Phase-preserving methods to visualise ultrasonic fields with schlieren imaging

Abstract

Schlieren imaging allows visualising local density modulations in optically transparent media, which enables to analyse ultrasonic wave propagation. In typical measurement setups, spatial filtering is applied. Commonly, phase information of the ultrasonic wave is lost, which e.g. prevents accurate tomographic reconstruction. In this work two methods are presented to preserve phase information. The first method relies on the combination of two different spatial filters, while the second method does not require any spatial filtering. The fractional Fourier transform is applied to explain and simulate the effects occurring if the position of one of the optical lenses in a schlieren measurement setup is varied. Exemplary images of ultrasonic waves are shown to demonstrate the application of both methods.