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Nonlinearities arising from spatial coherence effects in optical instruments occur when the coherence interval is comparable in size to the optical spread function of the system as measured in the object plane. This is the situation depicted schematically in Fig. 11.16(c). Since the spread function of the system determines the size of a resolution element in object space, we expect nonlinear effects from spatial coherence whenever the object is sufficiently small to diffract light outside of the collection angle of the instrument or when the coherence interval is large enough to enable two or more object resolution elements to interfere in image space, causing effects similar to those seen in Figs. 11.14 and 11.15. Therefore, to ensure that an instrument is operating in a linear mode, it is necessary to design or operate the instrument such that all of the diffracted light containing information about the object spatial frequencies of interest is collected. In general, these effects are of importance in the use of high-resolution analyzing instruments such as microdensitometers, microscopes, contact printers, enlargers, projection printers, and viewers. These instruments can be operated in a linear mode for low-resolution applications. Instruments such as cameras used with normal sunlight illumination, aerial cameras, the human eye, and conventional magnifiers generally operate in an incoherent linear mode.
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