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Laws governing image formation were presented in Chapter 2. They derive from geometrical optics in which, according to the Eikonal equation, diffraction is neglected. However, diffraction at the edges of apertures such as lenses, mirrors, and other optical elements causes deflections of light rays in directions different from those in accordance with geometrical optics, thereby giving rise to image blur. Diffraction effects are an example of physical optics, in which light is treated as waves rather than rays. In this chapter diffraction effects are considered in general, particularly Fraunhofer diffraction, which can be expressed in the form of spatially varying Fourier transforms. This is fundamental to optical data processing, the basic elements of which are introduced here. Chapter 8 then considers the effects of diffraction on image quality, expressed in the form of the optical transfer functions deriving from the Fourier optics described here.
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