# Basic Principles of Velocimetry

Laser Surface Velocimeters use the Laser Doppler Principle to evaluate the laser light scattered back from a moving object. Polytec's LSVs incorporate sophisticated detection circuits based on heterodyne detection to measure the Doppler shift of the light. Unlike conventional non-contact methods which measure only the magnitude of the velocity, Polytec's  heterodyne detection circuits are able to detect the direction of motion (+/-), changes in direction and standstill conditions. The measurement is precise enough to detect even minute displacements and changes in velocity, allowing tighter process control.

## The Differential Doppler Process

Polytec Velocimeters work according to the so-called Difference Doppler Technique. The technique involves using two coplanar laser beams that form a triangle with a vertex where the intersecting beams overlap on the surface being measured. The optical axis of the instrument bisects this vertex and is coplanar with the laser beams making an angle φ with each beam. The instrument is positioned so that the plane is perpendicular to the surface being measured and aligned with the velocity of the surface. 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 - the upstream beam is frequency shifted down and the downstream beam is shifted up.

## Graphic Representation of the Difference Doppler Technique

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 spacing Δs is a system constant which depends on the laser wavelength λ and the angle between the laser beams 2φ:

Δs = λ/(2 sin φ)

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 = (2v/λ) sin φ

• fD = Doppler frequency
• vp = Velocity component in the direction of measurement
• Δs = Fringe spacing in the measurement volume

The value λ/sinφ makes up the material measure for the velocity and length measurement. It is measured precisely for every sensor head and is printed on the identification label.