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200   Chapter Nine

                                        FOURIER
                   OBJECT     LENS A     PLANE      LENS B      IMAGE

        COLLIMATED/
        COHERENT
        ILLUMINATION


                         ƒ           ƒ          ƒ           ƒ
        Figure 9.19 An illustration with a transparent object located at the first focal point
        of a lens.





        sinusoidal grating has but a single diffraction order, the first. Now, if
        the object is illuminated by collimated/coherent light, that diffracted
        light will be focused as two points in the second focal plane of lens A
        (which is indicated as the Fourier plane, midway between the lenses
        in Fig. 9.19). The points will be laterally displaced by    f tan   from
        the nominal focus. Thus, if an annular zone in the Fourier plane is
        obstructed, all the spatial information of the frequency corresponding
        to the radius of the obstruction will be removed (filtered) from the final
        image. Thus it can be seen that the Fourier plane constitutes a sort of
        map of the spatial frequency content of the object and that this content
        can be analyzed or modified in this plane.


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