Logo PTB

Development of optical sensors to measure fluid flow velocities

Working Group 1.41
Beispiel eines Halbleiter-Laser-Doppler-Anemometer

Laser Anemometry is a non-intrusive method to measure precisely velocities in fluid flows. The method used is to create a spatially modulated light distribution in a well defined measuring volume in a fluid and to detect and analyze the light scattered by small particles seeded in the fluid. The frequency of the detected signal is directly proportional to the velocity of the scattering particle. There are many possibilities to create a light pattern like imaging a grating, a laser diode array or a bundle of fibres into the measuring volume. The commonly used way is to cross two laser beams to produce a measuring volume at the cross section. Whenever light from both sources gets scattered by a tracer particle crossing the measuring volume, its frequency is changed by the Doppler effect. Collecting and heterodyning the scattered light on a photodetector will result in an electrical signal that is modulated with the difference frequency of the Doppler shifted light beams. Therefore, this kind of sensor is called laser Doppler anemometer (LDA). To illustrate the LDA process animations for PCs can be downloaded.