ACOUSTIC IMAGING BY HOLOGRAPHY

A method for visualizing objects immersed in water is formulated analytically and demonstrated experimentally. The technique, called acoustic holography, is an adaptation of Gabors two-step imaging process known as wavefront reconstruction or holography. The hologram is first formed from coherent acoustic radiation and then the image is reconstructed optically using coherent light source. Acoustic holography has advantage over other schemes for imaging in optically opaque media in that lenses or other focusing devices are not required, and a complete amplitude and phase reconstruction of the scattered field may be obtained. Since instantaneous amplitude is an acoustic observable, the reference field may be simulated electronically. Moreover, by resorting to heterodyne or phase detection the cross product term between object and reference signals may be generated without the undesired extraneous terms which occur in conventional holography. A scanning technique for generating acoustic holograms of underwater objects in the laboratory is described in detail. Using this system acoustic holograms have been recorded which show angular resolution of 3.6 milliradians, approximately 1.5 times the Rayleigh limit. A variable contrast television display was used to view the acoustic holograms.