Photocopying is the production of copies of original documents; this is when the creation of the copies occurs by means of exposure to chemicals, electrostatic energy, heat or light. A sensitized surface records the images that result from the photocopying process.
Photocopying involves the use of an original document. Other processes of duplication do not use original documents. Such alternatives, all of which require the presence of a distinct master copy, include offset lithography and stencil reproduction. The American physicist Chester F. Carlson invented the modern xerographic photocopying process in 1938. Before then, people used blueprinting or whiteprinting processes to make copies of documents.