Principle of Operation

Anemos is a Greek word which means ��wind��. Consequently, a Laser Doppler Anemometer is a ��wind meter�� using a laser based on the physical effect that Christian Doppler discovered in 1842. However, the LDA cannot detect pure wind as a clean air stream; it needs to have particles moving with the wind, These particles move through two crossing laser beams. Due to the coherence of the laser a spatial interference pattern appears within the crossing zone which look like zebra stripes. When particles are moving through the stripes, they scatter the light in preferred directions.
When children play by striking the vertical bars of a picket fence with a stick it gives a characteristic burst like noise. The faster they run the shorter the burst is, however, the frequency of the strikes is higher. Yeh and Cummins exploited the same principle in 1964 when they invented their Laser Doppler Anemometer.
Instead of children playing, the set-up uses an ultra sonic particle seeder. An avalanche photo detector combined with a telescope is used to detect the scattered light. A storage oscilloscope is required to display and store the individual burst for subsequent analysis.