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Photocopiers



   

How Photocopiers Work Ė A Clear Picture

The purpose of photocopiers is to create a clear picture Ė a picture thatís an exact duplicate of the original. Look at the word Ė photocopy. Photo Ė picture of. Copy Ė duplicate. So, as we said, a photocopy is a duplicate of the original picture. But how could an exact copy be created? What kind of technology would it involve? Would it surprise you that the technology behind photocopying is based on two natural phenomena?

The amazing thing about how photocopiers work is the nature of these phenomena. These principles are:

  1. Materials of opposite electrical charges attract, and
  2. Some materials become better conductors of electricity when exposed to light.

When Chester F. Carlson applied these principles to his work Ė to find an alternative method of duplicating for the myriad of reports he had to produce Ė the result was the basis of todayís photocopier, printer and fax machine technology.

The basic principles of photocopying

Chester Carlsonís experiments followed a simple process:

  1. A photoconductive surface is given a positive electrical charge.
  2. The photoconductive surface is then exposed to the image of a document.
  3. The electrical charge dissipates in the exposed areas.
  4. Negatively-charged powder thatís spread over the surface sticks to the positively-charged areas through electrostatic attraction.
  5. A piece of paper is placed over the powder image and given a positive charge.
  6. The negatively-charged powder is attracted to the paper as itís separated from the photoconductor.
  7. Heat fuses the powder image to the paper, producing a copy of the original image.

Todayís copiers follow these principles

Carlsonís electrophotographic process, through much experimentation, has been greatly improved. Modern-day copiers are much advanced from the original model first put out by Xerox in 1948. Letís look at the electrophotographic process, after it was developed into what we know of today as the photocopier.

  • Inside every copier is a light-sensitive surface called a photoreceptor, consisting of a thin layer of photoconductive material thatís applied to a flexible belt or drum. This drum can be charged with a form of static electricity.
  • Also inside the copier is toner, a very fine black powder. The drum, when itís charged with static electricity, can attract the toner particles.
  • The drum can be selectively charged; that is, selected parts of it can be charged, thereby attracting toner to only those parts that are charged.
  • 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, or fused, to the paper with heat as soon as they come off the drum.

How do you like your copy?

So letís put all these principles together and see exactly what happens after you put your original on the platen (glass) and close the lid.

When the copier illuminates the sheet of paper on the glass surface of the 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 drumís surface to be neutralized. This leaves positive charges only where there are dark areas on the paper that didnít reflect light. These positive charges attract negatively-charged toner. The toner is then transferred and fused with heat to a positively-charged sheet of paper.

Thatís simply it! You get your copy exactly as you want it Ė exactly what you tell the photocopier to give you.

What about all those parts?

Donít you wonder how all those moving parts come together at exactly the right time to give you your copy? Letís see if we can see inside the photocopier to find out what happens when you press that ďStartĒ button.

  • For the photocopier to work, the surface of the photoconductive material must first be coated with a layer of positively-charged ions by the corona wire.
  • When you hit the ďStartĒ button, a strong lamp casts light onto the paper youíre copying, and the drum starts to rotate. As light reflects off blank areas of the paper, mirrors direct it onto the drum surface. The dark areas of the original absorb the light, and the corresponding areas on the drumís surface are not illuminated.
  • Where light strikes the rotating drum, the energy of the photons (small particles of electromagnetic energy) repels electrons away from the photoconductive atoms and towards the positively-charged ions coating the photoconductive layer. When one ion and one electron meet, they produce a neutral particle. Charged particles remain only in places where light didnít hit the drum because it wasnít reflected from the original.
  • Voltage is applied to the aluminum core of the drum, causing current to flow through the photoconductive layer while the drum is being illuminated. Then electrons which are 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 drumís surface. The plastic-based toner particles have a negative charge and are attracted to areas of positive charges that remain on the drumís surface.
  • The corona wire gives a sheet of paper an electrical charge.
  • The area of the drum freshly coated with toner spins into contact with the positively-charged sheet of paper. The electric field surrounding the paper exerts a stronger pull than the ions coating the drumís surface, and the toner particles adhere to the paper as the drum passes by.
  • Once the entire original has been recreated on toner in the page, the paper proceeds through the copier to the fuser. To fix the toner image in place on the paperís surface, the entire sheet is sent through the fuserís heated rollers. The heat melts the plastic material in the toner and fuses the pigment to the page.
  • The photocopier then prepares for the next cycle by again cleaning off the drumís surface and applying a fresh coat of positively-charged ions to it.

The principles of electrophotography are used in many machines today

All this from the dreams and the efforts of one man Ė Chester F. Carlson Ė the creator of xerography. Xerography is a unique process that depends on chemical, electrical, mechanical and software know-how. Besides photocopiers, this process is also used in printers and fax machines. The rapid and economical digital printing process most used today produces either one print or hundreds of identical prints in black and white or color. More importantly, the capability of xerography enables on-demand printing of complete documents such as brochures and books. Such on-demand printing can save time, reduce costs, and eliminate document obsolescence, overruns and warehousing.

So you can see, there are many reasons to gain a good knowledge and understanding of electrophotography and its subsequent technology, xerography. If you know whatís going on with your copier, youíll be able to get the maximum use of its features and benefits. And with that, youíre left with a clear picture.

About The Author

Gareth Marples is a successful freelance writer providing valuable tips and advice for consumers purchasing Xerox inkjet cartridges, pictures of light bulbs and . His numerous articles offer moneysaving tips and valuable insight on typically confusing topics.

This article on "How Photocopiers Work" reprinted with permission.

© 2004 - Net Guides Publishing, Inc.

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