Chapter 5 Noise Reduction in Amplitude-Fluctuation Electronic Speckle-Pattern Interferometry Sanichiro Yoshida, David Didie, Jong-Sung Kim, and Ik-Keun Park Abstract Amplitude-Fluctuation Electronic Speckle Pattern Interferometry is used in a variety of vibration analyses. The technique utilizes the fact that when the vibration frequency of the object is significantly higher than the frame rate of the imaging device, the interference term can be approximated by the lowest (0th) order of the first kind Bessel function. Since the 0th order Bessel function takes the maximum value of unity when the vibration amplitude is null, the amplitude of a given vibration can be estimated from the reduction of the interference term relative to the case when the object is still. In reality, however, various environmental noises, such as temperature fluctuation of air in the interferometric paths and floor motion transferred through the optical table, cause low frequency fluctuations of the interference term, and thereby compromise the integrity of data. In this paper, we discuss typical environmental noise on standard optical interferometer settings, and propose to reduce the effect of the noise on the signal by introducing a carrier fringe system and analyzing the fringe pattern in the spatial frequency domain. The effectiveness of the proposed method is assessed for our recent Michelson interferometer experiment in which vibration of thin-film specimen is characterized. Keywords Michelson interferometer • Amplitude-Fluctuation Electronic Speckle-Pattern Interferometry • Phase noise • Carrier Fringes • Thin-film adhesion 5.1 Introduction Optical interferometry is widely used in various subfields of experimental mechanics. An interferometer is a devise to convert phase to intensity. The relative phase difference between the two interferometric arms is read out as a change in the intensity of the combined optical fields. Since there is no theoretical limitation in the phase resolution other than Heisenberg’s uncertainty principle, the sensitivity in length measurement of an optical interferometer can be at the quantum physical level. This very advantage, however, makes an optical interferometer be vulnerable to environmental noise. It is important to deal with phase noise appropriately. Another point of argument is the data acquisition time. Since light travels fast, the temporal behavior of the relative phase difference can be analyzed fast. This also means that a fast photo-detector is necessary to resolve the fast phenomenon. If the phase information is one-dimensional, i.e., simply associated with the arm length change, it is relatively easy to use a fast photo-diode such as a PIN-photo-diode. However, if the phase information is to be analyzed two-dimensionally, e.g., a full-field analysis of fast deformation of an object, often the data acquisition speed becomes an issue. A digital camera is most likely used to record the optical intensity that contains the phase information. Recent technological advances make high frame rate digital cameras commercially available. It is possible to obtain a digital camera with a frame rate of the order of 106 frames per second or higher. However, the price of these fast devices is often beyond our budget, and in addition, there are still technical restrictions such as the limitation in the area of view in exchange for a high frame rate. When an oscillatory phenomenon is to be analyzed with a digital camera whose frame rate is significantly lower than the oscillation frequency, Amplitude-fluctuation Electronic Speckle Pattern Interferometry (AF-ESPI) [1, 2] is useful. Basically, this technique estimates the oscillation amplitude of the test object from the interferometric fringe pattern formed in a twodimensional, full-field image. Although the acquired image is time-averaged over the exposure time of the digital camera, S. Yoshida ( ) • D. Didie • I.-K. Park Department of Chemistry and Physics, Southeastern Louisiana University, SLU 10878, Hammond, LA, 70402, USA e-mail: syoshida@selu.edu J.-S. Kim Department of Chemistry and Physics, Southeastern Louisiana University, SLU 10878, Hammond, LA, 70402, USA Department of Mechanical and Automotive Engineering, Seoul National University of Science and Technology, Nowon-gu, Seoul, South Korea © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_5 27
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