The process of xerography is widely used for making photocopies. The basic idea behind the process was developed by Chester Carlson of USA. Carlson developed the technique in 1938 and was granted a patent for his invention in 1942. Carlson originally called his invention electrophotography. It was later renamed xerography.
In 1947, the Xerox Corporation launched a full-scale programme to develop automated duplicating machines using Carlson’s process. Carlson’s original process was cumbersome, requiring several manual processing steps with flat plates. It took almost two decades to develop a fully automated process. A dozen companies showed no interest and had rejected Carlson’s idea. This first commercial automatic copier, the Xerox-914 was released in 1960. The huge success of that development is quite evident. Today, practically all offices and libraries have one or more duplicating machines and the capabilities of these machines continue to evolve.
The basic principles of the xerographic process are based on simple concepts from electrostatics (a branch of physics dealing with the phenomenon involving stationary or slow-moving electric charges) and optics. What makes the process unique is that it uses a photoconductive material to form an image. A photoconductor is a material that is a poor conductor of electricity in the dark and becomes a reasonably good conductor in the presence of light. The widely used material is selenium or one of its compounds.
The steps involved in the xerographic process are as follows. First, the surface of a plate or drum is coated with a thin film of the photoconductive material and it is given a positive charge in the dark. The page to be photocopied is then projected onto the charged surface. The photoconductive surface becomes conducting only in areas where light strikes. The charges remain on those areas of the photoconductor not exposed to the light. This creates a hidden image of the object in the form of a positive surface charge distribution.
Next, a powder called toner is negatively charged and dusted on the photo-conducting surface. The negatively charged powder adheres only to the areas that are positively charged image. At this stage the image becomes visible. It is then transferred to the surface of a sheet of positively charged paper. Finally, the toner is fixed to the surface of the paper by heat. This results in a permanent copy of the original. The whole process is done within fraction of a second, as we know.
The steps for producing a document on a laser printer are essentially similar to those used in a photocopying process. There is one basic difference. The difference between the two techniques lies in the way the image is formed on the selenium-coated drum. In a laser printer, the command to print the letter A, for instance, is sent to a laser from the memory of a computer. A rotating mirror inside the printer causes the beam of the laser to sweep across the selenium-coated drum in an interlaced pattern. Electrical signals generated by the printer turn the laser beam on and off in a pattern that traces out the letter A in the form of positive charges on the selenium. Toner is then applied to the drum and the transfer to paper is accomplished as in a photocopy machine.
[The writer teaches at Engineering Department, Salalah College of Technology (SCT), Salalah, Sultanate of Oman. [email protected]]