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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. (1970). 9780852291351, William Benton, Encyclopædia Britannica Inc.. ISBN 9780852291351 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.
The process is obsolete, initially superseded by the diazo-based whiteprint process, and later by large-format xerography photocopiers. It has since almost entirely been superseded by digital computer-aided construction drawings.
The term continues to be used informally to refer to any floor plan (and by analogy, ). Practising engineers, architects, and drafters often call them "drawings", "prints", or "plans". (2025). 9781631260513, The Goodheart-Wilcox Company, Inc.. ISBN 9781631260513
This is a simple process for the reproduction of any light transmitting document. and drew their designs on cartridge paper; these were then traced on to tracing paper using India ink for reproduction whenever needed. The tracing paper drawing is placed on top of the sensitized paper, and both are clamped under glass, in a daylight exposure frame, which is similar to a picture frame. The frame is put out into daylight, requiring a minute or two under a bright sun, or about thirty minutes under an overcast sky to complete the exposure. Where ultra-violet light is transmitted through the tracing paper, the light-sensitive coating converts to a stable blue or black dye. Where the India ink blocks the ultra-violet light the coating does not convert and remains soluble. The image can be seen forming. When a strong image is seen the frame is brought indoors to stop the process. The unconverted coating is washed away, and the paper is then dried. The result is a copy of the original image with the clear background area rendered dark blue and the image reproduced as a white line.
This process has several features:Ralph W. Liebing Architectural Working Drawings, John Wiley & Sons, 1999 page 576
Introduction of the blueprint process eliminated the expense of photolithographic reproduction or of hand-tracing of original drawings. By the later 1890s in American architectural offices, a blueprint was one-tenth the cost of a hand-traced reproduction.Mary N. Woods From Craft to Profession: The Practice of Architecture in Nineteenth-Century America University of California Press, 1999 , pages 239–240 The blueprint process is still used for special artistic and photographic effects, on paper and fabrics.Gary Fabbri, Malin Fabbri Blueprint to Cyanotypes – Exploring a Historical Alternative Photographic Process Lulu.com, 2006 page 7
Various base materials have been used for blueprints. Paper was a common choice; for more durable prints linen was sometimes used, but with time, the linen prints would shrink slightly. To combat this problem, printing on imitation vellum and, later, polyester film (Mylar) was implemented.
In the early 1940s, cyanotype blueprint began to be supplanted by diazo prints, also known as . This technique produces blue lines on a white background. The drawings are also called blue-lines or bluelines. Blueprints replaced by whiteprints Other comparable dye-based prints were known as blacklines. Diazo prints remained in use until they were replaced by Xerography print processes.
Xerography is standard copy machine technology using toner on copy paper. When large size xerography machines became available, 1975, they replaced the older printing methods. As computer-aided design techniques came into use, the designs were printed directly using a computer printer or plotter.
In the case of construction plans, such as road work or erecting a building, the supervising workers may view the "blueprints" directly on displays, rather than using printed paper sheets. These displays include mobile devices, such as or Tablet computer. Software allows users to view and annotate electronic drawing files. Construction crews use software in the field to edit, share, and view blueprint documents in real-time.
Many of the original paper blueprints are archived since they are still in use. In many situations their conversion to digital form is prohibitively expensive. Most buildings and roads constructed before 1990 will only have paper blueprints, not digital. These originals have significant importance to the repair and alteration of constructions still in use, e.g. bridges, buildings, sewer systems, roads, railroads, etc., and sometimes in legal matters concerning the determination of, for example, property boundaries, or who owns or is responsible for a boundary wall.
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