In the previous chapters, we gave a good deal of attention to the phenomenon of Fraunhofer diffraction. This was necessary because of its importance to the image forming process in describing the impulse response of optical systems. However, in pursuing this aspect of diffraction, we have ignored the whole area of Fresnel diffraction. In this chapter, we discuss the general problem of Fresnel diffraction, which essentially occurs whenever results of the two conditions for Fraunhofer diffraction are not satisfied, i.e.,
1. when the plane of observation is not in the far field of the aperture or
2. when the plane of observation and the point source are not parallel conjugate planes of a well-corrected optical system.
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