Transmission Imaging: Forward Scattering and Scatter Reconstruction

Abstract

We give here a theory for the nature of the image if produced by the scanning laser acoustic microscope (SLAM) at a surface in the far field of the flaw. A physical model and an exact calculation of scattering of longitudinal acoustic waves by spherical defects are presented. The total displacement field (incident plus scattered) is evaluated at planes at a distance from the defect by using the Ying and Truell scattering theory. It is found that evaluation of the exact type and size of a defect is not possible from direct observation of the total field. A holographic type reconstruction technique using either 2-dimensional Fourier transform, or Rayleigh-Sommerfeld diffraction formulae, was used to reconstruct the field at the location of the defect from a knowledge of the field at an observation plane, a distance z away from the defect. It is found that accurate defect characterization can be made using this reconstruction technique.