In the introduction to Chapter 6, we stated that the determination of an image quality criterion has long been and still is a major problem in the field of image evaluation and assessment. If this statement was true when only incoherent objects had to be considered, it is doubly true now that we need to allow for coherent and partially coherent objects. To attempt to obtain some insight into the problems encountered with partially coherent light, we will discuss a number of simple objects that are often used in image evaluation. Clearly, the most simple object is the one that consists of just two points which are themselves not resolved, i.e., the amplitude impulse response of the imaging system in object space is much greater than the physical dimensions of the "point." Our discussions here relate directly to Chapter 6, which discussed the two-point imaging problem for incoherent light. The analysis was applicable to the problem of two stars, which can be considered incoherent with respect to each other. However, it is unlikely to apply to the viewing of two small adjacent cells in a microscope.
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