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Here and in Chapter 21, the various sources of noise in coherent imaging systems are reviewed and classified. This classification includes film granularity noise, film processing noise (including both phase images and film processing non-linearities), dust noise, and speckle noise. In Sec. 20.2, we review those film parameters that are necessary to define a maximum achievable system performance figure in terms of system resolution. The important film parameters are granularity, film MTF, and threshold modulation. In Chapter 21, we continue our study of photographic films with discussions of the types and sources of noise in coherent imaging systems, numerous speckle-averaging techniques, and a summary. A photographic emulsion is a suspension of silver halide crystals (called grains) in a gelatin medium. When a silver halide grain absorbs a photon, a hole electron pair is formed. On the average, at least four silver atoms are needed to form a stable silver speck in a grain, which means that a grain must absorb at least four photons to create such a stable silver speck. These stable silver specks are referred to as the latent image. The process of development reduces those grains that contain a latent image to metallic silver; the other grains (which do not contain a latent image) are not reduced to silver. In addition, the reaction rate determined by the strength of the developer used in the processing can cause spontaneous reduction of a grain having no latent image speck to metallic silver, thus forming a random or diffuse image called fog. This diffuse image is one source of photographic noise. In addition to the effects of spontaneous reduction, it is possible for the metastable latent image to undergo spontaneous decay back to silver halide. This phenomenon is referred to as latent image fade.
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