LM-0500 LDA - Laser Doppler Anemometer

When children are playing while striking the vertical bars of a picket fence with a stick it gives a characteristic burst like noise. The faster they are running the shorter the burst is however the frequency of the strikes is higher. The same principle Yeh and Cummins exploited 1964 when they invented their Laser Doppler Anemometer. Anemos is a Greek word and means “wind”. Consequently a Laser Doppler Anemometer (LDA) can be considered as a “wind meter” using a laser.
Consequently a Laser Doppler Anemometer (LDA) can be considered as a “wind meter” using a laser. However the LDA cannot detect pure wind as a clean air stream, it needs to have particles moving with the wind, These particles are guided to move through two crossing laser beams created from one source. Due to the coherence of the laser and thus also of the two beams a spatial interference pattern appears within the crossing zone looking like zebra stripes. When particles are moving through the stripes they scatter the light in preferred directions. Instead of playing children the set-up uses an ultra sonic particle generator. Via a nozzle the "dry" water particle are perpendicular crossing the interference zone. A photodetector combined with a fast and sensitive amplifier is used to detect the scattered light. A storage oscilloscope is required to display and store the individual burst for the subsequent analysis.