Optical Processing, 4

To filter (or more exactly to suppress) an unwanted event on the section which has a given dip and frequency, it is sufficient to mask the corresponding light spot by positioning an opaque patch in the correct position in the SPECTRAL PLANE. That is the plane on which is focussed the diffraction image made up of the previously mentioned spot. Figure 1 shows that, thanks to available lenses, it is possible to obtain an image of only the diffraction image which is produced between the object plane and the image plane. If this new section is photographed in the image plane and produced, it will be seen to contain no more than the unwanted event. This diffraction image may be masked out.