What is Schlieren imaging?

Schlieren imaging is a method to visualize density variations in transparent media.

The term "schlieren imaging" is commonly used as a synonym for schlieren photography, though this article particularly treats visualization of the pressure field produced by ultrasonic transducers, generally in water or tissue-mimicking media. The method provides a two-dimensional projection image of the acoustic beam in real-time. The unique properties of the method enable the investigation of specific features of the acoustic field , detection of acoustic beam-profile irregularities and on-line identification of time-dependent phenomena. Some researchers say that schlieren imaging is equivalent to an X-ray radiograph of the acoustic field.