A blueprint is a reproduction of a technical 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. Originally, blueprints consisted of white lines on a blue background, but a more recent process uses blue lines on a white background. The term "blueprint" is usually used to describe both printing methods, the blueprint and the diazotype. The blueprint process is now obsolete and has been largely replaced by digital computer-aided construction drawings. However, the term "blueprint" continues to be used less formally to refer to any floor plan or any type of plan. Practicing engineers, architects, and drafters often call them "drawings," "prints," or "plans". In general, a blueprint is a guide for making something, a design or pattern that can be followed.