3D Pulsed Digital Holography for the Simultaneous Acquisition of Vibration Displacements
Non contact full field optical techniques such as speckle and holographic interferometry are being developed in order to apply them to the study of displacements, crack detection or any other type of defects that are in areas hidden within an object, superficially or within its walls. Both techniques have been well received in the past as a solution to 3D measure static and dynamic events from object areas easily imaged and in many types of environmental conditions. A drawback to date has been the gaining of access to hidden areas, mainly because the imaging device, e.g., an endoscope, did not have enough image resolution and thus both techniques did not render significant results, Ref. 1. Besides, the interferogram analysis for speckle and holography had to be done using data stored on photographic and holographic films, respectively, Ref. 1-3. Good quality image endoscopes and high resolution CCD sensors appeared recently, making it possible to design systems capable of obtaining out-of-plane displacements, Refs. 4-6. Non endoscopic pulsed speckle and digital holographic interferometric systems have been used in the past to measure displacements along all three perpendicular coordinate axes X, Y and Z. The optical set up relied on the object surface being illumined from three diverging sources located at three arbitrary, but known, positions, Ref. 7. The majority of those systems acquire the information sequentially, meaning that the experiment has to be repeated several times. There are however, occasions in which the experiment cannot be repeated and then a system was designed to simultaneously acquire in a single CCD frame the three orthogonal displacement components, Ref. 8, from the easily seen surface of a harmonically vibrating object.
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