Polytec PSV-500-3D, Operating Instructions Manual

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4-5
4 Measuring
4.3 Setting Up Optimal Stand-Off Distance (Only for PSV-500 Standard)
4.3.1 Relation between Stand-Off Distance and Visibility Maximum
INFORMATION
With PSV-500 Xtra there are no visibility minima and therefore no periodic
fluctuations of the signal level depending on the stand-off distance.
Visibility
maxima
The signal-to-noise ratio (SNR) of the data is primarily determined by the
signal level. The higher the signal level, the higher the signal-to-noise ratio.
The height of the signal level depends on the surface properties of the object,
on random speckle effects, and on the stand-off distance. Rule of thumb: The
higher the stand-off distance, the lower the signal level. This coherency may
be superimposed by periodic fluctuations that are described in the following.
The light source of the instrument is a helium-neon laser. This is a multi-mode
laser, in which there can be typically two longitudinal modes. Interference
between the laser modes can result in a lower signal level. The degree of this
lowering depends on the stand-off distance and the temperature. The laser
cavity length varies caused by changes in temperature. Yet small changes in
temperature can lead to considerable changes in the ratio of the intensities of
the laser modes and thus to important changes of the signal level.
The stand-off distances where the signal level is at its maximum are called
visibility maxima. The visibility maxima recur every 204 mm (± 1 mm)
corresponding to the laser cavity length.
The diagram in F
IGURE
4.3 shows the signal level depending on the stand-off
distance. At certain distances (visibility maxima) the signal level is constantly
high. Between two visibility maxima there is a visibility minimum each in which
under certain circumstances the signal level may drop completely. Depending
on the ratio of the intensities of the laser modes different progresses will
result. At equal intensity of both modes (black line) you have the strongest
loss of signal level. One of both modes dominates, then depending on the
stand-off distance the signal level hardly fluctuates (gray line) or does not
fluctuate at all (dashed gray line).
To get the best signal level independent of the laser state, choose a stand-off
distance near the visibility maximum.
For larger objects it is not possible for all sample points to position the
scanning head into the visibility maximum. Since the laser for sample points
on the edge of the area of interest usually hits the surface at an unfavorable
angle, the signal level is lower there. In this case, optimize the stand-off
distance for sample points on the edge of the area of interest.