Laser Doppler anemometer

Laser Doppler anemometry is a non-contact optical measurement method to obtain the velocity of a flow (liquid, gas). In the experiment P5.8.8.4 a laser Doppler anemometer is assembled. Among other things, this method is used for the calibration of flow sensors. A laser beam is split into two parts. Focused back to one spot, the laser beams create an interference pattern. Particles in the fluid flow move through the bright and dark zones of the pattern and the scattered light is modulated according to the speed of the particle.
Alternatively, the same setup can be explained in terms of Doppler shifted light.
In this experiment, water droplets inside an air stream are used as the scattering particles. The water droplets are too small to stick to surfaces nearby and to wet them. They will just evaporate or bounce off due to surface tension.