Particle Image Velocimetry
Particle Image Velocimetry (PIV) measures whole velocity fields by taking two consecutive images and calculating the distance individual particles travelled within this time. The fluid is seeded with microscopic particles that are illuminated as they pass through a laser light sheet. The laser and cameras are fired simultaneously and sequential images are taken of the particles in the light plane. In essence, the laser acts as the camera “flash.” The computer takes the raw images and uses a cross correlation algorithm to calculate average flow fields and their magnitudes. Both two and three dimensional fields can be generated, depending on the experiment requirements. See Fig 4 PIV
Since the flow can be quite fast, one has to avoid blurred images and that’s one reason to use laser pulses. The other reason is that only laser light can be focused into a thin enough light sheet so that only particles in that plane are imaged. Otherwise the scattered light from particles in other planes would make this measurement impossible.
A special camera is utilized so that it can store the first image (frame) fast enough to be ready for the second exposure. The image exposures are only 6-10 ns long and freeze any motion. The "dead" time between the frames when the camera is "blind" is very short; down to 100 ns. From the known time difference and the measured displacement the velocity is calculated.