Data Acquisition for Scanning Tomographic Acoustic Microscopy
Acoustic microscopes are valuable in non-destructive evaluation because of their ability to provide high-resolution images of microscopic structure in small objects. When such a microscope operates in the transmission mode, the micrographs are simply two-dimensional shadowgraphs of three-dimensional objects and the resultant images are frequently difficult to comprehend because of diffraction and overlapping. This is especially true in the case of objects of substantial thickness with complex structures. We have developed a scanning tomographic acoustic microscope (STAM) to overcome these problems.
We have proposed two different rotation schemes to obtain projections for reconstructing the tomograms. The first involves rotating the transducer and the second, rotating the object. To avoid phase errors, the distance between the centers of the transducer and the object should be kept constant, or at least accurately known, throughout the rotations.
In this paper, we examine the stringent geometrical requirement for these schemes. We show, by computer simulation, that small misplacements of the order of a fraction of a wavelength are capable of destroying the image. We therefore propose a third approach which eliminates this problem since it does not require rotating or moving either the transducer or the object.
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