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The time period prior to the experimental discovery of the laser witnessed the theoretical development and experimental verification of classical coherence theory, as well as the application of communication and information theory concepts, in the design of optical systems. Communication techniques applied to optical systems led to the introduction of the optical transfer function as a viable tool in the design and analysis of incoherent optical imaging systems. The work in coherence theory was primarily concerned with the propagation of radiation. However, application of this theory to the imaging problem determined the parameters and conditions of coherence for which the optical system is linear. The advent of the laser stimulated considerable research using a coherent optical source. In imaging systems equipped with laser sources, researchers observed deleterious effects in the image, such as edge ringing, edge shifting, and the presence of speckle noise. Simultaneously, the long coherence length of the laser led to a rebirth of holography, since improved three-dimensional imaging effects were observed. The subsequent use of holograms as filters also revived interest in optical data processing. The incoherent and coherent limits of the theory of partial coherence were utilized to describe these various incoherent and coherent imaging phenomena. Techniques for linearizing the coherent imaging system and reducing the effect of speckle noise were also developed. In addition, interest in high-resolution optical analyzing instruments led to the examination of the partially coherent imaging problem. This resulted in a generalized treatment of the problem in which it was shown that optical imaging systems become nonlinear when the degree of spatial coherence in the object plane becomes comparable in size to the resolved object. Techniques for avoiding these system nonlinearities were subsequently developed.
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