The term photocopying refers to the duplication of an existing document by using light. There are three major types of photocopying: photographic copying, which uses photographic techniques to develop copies on photosensitive materials; thermography, which is a dry-copying process that uses the action of heat on heat-sensitive chemicals; and xerography, which uses electrostatic charges, dry ink, and heat to fuse an image onto paper.

Although the process of photocopying is commonly thought to be a recent invention, the foundation for modern photocopying was laid before the nineteenth century. In 1780 the Scottish engineer and inventor James Watt took out a patent on a letter-copying machine after becoming frustrated with the inefficiency of making hand written copies of letters. The device consisted of a roller press which created a reverse impression on the copy paper. The paper which was used in this process was thin enough that the document could...