An interferometry system is presented that is useful in large-scale measurements such as those of topographic maps. The envelope of a propagating wave is used to create the desired fringe patterns, representing range contours, through the interference of a light modulation function and an image modulation function. A number of variations are shown that either provide altitude contours or place the contour information on a spatial frequency carrier. This latter system offers the flexibility of electronic processing when the image is scanned. The comparison of contour patterns taken at different times provides an accurate indication of subtle surface deformations.
Albert Macovski, "Envelope Interferometry for Large-Scale Processing," Appl. Opt. 13, 2689-2692 (1974)