The document discusses particle image velocimetry (PIV), which is a non-intrusive method for measuring fluid flow velocities. PIV works by seeding the fluid with particles and using a laser sheet and camera to capture particle images. Software then tracks the particle movements between images to calculate velocity vectors across the flow field. PIV has applications in analyzing flows around objects like fish, helicopters, and prosthetic heart valves. Advanced PIV systems are being developed that can perform 3D motion tracking.