Optical Systems for Flow Measurement: Shadowgraph, Schlieren, and Interferometric Techniques

Optical techniques that have been used in the measurement of flow include (1) direct visualization, where some type of marker (e.g., dye, bubbles, solid particles) is followed along with the fluid motion; (2) laser-Doppler systems, in which the frequency shift of scattered illumination from such a marker—usually a particle — is measured; and (3) measurement of what might be called index of refraction of a medium, from which some property or properties of the flow are determined.