8. Ultrasonic Imaging and Microscope
The article describes the results of two methods: the Fresnel approximation and the convolution method used for the exact Rayleigh-Sommerfeld equation. Curves for the Fresnel approximation are shown as a function of flaw depth and number of sample points. Despite a violation of the fresnel condition, an image can be achieved without visible phase distortion. This and its limits are shown on synthetic and experimental data. By shifting the hologram and not changing the symmetric phase factor, calculation time is saved because the rearrangement in the image space is achieved automatically. The exact method "angular spectrum" has been simplified in three steps. For different insonification angles in shear wave holography, the data set must be multiplied by an aperture function before being Fourier transformed. Examples demonstrate the usefulness of contact technique probes. To improve the recording time, a 140 element array will be multiplexed electronically and moved mechanically.
Schmitz, V.; Kiefer, R.; and Schäfer, G., "Advances in Computer Reconstruction of Acoustical Holography" (1981). Proceedings of the DARPA/AFWAL Review of Progress in Quantitative NDE, October 1979–January 1981. 31.