In Chapter 29 we demonstrated that a lens can be used to perform optical Fourier transformations. As discussed previously in Chapter 19, a second lens can be used to perform a second Fourier transform to produce an image, and controlled manipulations can be performed in the first transform plane. This system, shown in Fig. 30.1, is often used for optical spatial filtering. This system is also referred to as a one-to-one imaging system since a nonmagnified image of the object appears in the back focal plane of the second lens. Magnification can be introduced by varying the ratio of the focal lengths of lenses L2 and L3. It should be noted that a one-lens system could also be used to perform this filtering function with magnification; however, we have chosen the system in Fig. 30.1 for simplicity.
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