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We conclude the discussion we began in Chapter 20 about photographic films with a review here of the sources of coherent noise and their reduction. In Sec. 21.2, we discuss the types and sources of noise in coherent imaging systems. These include dust noise, phase image noise, and speckle. Various techniques for reducing the effects of dust noise are described. Control of the spatial and temporal coherence of the system radiation, which reduces the effect of system speckle noise, is discussed and demonstrated. In Sec. 21.3, several of the numerous speckle-averaging techniques that have been introduced in the literature are put into the broad categories of spatial and temporal coherence control techniques and further subclassified in terms of ensemble- and temporal-averaging techniques. Results are summarized in Sec. 21.4 by listing five experimental procedures to be considered when designing a coherent optical system in which all noise effects will be minimized. Two experiments obeying these procedures are discussed and the results are shown.
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