Associating particle tracking with laser fringe anemometry

In order to obviate the complementary limitations of velocimetry by laser fringe anemometry and by photographic particle tracking, a family of optical systems is described which is suitable for both types of measurement over extended test regions. It is based on a thin sheet of light from a powerful CW laser which is broken into fine interference fringes in its plane and interrupted at a known frequency. Depending on the divergence and energy density of the beam, various applications are considered ranging from test areas as large as those required in fire research to those more characteristic of high velocity applications which involve test space dimensions of several centimeters. The conflicting criteria of visibility of tracer particles and their ability to follow variations in gas flow are examined for these regimes. Among other capabilities of such systems are the addition of a third dimension by using multiple sheets of light, and the use of the fringes as visible 'ray trajectories' in deflection tracing through refractive index gradients.