photograph

A photograph (also known as a photo) is an image created by light falling on a photosensitive surface, usually photographic film or an electronic image sensor, such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of what the human eye would see. The process and practice of creating such images is called photography. The word photograph was coined in 1839 by Sir John Herschel and is based on the Greek φῶς (phos), meaning "light," and γραφή (graphê), meaning "drawing, writing," together meaning "drawing with light."

The first permanent photograph, a contact-exposed copy of an engraving, was made in 1822 using the bitumen-based "heliography" process developed by Nicéphore Niépce. The first photographs of a real-world scene, made using a camera obscura, followed a few years later, but Niépce's process was not sensitive enough to be practical for that application: a camera exposure lasting for hours or days was required. In 1829 Niépce entered into a partnership with Louis Daguerre and the two collaborated to work out a similar but more sensitive and otherwise improved process.

After Niépce's death in 1833 Daguerre concentrated on silver halide-based alternatives. He exposed a silver-plated copper sheet to iodine vapor, creating a layer of light-sensitive silver iodide; exposed it in the camera for a few minutes; developed the resulting invisible latent image to visibility with mercury fumes; then bathed the plate in a hot salt solution to remove the remaining silver iodide, making the results light-fast. He named this first practical process for making photographs with a camera the daguerreotype, after himself. Its existence was announced to the world on 7 January 1839 but working details were not made public until 19 August. Other inventors soon made improvements which reduced the required exposure time from a few minutes to a few seconds, making portrait photography truly practical and widely popular.

The daguerreotype had shortcomings, notably the fragility of the mirror-like image surface and the particular viewing conditions required to see the image properly. Each was a unique opaque positive that could only be duplicated by copying it with a camera. Inventors set about working out improved processes that would be more practical. By the end of the 1850s the daguerreotype had been replaced by the less expensive and more easily viewed ambrotype and tintype, which made use of the recently introduced collodion process. Glass plate collodion negatives used to make prints on albumen paper soon became the preferred photographic method and held that position for many years, even after the introduction of the more convenient gelatin process in 1871. Refinements of the gelatin process have remained the primary black-and-white photographic process to this day, differing primarily in the sensitivity of the emulsion and the support material used, which was originally glass, then a variety of flexible plastic films, along with various types of paper for the final prints.

Color photography is almost as old as black-and-white, with early experiments including John Herschel's Anthotype prints in 1842, the pioneering work of Louis Ducos du Hauron in the 1860s, and the Lippmann process unveiled in 1891, but for many years color photography remained little more than a laboratory curiosity. It first became a widespread commercial reality with the introduction of Autochrome plates in 1907, but the plates were very expensive and not suitable for casual snapshot-taking with hand-held cameras. The mid-1930s saw the introduction of Kodachrome and Agfacolor Neu, the first easy-to-use color films of the modern multi-layer chromogenic type. These early processes produced transparencies for use in slide projectors and viewing devices, but color prints became increasingly popular after the introduction of chromogenic color print paper in the 1940s. The needs of the motion picture industry generated a number of special processes and systems, perhaps the best-known being the now-obsolete three-strip Technicolor process.