Multiple-Exposure Laser Speckle Velocimetry for Fluid Flow Measurement

In laser speckle velocimetry or LSV a sheet of laser light is used to illuminate a plane of interest within a volume of moving fluid which has been properly seeded to assure efficient scattering. An accurately focussed photograph of the resulting side scattered intensity field provides a pattern of (depending on the effective seeding concentration) either particle images or coherent speckle which is directly related to the existing two-dimensional flow field in the plane of interest. If the illumination is appropriately interrupted while recording particle images, the resulting photograph or LSV specklegram will include many discrete recordings of each particle image. A positive transparency print of this multiple exposure LSV particle image specklegram may readily be analyzed optically by either a point-by-point "Young's fringe" technique or a full-field "Fourier filtering" technique to produce quantitative velocity field information with superior signal-to- noise characteristics. Computer aided image processing and data interpretation techniques are also used to provide further enhancement and convenience. In this paper the sources and consequences of signal decorrelation and noise are discussed in detail, and compared with those obtained using double exposure LSV (either coherent speckle or particle image).