Photography
    
    Photography, technique of producing permanent images on sensitized surfaces by means of the photochemical action of light or other forms of radiant energy.
    In today's society, photography plays important roles as an information medium, as a tool in science and technology, and as an art form, and it is also a popular hobby. It is essential at every level of business and industry, being used in advertising, documentation, photojournalism, and many other ways. Scientific research, ranging from the study of outer space to the study of the world of subatomic particles, relies heavily on photography as a tool. In the 19th century, photography was the domain of a few professionals because it required large cameras and glass photographic plates. During the first decades of the 20th century, however, with the introduction of roll film and the box camera, it came within the reach of the public as a whole. Today the industry offers amateur and professional photographers a large variety of cameras and accessories.
    Basic Technology
    Light is the essential ingredient in photography. Nearly all forms of photography are based on the light-sensitive properties of silver-halide crystals, chemical compounds of silver and halogens (bromine, chlorine, or iodine). When photographic film, which consists of an emulsion (a thin layer of gelatin) and a base of transparent cellulose acetate or polyester, is exposed to light, silver-halide crystals suspended in the emulsion undergo chemical changes to form what is known as a latent image on the film. When the film is processed in a chemical agent called a developer, particles of metallic silver form in areas that were exposed to light. Intense exposure causes many particles to form, while weak exposure causes few to form. The image produced in this manner is called a negative because the tonal values of the subject photographed are reversed-that is, areas in the scene that were relatively dark appear light, and areas that were bright appear dark. The tonal values of the negative are reversed again in the photographic printing process or, when preparing color transparencies (slides), in a second development process.
    Photography, then, is based on chemical and physical principles. The sensitivity of silver halides to light is the primary chemical principle. The physical principles are governed by the physics of light . The generic term light refers to the visible portion of a broad range of electromagnetic radiation, which includes radio waves, gamma rays, X rays, infrared, and ultraviolet rays. The human eye is sensitive to only a narrow band of electromagnetic wavelengths, called the visible spectrum. The spectrum comprises the full range of color tones. To the eye, the longest wavelengths register as red, the shortest as blue.
    Photographic Film
    Photographic films vary in the way they react to different wavelengths of visible light. Early black-and-white films were sensitive only to the shorter wavelengths of the visible spectrum-that is, to light perceived as blue. Later, colored dyes were added to film emulsions to make the silver halides responsive to light of other wavelengths. These dyes absorb light of their own color, making silver halide particles sensitive to light of that color. Orthochromatic film, incorporating yellow dyes in the emulsion and sensitive to all light but red, was the first improvement on simple blue-sensitive film.
    In panchromatic film, the next major improvement, red-toned dyes were added to the emulsion, rendering the film sensitive to all visible wavelengths. Although slightly less sensitive to green tones than the orthochromatic type, panchromatic film is better able to reproduce the entire range of color tones. Thus, most films now used by amateur and professional photographers are panchromatic.
    Two additional varieties of black-and-white film-process and chromogenic-have their special uses. Process film is used primarily for line reproduction of copy in the graphic arts. Such films have extremely high contrast, producing images with no tonal values between black and white. Chromogenic film produces dye images rather than silver images on the negative. Using dye couplers and silver halide in the emulsion, it can be developed by standard color-negative development processes. After development, the silver is bleached out of the film, leaving a black-and-white dye image.
    Special-purpose films are sensitive to wavelengths beyond the visible spectrum. In addition to visible light, infrared film also responds to the invisible, infrared portion of the spectrum (see Infrared Photography, below).
    Instant film, introduced by the Polaroid Corporation in the late 1940s, provides photographs within seconds or minutes of the taking of the picture, using a camera specially designed for this purpose. In instant film the processing chemicals and emulsion are combined in a self-contained envelope or on the print itself. Exposure, development, and printing all take place inside the camera. Polaroid, the leading manufacturer of this film, uses a conventional silver-halide emulsion. After the film is exposed and a negative image produced, the negative is sandwiched with photographic paper and processing chemicals, and a fogging agent transfers the negative image to the paper, producing a print. A number of instant films are manufactured in a 35-millimeter format, both in black and white and in color.
    Color Film
    Color films are more complex than black and white because they are designed to reproduce the full range of color tones as color, not as black, white, and gray tones. ............