Electrooptical image shifting for particle image velocimetry

The direction of fluid motion can be determined in particle image velocimetry (and laser speckle velocimetry) if the image field is spatially shifted between the first and second exposures, so that the most negative fluid velocity still produces a positive particle image displacement. Two new techniques are described which accomplish very fast accurate image shifting using electrooptical hardware. These methods involve regulating the polarization of light scattered from the seeding particles by switching the polarization of the illuminating beam, and placing a birefringent unaxial plate in front of the photographing lens to displace the particle image between illuminating pulses.