NDE of Solids with a Mechanically B—Scanned Acoustic Microscope

Acoustic microscopy of the interior of solids is hampered by spherical aberration introduced at the sample surface. The incorporation of a spherical coupling piece in front of a conventional acoustic lens radically reduces the aberration. This is so not only when the coupling piece is concentric with the lens, but when the lens is axially displaced. This means that diffraction limited focussing can be achieved over a wide range of depths, typically up to several hundred wavelengths. Thus a B-scan imaging system with mechanical scanning of the lens focus can be used to image a slice through a solid specimen. Alternatively, the lens may be focussed at a constant depth and transversely C-scanned in the normal way. This paper reports on theory and experiments using this system for non-destructive imaging of cylindrical specimens.