Continuous scanning laser Doppler vibrometry and speckle noise occurrence

Continous scanning laser Doppler vibrometry (CSLDV) is a type of ‘‘spatial field’’ non-contact technique for measuring structural vibrations by employing a laser Doppler vibrometer whose laser beam is moving continuously on the structure surface. When an LDV is scanned continuously along an arbitrarily line, the LDV output is an amplitude-modulated sine wave according to the structure operational deflection shape. Smooth mode shapes, which can be defined by polynomial functions across the scanned area, may be recovered as a set of polynomial coefficients derived from the LDV output analysed in the frequency domain, which spectrum comprises sidebands centred on the excitation frequency and spaced at multiples of the scan frequency(ies). This paper concentrates its attention to the influence of the speckle noise on the measured data quality, the speckle being an unavoidable phenomenon occurring when a coherent light beam is scattered back from an optically rough surface.