Visualization and measurement of ultrasonic wavefronts

The transmission of ultrasonic wavefronts through a medium and the reflection of these wavefronts from an object or interface depend upon the bulk mechanical properties of the medium or object. Measuring or visualizing the wavefront yields important information about various transmission media and objects under study and has found application in nondestructive evaluation, medical diagnosis, underwater imaging, and acoustic microscopy. This paper reviews the various methods by which ultrasonic wavefronts are detected, measured, and visualized. Special emphasis is placed on optical diffraction by ultrasound and optical interferometric detection of acoustic particle displacement. Shorter summaries are given of chemical, thermal, liquid crystal, electromechanical, and mechanical methods. The distinction between these methods are not always clear. For example, the displacement of an air/water interface by radiation force is a mechanical method even though the displacement may be recorded optically. Therefore, an attempt is made to distinguish between the physical effect of the wavefronts on the medium or measurement device as opposed to the process used to record that effect.