Electronic reconstruction of acoustic holograms

Abstract

A method is described for reconstructing acoustic hologram images using an electronic means which avoids multiple image formation. When the dimensions of the hologram are small compared to the dimensions of the viewing area, the incident waves are approximately plane rather than spherical. In this case the wavefront pattern can be resolved into components along two orthogonal directions, e.g., the x and y axes. Thus the information can be received by an array of transducers along only the two axes, instead of over the entire hologram surface as is customary. A disadvantage which occurs with reconstruction from plane waves is an ambiguity between right and left, resulting in the formation of multiple images. This effect may be avoided if the usual method of forming a diffraction pattern at the hologram surface by adding a reference wave is not carried out. Instead, the information is received and processed at the sound frequency f 0 . Spatial sine (V s ) and cosine (V c ) transforms are carried out first, and then demodulated. The essence of the unambiguous decoding method is to demodulate V s and V c by both in-phase and quadrature demodulation, forming four quantities which can be combined to give the required intensity. The combination of the four quantities is performed such as to effectively phase out the information from all directions except one and this avoids the formation of multiple images. The procedure is to be applied to both x and y axis information and the results multiplied to give a single reconstructed image.