Zooming lens for acoustic microscopy and analysis of non destructive testing images

Acoustic microscopy at 100 MHz frequency is a very efficient tool for detecting defects in ceramic materials. For non destructive evaluation in the bulk of the samples, the major inconvenient of acoustic microscopy is the requirement of a collection of focusing lenses for covering all the depth. The design and construction of a variable focus lens enables now to explore ceramic material over depths comprised between 1 and 10 mm using a single lens, with a convenient spatial resolution. The experimental results are compared to those derived theoretically from a simple geometrical model. Another difficulty encountered in non destructive applications of acoustic microscopy consists in the extraction of a quantitative informations from the image obtained. An interpretation based on a model of the matrix medium for different ceramics is presented. The physical parameters (structure, grain size and density)characterizing the material, may be deduced.