An Optical Technique for Measuring the Velocity of a Fluid

Abstract

The technique described is based on a refraction phenomenon. Objects observed through a rotating transparent parallel-faced prism have images which move relative to the object. When the objects are small tracer particles suspended in a moving fluid, the velocity of the fluid can be measured by varying the rotational speed of the prism until the component of the image velocity normal to the prism axis is cancelled. The point in space at which the measurement is made is fixed by the intersection of the line of sight of the viewing telescope with a thin transverse beam of light used for illuminating the tracer particles.