optical fluid-flow measurement

optical fluid-flow measurement [′äp·tə·kəl ′flü·əd ¦flō ‚mezh·ər·mənt]

(engineering)

Any method of measuring the varying densities of a fluid in motion, such as schlieren, interferometer, or shadowgraph, which depends on the fact that light passing through a flow field of varying density is retarded differently through the field, resulting in refraction of the rays, and in a relative phase shift among different rays.