Background rejection in schlieren visualization

Schlieren visualization is one of the simplest ways of determining the geometric parameters of an ultrasonic beam generated by diagnostic transducers, and is recommended for evaluating the potential and the limitations of ultrasonic equipment used in diagnostic applications.’ Apart from some special cases, for example, dynamically focused and/or scanned receiving arrays, the shape of the transmitted beam is the most important factor determining the attainable lateral resolution, The schlieren image of the generated ultrasonic field yields a good amount of information, such as the beam diameter along the axis, and the approximate direction and level of possible side-lobes. Unfortunately, these data must usually be measured at a rather low level of ultrasound, when it is difficult to discriminate between the light deflected due to the acoustically generated changes in the refractive index of the medium and interfering light diffracted by optical impurities in the background.