Fringe image technique for the measurement of flow velocities

One interpretation of the differential Doppler laser technique for measuring flow velocity is that real fringes are formed in the flow region. Particles crossing alternate bright and dark interference fringes produce a modulation of the scattered radiation. Measurement of the modulation frequency determines the flow velocity. The fringe image technique is similar in principle except that it uses localized fringes formed by the optical image of a grating (e.g., a Ronchi grating) illuminated with an incandescent source.