The PSV technique is a variant of PIV that utilizes direct in-line volume illumination and an imaging-optics setup that produces a narrow depth-of-field (DOF) for 2D plane imaging. PSV employs a fundamentally different approach that does not rely on principles such as fluorescence, scattering, coherence, Doppler, defocusing, or tagging but on the simpler particle shadow cast on a bright background. This is a consequence of the in-line, zero-degree-deviation direct-illumination setup. Figure is a schematic of the differences between collection-of-scattering and collection-of-extinction (shadow-casting) alignment setups.
Fig: Schematic of imaging alignment for scattering mode (a) and PSV mode (b), showing spherical particle, its scattering (dashed), and its shadow from background light directed from left to right
In the PSV mode the angle between the components is zero. A particle lies between the source and the detector (a camera imaging system in this case) and casts a shadow of a certain area given by the light extinction characteristics that can be considerably greater or smaller than the geometrical shadow of the particle. Contrast changes yield particle-shadow-diameter variations and permit the diameter to be adjusted by varying the intensity of the incident light. A brighter light produces a smaller particle shadow and, as a consequence, yields a sharper DOF.
In PIV, velocity is found by calculating the particle ensemble displacements between two instantaneous time snap-shots, which is generally accomplished through correlation techniques such as using FFT on the image signal. The signal information is generated from changes in intensities; therefore, the same technique can be used for PSV since it is based on information from particle ensembles, although the intensity information is inverted to that from PIV. A subtle difference is that in PIV the particles intensities have a shape such as Gaussian whereas in PSV this shape has not been determined yet and depends on the aforementioned light-particle interaction characteristics. Moreover, since the interest is in the shift or displacement between two signals rather than the signal characteristics themselves, techniques such correlation are applicable to that effect. The PSV image with particles having the lowest intensity compared to the maximum intensity of the background can be readily inverted and generate a PIV-like signal if desired.Courtesy:
1. PIV with LED: Particle Shadow Velocimetry (PSV) Jordi Estevadeordal* and Larry Goss Innovative Scientific Solutions, Inc. 2766 Indian Ripple Rd.,Dayton, OH 45440, US.