Improving pulsed dark field method to study ultrasonic phenomena

Using pulsed dark field (PDF) method to analyze ultrasound fields presents some advantages with respect to the traditional Schlieren method, and with respect to the non-pulsed dark field method by I. Nunez and C. Negreira (1995) and I. Nunez and C. Negreira (1996). First, the circular black spot used to stop the zero-order of diffracted light (dark field method) allows us to observe phase modulations in all directions perpendicular to the propagation of the beam. The classical "knife edge" of the Schlieren method shows phase modulations principally in one direction. This one is normal to the edge of the knife and normal to the light beam. On the other hand, PDF method uses short time illumination triggered by the acoustic pulse. This allows observing elastic fields in pulsed regime and we can evaluate the evolution of such pulse inside homogeneous and heterogeneous media. In this paper, we apply PDF method to determine the conditions of change from single to multiple scattering. A 2.25 MHz transducer in water emits a wide-band ultrasonic pulse. By varying the distribution of scatterers (cylindrical wires of 1.2 mm diameter), it is possible to generate the conditions necessary for loosing coherence in the pulse. We use a 10 mW He-Ne, an acousto-optic modulator (Neos Bragg cell driven by Neos N21080) triggered by the acoustic pulse generator (Sonic), two lenses of focal length 850 mm and diameter 74 mm, a black spot of diameter 0.5 mm, a CCD camera (EDC-1000HR, Electrim Co.) with resolution 244x753 pixels.