Laser Doppler Vibrometers (LDVs) operate on a variety of surfaces. It is important that the amount of light scattered back from the different surfaces is sufficient for further signal analysis.
Specular surfaces, i.e. highly reflecting surfaces, obey the law: angle of incidence = angle of reflection. When making measurements from such surfaces, the optics of the LDV need to be aligned so that the reflected light returns within the aperture of the collecting optics. Typical aperture values are 10mm diameter for the single point 505/503 systems and 3mm diameter for the 551/552 fiber optic systems.
Diffuse surfaces scatter the incident light over a large angular area. The intensity of the scattered light power per unit solid angle follows Lambert's cosine law. It can vary greatly between shiny surfaces and dull black surfaces that absorb most of the light.
It is possible to increase the reflectivity of a surface using retro-reflective tape or paint. This material consists of small glass spheres (approximately 50 µm in diameter) that are fixed with an elastic epoxy to the base material. Each sphere acts as a small "cat's-eye" scattering light back along the path of the incident beam. The incident light is returned to the LDV over an angle of a couple of degrees. As the laser beam usually hits several glass beads at one time, each of the beams can interfere with the others and produce a granular pattern of intensity as the distances traveled differ by a small amount. This is called a speckle pattern.
Speckle patterns are always produced when a coherent light source is focused onto a rough surface. It is caused by interference effects between the beams originating at the different scattering centers on the surface. If the focused spot is very small, the number of scattering centers is small and the angular dependence of the path length differences in a given direction is also small. This leads to a large angle over which the interference condition is reasonably constant and thus a large solid angle for the speckle.
The solid angle corresponding to one speckle is given by: dW = D/ -l2
Where dW is the solid angle corresponding to one speckle , D is the spot diameter on the surface and l the wavelength of the laser.
The problem with this phenomenon is that between each region of constructive interference is a region of destructive interference, i.e. little optical signal is detectable. The fact that there is destructive interference means that the phases of the adjacent intensity maxima are at least significantly different, if not in antiphase.
As most targets are not perfectly stationary, the LDV will not stay within one bright speckle during the measurement. The detector will see different dark and bright speckles. As the speckles have different phases, this mechanism causes noise in the vibrometer output. The speckle noise produced depends on the rate of change in the speckle pattern and is therefore strong when measuring from rotating targets or surfaces that show a strong in-plane motion.
The electronics in Polytec's signal processors incorporate sophisticated circuits to minimize the effects produced by speckle "dropouts". In addition, speckle tracking routines and optimized scan mirror control are available with Polytec's scanning vibrometer to minimize noise effects coming from speckles. When the laser beam is not brought to rest immediately after a measurement point has been reached, a lateral movement of the beam will produce noise because of speckle effects.
|
