) and MFDs (MultiFunctional Device). With the ability to function as high-speed colour scanners and typically offering the ability to send colour documents via email or to make them available on file servers these Document Managers - as they are also aften called - truly are Multifunctional, replacing other such office utilities such as colour printers, fax machines, scanners and pdf writers.

A great advantage for colour photocopiers by way of modern digital technology is 'scan once print many'. It's old news nowadays but with an old analogue copier, either each page would have been scanned twenty times (a total of 400 scans), making one set at a time, or twenty separate output trays would have been used for the twenty sets. With modern digital scanning technology a digital copier scans each page only once then uses the stored information to produce twenty sets and releases the copies into the exit tray or finisher by digital offset for easy retrieval.

So how does a Colour Photocopier Work?

What is Inside a Photocopier?

If you dismantle a photocopier you will find how many different components make the machine. However, when it comes to the actual photocopying process only a few key elements are necessary: Photoreceptor drum (or belt); Corona wires; Lamp and lenses; Toner; Fuser

In the following sections, we explain each of these parts.

Photoreceptor Drum

Photoreceptive photocopier drumThe 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 selenium or 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 neutralises the positive charge coating the drum to form the latent image.

Corona Unit to transfer Charge to DrumCorona 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

Colour Photocopier Toner ColoursToner 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 colour 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.

Fuser unit in PhotocopierThe 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: 1. Melt and press the toner image into the paper 2. 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.

How a Photocopier works

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. Diagram of PhotocopierAs 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. ThereĀ“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!

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Read further information on How Colour Printers Work - the Physics (more technical).

 

Colour Photocopiers

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