Schlieren imaging

The parallel beam may be achieved by a point-like light source (a laser focused into a pinhole is sometimes used) placed in the focal point of a collimating optical element.

The optical stop may be realized by a razor placed horizontally or vertically in the focal point of the focusing element, carefully positioned to block the light spot image on its edge.

[citation needed] The rays cross through the transparent medium while potentially interacting with the contained acoustic field, and finally reach the focusing element.

Thus, the population of rays crossing the focal plane of the focusing element can be divided into two groups: those that interacted with the acoustic field and those that didn't.

The latter group is undisturbed by the acoustic field, so it remains parallel and forms a point in a well-defined position in the focal plane.

The phase acquired by electromagnetic wave traveling through the medium is related to the line-integral of the wavenumber along the propagation line.

However, scanning the acoustic field with a hydrophone suffers from several limitations, giving rise to supplementary evaluation methods such as the schlieren imaging.

The importance of the schlieren imaging technique is prominent in High Intensity Focused Ultrasound (HIFU) research and development.