Laser speckle photography in a fluid medium

Abstract

A TECHNIQUE for obtaining quantitative velocity data from hydrodynamic flow fields using laser speckle photography (LSP) has been developed and uses the scattered light from the interior of a suitably seeded liquid which is illuminated by a coherent beam from a pulsed ruby laser. The resulting speckle pattern can be photographed on high resolution film. A doubly exposed photograph of the correlated speckle patterns produced by fluid dynamic motion contains all the information necessary to describe the motion throughout a selected plane. When the speckle photographs are optically interrogated distinct fringe patterns are produced whose geometries are related to the velocity field. Here we describe how a Poiseuille flow was used to demonstrate this novel technique. Doubly exposed speckle photographs and typical fringe patterns illustrating the velocity distribution are given, analysed and compared with the classical theory.