The photocopiers commonly found in offices and classrooms use electric charges to transfer the image of an original document to a plain piece of paper. The document to be copied is placed face down on the platen and illuminated by a lamp. Its image is directed to a negatively charged metal drum (the electrostatic drum) by a series of mirrors. Where light strikes the drum, the charge disappears, so that dark areas remain charged. Next, positively charged particles of toner powder are brushed onto the drum. These stick only to the charged areas. The image on the drum is then transferred to a piece of paper that has been given a negative charge. Finally, a heater is used to seal the toner to the paper. Unfortunately, the process produces either black or white images and cannot produce greys very well.

To overcome this limitation, the latest copiers use digital images made up of a myriad of dots each recorded as a computer record. These are then reproduced from the computer file at resolutions up to 600 dots per inch.

Xerography is the electrostatic dry-printing process for the reproduction of images or documents which is widely employed in the office systems of commerce and industry in copying machines. However, it is being superceded by newer processes associated with the latest digital copiers.

The Xerograph process was invented by the American printer Chester F. Carlson in 1937 and was first developed commercially in 1950. It is based the principle of photoconductivity in which certain substances resist the passage of an electric current except when struck by light. Silicon, germanium, and selenium are normally poor conductors of electricity, but light energy is absorbed by the surface electrons and then passes from one atom to another, thus enabling a current to flow when a voltage is applied. When the light is removed, their conductivity again becomes low.

Xerography employs a photoconductive insulating layer such as selenium on a conductive metal support, eg aluminium. The layer is charged electrostatically, either with positive or negative ions, the polarity depending on the type of material employed. When the plate is exposed, in a camera or photographic machine, those areas of the coating subjected to light lose a varying portion of the charge, depending upon the intensity of the illumination. Thus, the variation of the amount of charge retained on the coated metal plate is established as an electrical or electrostatic pattern of the image.

The image is created by sprinkling over the exposed plate a special charged powder (the ink) which carries an opposite charge to the initial charge applied to the plate and insulating layer. The powder adheres to those areas that have retained their charge and but not to the other areas. The print is obtained by covering the plate with paper and applying to the back of the paper a charge of the same polarity as the initial charge applied to the photoconductive insulating layer. In this way the opposite charged powders are transferred to the paper surface. The powder image is then fused onto the paper by exposure to solvent vapours or heat to make the image permanent.

The entire xerographic process can be carried out, in high-speed mechanised equipment, in less than 5 seconds, and it is comparatively inexpensive to make these images because the photoconductive insulating layer can be recycled many thousands of times. The process has found its primary usefulness in copying office documents and in low-volume duplication of data.

A variety of automatic xerographic machines are available today that make office copies on the push of a button. Xerographic machines can reproduce half-tone photographs and can enlarge or reduce copy. They can be set to print on either one or both sides of paper, to produce automatically a large or small number of copies without further intervention by the operator, and to sort and collate. As in so many examples of technological development, problems have accompanied the benefits of the new process. The economy, versatility, and flexibility of the xerographic process have created legal problems relating to copyright and helped stimulate numerous changes in the copyright laws in many countries.

Xerography is essentially an analogue process depending on the intensity of the light received by the photoconductive layer. However modern computer technology is based on digital processes in which an image is produced by specifying the colour/intensity of a myriad of dots. The quality of the image depends on the number of dots per square inch and the accuracy with which each dot is produced.

With the increasing speed of modern computers it is now possible to digitally scan an image at 600 dots per inch and replicate those dots on a sheet of copy paper almost as fast as the traditional analoge Xerography process but with graeter accuracy. It is this capability that has resulted in the launch of high speed digital copiers which can copy original documents or produce copies from a computer file at rates in excess of 3500 A4 copies per hour, almost as fast as the best analogue copiers. A major advantage of the digital copiers is that over a hundred pages can be stored in the machines memory so the original need be scanned only once to produce hundreds of exact copies in the same order as the original pages. Gone are the days of recycling originals with all the streaks associated with multiple scanning as too are the days of the mechanical colators.

Colour xerography was developed in the late 1970s and, like colour printing, uses four basic colours to produce a full colour image. The original document is scanned for each of the four colours and the four images are sent via four corresponding print heads and, hence, to the copy. Not surprisingly, colour copiers are significantly slower than black only copiers but they are catching up quickly.