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In Chapters 29 through 33 on analog optical computing, we considered filters that modify the amplitude and/or phase of the optical wave field in the Fourier transform plane of the system shown in Fig. 35.1. The most general filter function is one that modifies both the amplitude and the phase of the optical wave field in the Fourier transform plane. These filters are known as complex filters and, as pointed out previously, are quite difficult to fabricate. A special case of the complex filter is a filter that modifies only the optical path difference or phase of the optical wave field in the Fourier transform plane.
Both phase and complex filters have been successfully utilized in phase contrast imaging for purposes of making phase objects (which are invisible with normal incoherent light) observable. In this chapter, discussions of these various techniques are given as additional examples of complex optical filtering.
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