Deep Thermoacoustic Imaging Using Scanning Electron Acoustic Microscopy
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Begun in 1973, the Review of Progress in Quantitative Nondestructive Evaluation (QNDE) is the premier international NDE meeting designed to provide an interface between research and early engineering through the presentation of current ideas and results focused on facilitating a rapid transfer to engineering development.
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There has developed over the past few years some controversy  over the meaning and use of terms such as “thermal wave imaging” vs. “thermoacoustic imaging” or “Thermal Wave Microscopy” as opposed to “Scanning Electron Acostic Microscopy” (SEAM). The issue in question is the extent of the role played by acoustics directly on the imaging. In the present work, SEAM studies of a prepared test standard are presented which show that macroscopic elastic properties can play an important role in imaging. The SEAM technique uses Coordinate Modulation (CM) of the electron beam  rather than the more commonly used intensity modulation via beam blanking. Defects as deep as 26 thermal diffusion lengths in stainless steel have been clearly imaged with this technique. This work strongly supports the Jackson -Amer thermal bending model  for low frequency (up to approximately 100kHz in small specimens) acoustic detection in the solid as compared to the theory of Opsal and Rosencwaig  which does not incorporate total specimen response to the thermal source. This problem has more recently been theoretically investigated by Favro [5,6], who has developed a more general theory encompasing both the Opsal-Rosencwaig short wavelength limit and the Jackson-Amer long wavelength limit. Favro’s theory also accounts for features in the image shown.