A Two-Stage Spatial Frequency Filter.
DEPARTMENT OF THE NAVY WASHINGTON DC
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A two stage spatial filter, for use with a transparency of an object framed by a rectangular field stop, for separating the spectrum of the Fourier transform of the object from the superposed spectrum of the Fourier transform of the rectangular field stop. The transparency is disposed in an object lane of a focusing lens and illuminated by a beam of coherent light. The first stage of the spatial filter is a high pass filter in the form of an opaque cross adapted to be disposed in the Fourier transform plane of the lens to block the lower spatial frequency components of the Fourier transform of the rectangular field stop. The second stage of the spatial filter is a low-pass filter in the form of an opaque screen having a rectangular aperture and adapted to be disposed in the image plane of the lens conjugate to the object plane for blocking the detail in the real image of the transparency coming from the unfiltered higher spatial frequency components of the Fourier transform of the rectangular field stop. A second focusing lens behind the low-pass filter reimages the Fourier transform of the transparency after filtering. Author
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