The Relative Importance of Phase and Amplitude in Acoustical Holography

Abstract

This chapter describes an investigation to determine qualitatively the effect of recording a hologram by detecting only the phase of the object wave rather than both the phase and the amplitude, as in conventional holography. The “phase-only” hologram offers three advantages over a conventional hologram in acoustical or microwave holography: first, the amount of data to be acquired is reduced because only the phase needs to be measured; second, the full dynamic range of the recording equipment can be used over the entire hologram aperture; and finally, there is a possibility that image resolution may be better. In experiments conducted with sound using a frequency of 18 kHz (0.75-in. wavelength) in air, holograms were made using both “phase-only” and conventional techniques. Objects used were (1) a cutout of the letter “R,” and (2) three point sound sources at different relative intensities. Reconstructed images of the objects made from both types of holograms did not show any noticeable differences either in image geometry or in image grey scale. For the geometry and relative intensity levels in the object image to be preserved it appears that the following two conditions are necessary: the surface of the object should be made up of points which radiate spherical waves; and all the points on the object surface that are visible at one point in the hologram plane should also be visible at all other points in the hologram plane. Unless these conditions are satisfied, the reconstructed image from a “phase-only” hologram will still be geometrically correct, but the grey scale, though still in evidence, will appear distorted. A new technique for completely eliminating the conjugate image in reconstructing a true image of the object in the Gabor (on-axis) type of arrangement is also discussed. This consists of (1) blocking the defocused conjugate image with an aperture placed at the focus of the true image of the source which “illuminates” the object, and (2) blocking the zero-order wave with a stop.