To perform a speed measurement on moving objects, which can in principle be of any length, you require a measurement setup with a sensor which is mounted perpendicular to the surface it is measuring on. Polytec velocimeters work according to the so-called Differential Doppler Technique. Here, two laser beams – which are each incident to the optical axis at an angle φ – are superimposed on the surface of the sample. For a point P, which moves at velocity v through the intersection point of the two laser beams, the frequencies of the two laser beams are Doppler shifted in accordance with the above formula.
Both the laser beams are superimposed in the measurement volume and, in this spatial area, generate an interference pattern of bright and dark fringes. The fringe space Δs is a system constant that depends on the laser wavelength λ and the angle between the laser beams 2φ:
If a particle moves through the fringe pattern, then the intensity of the light it scatters back is modulated. As a result of this, a photo receiver in the sensor head generates an AC signal, the frequency fD of which is directly proportional to the velocity component of the surface in measurement direction vp, and it can be said that:
- fD = vp/Δs = (2vp/λ) sin φ
- fD = Doppler frequency
- vp = Velocity component in the direction of measurement
- Δs = Fringe space in the measurement volume
The value λ/sin φ makes up the material measure for the velocity and length measurement.