17 June 2009 Industrial inspections by speckle interferometry: general requirements and a case study
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Digital speckle pattern interferometry is potentially capable to solve a large variety of measurement and inspection demands in industrial applications. However, it is not so widely used in industry due to some special requirements that are not easily fulfilled on the shop floor. This paper presents some reflections about what would be necessary for an interferometer to be successfully applied in industrial environments: it must be robust, flexible, compact, stable, friendly and cooperative. Next, a case study, that fulfills those requirements, is presented in details. It involves a digital speckle interferometer designed to measure residual stresses in-field. It was developed using an axis-symmetrical diffractive optical element in such a way that it is not sensitive to the laser wavelength at all. It produces radial in-plane sensitivity on a circular region. The interferometer was accommodated in a compact construction that made it robust enough for infield applications. A magnetic kinematic mounting base is used to firmly attach the interferometer to the surface where residual stresses have to be measured. The same kinematic base is used for positioning an ultra-high speed pneumatic drilling unit. In order to measure residual stresses, a reference phase pattern is first acquired from a sequence of four 90° phase-shifted images. After that, a small blind hole is drilled on the surface to be measured. The residual stresses are then relieved at the borders of the blind hole, what produces a local displacement filed. A second phase pattern is then acquired. The radial displacement filed is computed from the phase difference and it is fitted to a mathematical model. The principal residual stresses are then determined. The interferometer was used outside of the laboratory for residual stresses measurement in a gas pipeline in a risky area. The goal was to investigate the effectiveness of a repair.
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Matias R. Viotti, Armando Albertazzi G., "Industrial inspections by speckle interferometry: general requirements and a case study", Proc. SPIE 7389, Optical Measurement Systems for Industrial Inspection VI, 73890G (17 June 2009); doi: 10.1117/12.827585; https://doi.org/10.1117/12.827585


Diffractive optical elements






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