in a closely related technique, the presence of cracks in a solid structure can be detected by looking for differences in resonance frequency, bandwidth and resonance amplitude compared to a nominally identical but non-cracked structure. this technique, called ruv (resonance ultrasonic vibrations), has been developed for use in the photovoltaics industry by a group of researchers from the university of south florida, ultrasonic technologies inc. (florida, us), and isofoton s.a. (spain). the method was able to detect mm-size cracks in as-cut and processed silicon wafers, as well as finished solar cells, with a total test time of under 2 seconds per wafer.