A blueprint is a reproduction of a technical drawing or engineering drawing using a contact print process on light-sensitive sheets introduced by Sir John Herschel in 1842. The traditional white-on-blue appearance of blueprints is a result of the cyanotype process, which allowed rapid and accurate production of an unlimited number of copies of an original reference. It was widely used for over a century for the reproduction of specification drawings used in construction and industry. Blueprints were characterized by white lines on a blue background, a negative of the original. Color or shades of grey could not be reproduced.