Ultrasound field characterization using synthetic schlieren tomography

Synthetic schlieren imaging, also known as background oriented schlieren imaging, is used to determine the acoustical field of a focused ultrasound transducer operating at 1.01 MHz frequency with peak pressure amplitude of 0.97 MPa. The measurement setup is composed of a commercial off-the-shelf digital single-lens reflex (DSLR) camera with an ordinary objective, a high power light-emitting diode driven in pulsating mode, water tank, ultrasound transducer, rotation stage, and driving electronics. Measurements are performed in tomographic fashion by rotating the ultrasound transducer within the water tank and photographing an imaged target behind the ultrasound field. The photographs are processed with a Horn-Schunck-type algorithm, commonly used in optical flow analysis, in order to determine the deflection of light rays as caused by ultrasound field induced acousto-optic effect. Inverse Radon transform is then used, with the deflection data, to obtain three-dimensional spatial distribution of the pressure field gradient, from which an approximation of the ultrasonic pressure field is computed. The pressure field obtained with synthetic schlieren tomography is then compared to hydrophone measurements mainly qualitatively.