Analog photography is the use of light-sensitive materials to produce permanent visible images (photographs). The most familiar photographic processes depend on the light-sensitivity of the silver halides. A photographic emulsion is a preparation of tiny crystals of these salts suspended in a thin layer of gelatin coated on a glass, film, or paper support. On brief exposure to light in a camera or other apparatus, a latent image in activated silver salt is formed wherever light has fallen on the emulsion. This image is made visible in development, when the activated silver halide crystals (but not the unexposed ones) are reduced to metallic silver (black) using a weak organic reducing agent (the developer). The silver agent is then made permanent by fixing, in the course of which it becomes possible to examine the image is light for the first time. Fixing agents (fixers) work by dissolving out the silver halide crystals which were not activated on exposure. The image made in this way is densest in silver where the original subject was brightest, and lightest where the subject was darkest; it is thus a "negative" image. To produce a positive image, the negative (which is usually made on a film or glass (plate) support) is itself made the original in the above process, the result being a positive "print" usually on a paper carrier. An alternative method of producing a positive image is to bleach away the developed image on the original film or plate before fixing, and re-expose the unactivated halide in diffuse light. This forms a second latent image which on development produces a positive image of the original subject (reversal processing).
The history of photography from the earliest work of Niépce, Daguerre (see daguerreotype, and Fox Talbot to the present has seen successive refinements in materials, techniques, and equipment. Photography became a popular hobby after George Eastman first marketed roll film in 1889. The silver halides themselves are sensitive to light only from the blue end of the spectrum so that in the earliest photographs other colors appear dark. The color-sensitivity of emulsions was improved from the 1870s onward as small quantities of sensitizing dyes were incorporated. "Orthochromatic" plates became available after 1884 and "panchromatic" from 1906.
Motion picture photography dates from 1890, when Thomas Edison built a device to expose Eastman's roll film, and motion pictures rapidly became an important art form. Not all modern analog photographic methods employ the silver-halide process; xerography and the blueprint and ozalid processes work differently. False-color photography and the diffusion process used in the Polaroid Land camera (invented by Edwin Land) are both developments of the silver-halide process.
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