Digital Reconstruction of Acoustic Holograms.
PENNSYLVANIA STATE UNIV UNIVERSITY PARK APPLIED RESEARCH LAB
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The subject of this study is the development of a computer program capable of reconstructing images from acoustic holograms. The theory on which the program is based is that of Gabor type on-axis holography. A hologram is an intensity recording of the interference of an object wave and a reference wave. A reconstruction from the hologram contains four components, namely the reference wave squared, the object wave squared, the conjugate image and the true image. Digital techniques for isolating the four components are presented along with examples of reconstructions. Reconstructions from digitally generated holograms of point sources are discussed. It was found that interference between images of point sources can drastically affect image intensity. Image intensity is also dependent on source position relative to the hologram aperture. A source located near the edge of the aperture reconstructs with only half the intensity of the same source located at the aperture center. Methods of hologram generation are compared for image quality. Phase-only and binary techniques add spurious high frequency information to the images, but in the application of source location this is not a serious handicap. Digitization range -1, 1 was found superior to 0, 1 in that it eliminates two of the unwanted image noise terms in tha phase-only and binary cases. Holograms containing only the conjugate image component give vastly improved signal to noise ratios compared with the other types, but they require knowledge of the amplitude and phase at every sample point rather than just the intensity.
- Computer Programming and Software