Drop-out noise of laser vibrometers measuring on varnished steel surfaces of appliance cabinets for industrial diagnostics

摘要

It is presented the importance of signal drop-out when using laser Doppler vibrometry on varnished metal surfaces, which exhibit a large specular reflectance. The relevance of this class of surfaces is presented in the context of diagnostics of appliances, where laser vibrometry is being used in an increasing variety of applications. The paper recalls the state-of-art on speckle noise and drop-out noise and then presents experiments aimed to put in light the relationship between laser light scattering on varnished metal sheet, with Doppler signal and drop-out noise. It is introduced a non-dimensional quantity called Signal Quality (SQ), defined as the RMS value of the Doppler signal amplified logarithmically, and it is shown its correlation to drop-out noise. It is shown how drop-out noise depends on both the value of SQ and its time variations. Spatial distributions maps of SQ are presented; their analysis allows to highlight a possible strategy to displace the laser beam in the surrounding of a target measurement point in order to keep SQ large and stable during the measurements, so to reduce signal drop-out to a manageable level.