A blueprint is a reproduction of a technical drawing or engineering drawing using a contact print process on light-sensitive sheets. The process was introduced by Sir John Herschel in 1842 and allowed for rapid and accurate production of an unlimited number of copies. The blueprint process is based on a photosensitive ferric compound, which allows the image to be printed on a piece of paper. The paper is impregnated with a solution of ammonium ferric citrate and dried. When the paper is illuminated, a photoreaction turns the trivalent ferric iron into divalent ferrous iron, producing a negative image with the drawing appearing in white against a dark blue background.
Blueprints are used to copy large architectural and construction drawings. They are also used for copying maps, mechanical drawings, and architects plans. The term "blueprint" is usually used to describe two printing methods, the blueprint and the diazotype. The blueprint process uses white lines on a blue background or blue lines on a white background.
Although traditional blueprints became obsolete when less expensive printing methods and digital displays became available, the term is still used to refer to any type of floor plan or technical drawing.
