The stand-off distance is measured from the front panel of the scanning head.The stand-off distance is measured from the front panel of the scanning head.
The optimal stand-off distances are:The optimal stand-off distances are:
i.e. at 14mm; 217i.e. at 14mm; 217mm; 420mm; 623mm; etc.mm; 420mm; 623mm; etc.
The light source of the PSV is a helium neon laser. This is a multimode laserThe light source of the PSV is a helium neon laser. This is a multimode laser
in which a maximum of two modes can exist. The interference of the twoin which a maximum of two modes can exist. The interference of the two
modes leads to the modes leads to the intensity of the resulting optical signal varying intensity of the resulting optical signal varying periodicallyperiodically
with the stand-off distance. The intensity increases to a maximum, i.e. awith the stand-off distance. The intensity increases to a maximum, i.e. a
maximum of visibility is present if the optical maximum of visibility is present if the optical path difference is an even-path difference is an even-
numbered multiple of the length of the laser cavity (203numbered multiple of the length of the laser cavity (203 mm). As the opticalmm). As the optical
path difference is equal to twice the stand-off distance (the beam goes therepath difference is equal to twice the stand-off distance (the beam goes there
and back), a maximum of and back), a maximum of visibility is present once per laser visibility is present once per laser cavity length.cavity length.
In practice, it is not usually necessary to search for the maximum of visibilityIn practice, it is not usually necessary to search for the maximum of visibility
as the PSV is sensitive enough to make a measurement even close to theas the PSV is sensitive enough to make a measurement even close to the
minimum. A minimum is indicated during the minimum. A minimum is indicated during the warm-up phase by periodicwarm-up phase by periodic
fluctuation on the signal level display.fluctuation on the signal level display.
1414mm + nmm + n
⋅⋅
220033mmmm,, n n = = 00; ; 11; ; 22; .; .....
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