Photocopying

Thermography ("heat writing") is a method of copying that is based on the fact that dark regions of a document absorb heat more readily than do light spaces. If heat is applied to this page, for example, it will be absorbed more readily by the letters on the page than by the white spaces between the letters. As with electrostatic copying, themographic copying requires the use of specially treated paper. The paper used in thermography is coated with ferric [iron(III)] compounds in an acidic environment. When the paper is exposed to heat, a chemical reaction occurs that produces a dark image.

In use, a thermographic copy machine requires that the original document and the copy paper be placed into the machine in contact with each other. Some machines also use a "transfer sheet" placed in contact with the copy paper on the opposite side of the original document. A beam of infrared light is then shined through the document-copy paper (or document-copy paper-transfer sheet) combination. The infrared light heats dark spaces on the original document more strongly than light spaces. These heated areas—the places where text occurs on the document, for example—then cause darkening on the copy paper, producing a positive image copy of the original document.