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Photographic hypersensitization
Photographic hypersensitization refers to a set of processes that can be applied to photographic film or plates before exposing. One or more of these processes is often needed to make photographic materials work better in long exposures.
Most photographic materials are designed for snapshot exposure of much less than one second. In longer exposures, such as those used in astrophotography, many such materials lose sensitivity. This phenomenon is known as low-intensity reciprocity failure (LIRF) or the Schwarzschild effect. The reciprocal relationship between flux and exposure time for photographic film implies that at a given light flux, doubling the exposure time would double the photographic effect. This holds with exposures up to a second or so, but in general does not hold over exposure times of minutes or hours. Several hypersensitization or "hypering" techniques have been developed to overcome this failure of the reciprocity law, and what follows refers mainly to work in astronomy.
== Causes of reciprocity failure ==
A developable photographic latent image forms when crystals of silver halide in an emulsion layer are exposed to light. The initial nucleation phase is chemically and thermodynamically unstable; it is thus temperature sensitive, and involves the production of one, or very few silver atoms as sub-latent image specks in each silver halide crystal. Once a clump of a few silver atoms has formed at one site within a crystal it is capable of triggering the development of the whole crystal. This greatly amplifies the effect of relatively few photons to produce a metallic silver image "grain". With low-intensity light, the sub-latent image speck may rapidly revert to silver halide before sufficient photons have been absorbed to make it stable. Hypersensitization techniques are intended to lengthen the lifetime of the unstable sub-latent image, to increase the chances of the silver halide crystal receiving enough light to form an image that will catalyze the action of the developer.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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