In order to understand how instant photographs are developed within a few seconds, some information on the structure of traditional photographic film, as well as film used in polaroid cameras has to be given. Traditional color films used in photography consist of a plastic base coated with three layers of light sensitive silver compounds. The top layer is sensitive to blue light, the middle one to green light and the bottom layer to red light. When the film is exposed to light the sensitive grain on each layer reacts to it, forming metallic silver at that layer. Therefore, a chemical record of the light and color pattern is created. In order to receive a picture from this, the film has to be developed.
The developing process in polaroid cameras is similar to that of normal film, but the developing chemicals are already present in the film itself. It has its own built-in developing studio. The undeveloped polaroid has different layers. The base is formed by the black layer, followed by three light-sensitive layers in red, green and blue, which are separated from each other by dye coupler layers. On the very top there are the image, timing and acid layers.
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The chemical that is needed to set off the developing process is called reagent. It is situated between the light-sensitive layer and the image layer. The reagent is a mixture of opacifiers, which block light, alkali and white pigments that make the undeveloped polaroid opaque, to prevent light from further exposing the photograph. When the picture is ejected by the camera in between two metal rollers, the chemical packets in the camera are pinched on bottom of the film, broken open and the reagent is spread over the surface of the image, starting a series of chemical reactions. The reagent chemicals move downwards, changing the exposed particles in each layer into metallic silver. The reagent also dissolves the dye couplers, causing them to move towards the image layer. The silver compounds stop the dye from moving further. Therefore, only dye from the unexposed sections continues to move upwards to the image layer to form the picture. In the meantime the alkali and the opacifier in the reagent react with the acid layer of the paper, making the opacifier layer become clear. In this vein the image starts to appear. The timing layer is used to slow down the reagent on its path to the acid layer, so the opacifiers do not turn clear before the image is fully developed underneath. As soon as the opacifiers are cleared up by the acid layers, the fully developed photograph can be seen.
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