Xerography (or Electrophotography) is a photocopying technique developed by Chester Carlson (Born Feb 8 1906 - Died Sep 19 1968) in 1938 and patented on October 6, 1942. (Patent Number(s) 2,297,691).

The Xerographic Process

Making a copy follows a set sequence of events involving the same components.

Step 1. Charging An electrical charge emitted from a charge corotron (.aka scorotron) is applied to a cylinder called a drum or a photoreceptor. This is done so that an even charge is present evenly across the drums surface.

Step 2. Exposure A document is passed over a lens at the same speed that the drum turns and a light is flashed at the document. Where there is text or image on the document, light will not penetrate to the drum however the opposite is true where there is no image. The light that penetrates serves to neutralize the charge on the drum where there is no image. The resulting charge that remains on the drum is called the 'latent' image and is a positive of the original document.

Step 3. Image Transfer As the drum continues to rotate, it passes over the 'toner fountain'. Toner is nothing more than fine particles of carbon. Developer is nothing more than iron particles. Developer attracts toner particles to it and it collects on the rough edges of the developer particle. The charge that is present on the drum is opposite to the charge of the toner particle and greater in strength so the toner particles are attracted to the drum in the latent image area. At this point, the paper is introduced into the equation and it travels between the drum and an electrical device known as a transfer corotron. Because of the principle in physics where equally charged particles repel and oppositely charge particles attract, the electrical force of the charge on the transfer corotron is greater than that of the drum causing the toner to be attracted from the drum and travel towards the paper.

Step 4. Fusing The paper with the toner deposited onto it now travels into the fusing unit where the toner that has been deposited onto it is melted using either heat or a combination of heat and pressure to fuse the toner particles into the fibre of the paper. The xerographic image transfer is complete and the drum rotates past a 'discharge' circuit that neutralizes the drum preparing it for the next revolution. In this final phase, the drum is also cleaned with a wiper blade that removes any residual toner that did not jump from the drum to the paper. In this way, there is no possibility that the residual toner will 'dirty' the next print.

External link

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• Xerography, The invention that no one ever wanted