Doppler principle

Everyone has experienced the Doppler effect in daily life when the siren of a passing police car is heard. When the moving car is heard approaching, the frequency is heard at a level which is higher than it actually is, and when the car moves away, the frequency of the siren seems to be lower. The frequency which is heard by our ear is defined as a number of vibrations per second. If the car and the listener are stationary, i.e., at a fixed spot, the ear picks up the true frequency of the siren. Once the car moves away, every sound (every frequency) is transmitted from an ever greater distance. The frequency which reaches the ear becomes lower i.e., the sound is deeper. This so-called Doppler effect does not only apply to sound waves, but also for any other type of waves, i.e., light waves as well. 

Laser Doppler velocimetry is the method which is most frequently used to measure the speed of solid surfaces without contact. A laser beam is split into two beams of equal intensity using a beam splitter. These two laser beams are processed using optical components, and at a measurement distance, they are placed at the intersection via an adjustable mirror. When two laser beams intersect at an angle, a striped pattern of light and dark strips arises in the measurement volume. The photo receiver sees this pattern of strips and recognises the Doppler frequency in the light which is scattered back, i.e. the frequency caused by the movement of material.

The way a LDV system works is easy to understand if we first consider a single particle, which runs through the striped pattern. In this case, it always transmits a bright pulse of a light when it passes through a light coloured field. The photo receiver records this flash frequency and converts into an electrical frequency in the evaluation unit. This original Doppler frequency is made available to our pulse output from the evaluation unit. Although solid surfaces do not have individual particles, the surface structure behaves exactly the same.