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7 September 2018 Grating (moiré) interferometry with decreased sensitivity
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Proceedings Volume 10834, Speckle 2018: VII International Conference on Speckle Metrology; 1083426 (2018)
Event: SPECKLE 2018: VII International Conference on Speckle Metrology, 2018, Janów Podlaski, Poland
Grating Interferometry well known also as High Sensitivity Moiré Interferometry is the optical method for in-plane displacement and strain measurement. The sensitivity of this method depend on spatial frequency of the diffraction grating attached to the object under test. For typical specimen grating, with spatial frequency 1200 lines/mm, the basic sensitivity is 0.417 μm/fringe. On the other hand, the high sensitivity makes the measuring range to small for many applications in experimental mechanics and material engineering. In the paper the simple idea for decreasing sensitivity of the Grating Interferometry by using the gratings with lower spatial frequencies of 100 - 200 lines/mm is presented. Additionally, in this case the measurement sensitivity can be controlled by matching the illumination angles to the higher diffraction orders of the gratings, e.g. for grating frequency 200 lines/mm, the basic sensitivity is 2.5 μm/fringe for the first and 1.25 μm/fringe for the second diffraction orders. Such sensitivities can be delivered by other optical techniques, e.g. Digital Image Correlation but the Grating Interferometry has the unique feature - the specimen grating "remembers" its initial state. It means that all successive measurements have the same reference (moment when the grating is attached to the object surface) what allows to obtain information about the cumulative displacement between any measurement periods and monitoring the state of the object under test at any time. Theoretical analysis of proposed technique and concept of the Decreased Sensitivity Grating Interferometer design are described and discussed.
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Leszek Salbut "Grating (moiré) interferometry with decreased sensitivity", Proc. SPIE 10834, Speckle 2018: VII International Conference on Speckle Metrology, 1083426 (7 September 2018);

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