Understanding velocity is crucial in most fluid applications. Both magnitude and direction are needed to describe the process, determine mixing zones, shear rates, and more.

While there are several methods and devices to measure the local velocity of fluids, Particle Image Velocimetry is a technique widely used to measure entire velocity fields.

To track the movement of the fluid, small particles are suspended within the fluid. If the fluid is a gas, droplets can also be used. Their motion is captured with a camera in short, known intervals. An algorithm correlates two succeeding images to identify which particle of the first image has moved where in the second image to calculate the distance this particle has travelled. Since the time interval between the two images is known, the velocity of the particle can then be derived.

While technically any light source with enough power can be used, it can be problematic if all particles are illuminated. The particles that are closer to the camera obscure the field of view, so only surface measurements are possible. Lasers however, can be formed into a very narrow sheet that selectively illuminates a section of the flow. That way, velocity information from the inside of the flow can be derived.