From a user’s standpoint, a photocopier is one of the most self-explanatory pieces of office equipment. An original document is placed on a glass plate, a button is pushed and duplicate copies emerge within seconds. But the technology under the hood of a photocopying machine can be very complicated indeed.
The process begins with the original document. To a photocopier, all printed materials are patterns of light and dark. When the paper is placed on the glass table, it is in position to be bombarded with light. There is nothing especially exotic about the light source in a typical photocopier- it is either a fluorescent or incandescent bulb. This light source must be powerful enough to reflect the light from all the white areas of the paper into a mirror located beneath the glass table.
Just before a copy is made, several electrical processes must occur. Photocopiers feature a photosensitive cylinder or plate called a drum. This drum is coated with semiconductive materials such as silicon or germanium. In order to properly receive the light signals from the mirror, this drum must be charged with positive ions. These ions are generated by a ‘corona wire’- a length of wire stretched directly in front of the drum.
When high voltage electricity is sent through the corona wire, it releases a supply of positive ions to the entire surface of the drum. The drum would then be considered ‘charged’. The light sent through the mirror strikes the drum in small strips. The white portions of the text cause photons to strike the photosensitive material on the drum’s surface. Any area which receives these photons becomes negatively charge- a point which will become very important later in the process.
The drum in most photocopiers is smaller in diameter than a whole sheet of paper, so the charging process is repeated over and over again as the drum and mirror work in tandem. Even though it may appear to be a continuous process, the drum is actually charged several times during an average copying session.
Now that the drum contains enough positive and negative areas to make a good copy, the copy paper itself is also charged by a corona wire, but this time the charge is negative. As the paper passes over the drum, the positive charges on the drum are attracted to the paper’s negative charges. The copy paper now contains positively charged areas and negatively charged areas. The next ingredient is now ready to be added to the copying process.
Toner is a negatively-charged fine black powder that is very sensitive to heat; both of these properties allow it to be the perfect medium for photocopying. As the charged copy paper passes over the container of toner, the effect is similar to black pepper being attracted to a statically-charged balloon. The individual toner grains are instantly attracted to the positively-charged areas of the copy paper. The toner will not cling to any negative charges, so those areas will remain white. The result at this point is a very dusty piece of copy paper that could be erased easily.
The final step is to permanently fix the toner to the paper. Because toner is heat-sensitive, a photocopying machine uses a set of heated rollers to melt the toner into the paper. These rollers are properly called fusers because they fuse the toner and paper together. Rollers carry the fused paper out of the machine and into the hands of the operator.
Meanwhile, the drum has been recharged by the corona wire, a new piece of copy paper has been negatively charged, and fresh toner leaps onto this paper through static electricity. The process is repeated until the required number of copies, determined by an onboard computer, has been reached.
The drum is then allowed to lose its charge and the machine becomes dormant. This is why you cannot reprint the last document the machine copied- the drum becomes a blank slate after each use.