Chapter 10 Holographic Measurement of Semi-transparent Tympanic Membrane Shape Using Multiple Angle Illuminations H. Tang, P. Psota, J. J. Rosowski, J. T. Cheng, and C. Furlong Abstract The shape of the tympanic membrane (TM) plays an important role in conducting acoustic energy from the environment to the ear for hearing. Changes of the TM shape also have the diagnostic value, e.g., bulging of the TM is a sign of middle ear effusion. Previously we developed a high-speed holographic system employing a tunable wavelength laser for rapid TM shape measurement. However, because of the semi-transparency of the TM and the short-exposure time required for the high-speed acquisition, the illumination intensity (0.01 mW/mm2) of the tunable laser was insufficient for measurements on the semi-transparent TM surface. Here, we describe a multi-angle illumination technique that allows us to use a single wavelength (532 nm) laser with higher illumination intensity (0.06 mW/mm2) to perform shape measurements of the semitransparent TM. The accuracy of the proposed method is demonstrated by measurements of a stepwise gauge provided by the National Institute of Standards and Technology. We successfully applied the abovementioned method to resolve the shape of the fresh postmortem human TM. We see a potential for miniaturization of the apparatus into a holographic otoscope for in-vivo measurements. Keywords High-speed holography · Human tympanic membrane · Single wavelength shape measurement 10.1 Introduction Full field of view and noninvasive measurement of the surface shape of the tympanic membrane (TM) with high resolution is of interest for hearing research. The vibration of the TM initializes sound energy transmission from the environment to the inner ear [1, 2]. The TM’s complex geometry (shape and thickness) affects the energy transmission [3] and is essential to understanding the mechanics of the TM. The shape information also has diagnostic values for middle ear pathologies, e.g., bulging of the TM is a sign of middle ear effusion. Our group has developed a high-speed multiple wavelength Digital Holographic (DH) system using a tunable laser to measure the TM shape in cadaveric human ears [4, 5]. However, the tunable laser illumination is not sufficient to measure the shape of the TM without surface preparation due to the semitransparency of the TM and short exposure time for high-speed image acquisition. To overcome this limitation and enable the high-speed holographic system for in vivo measurement, we apply a high-power single wavelength laser to resolve the shape with a multiple angle DH method [6] together with a temporal phase unwrapping approach to perform robust and H. Tang ( ) Center for Holographic Studies and Laser micro-mechaTronics (CHSLT), Worcester Polytechnic Institute, Worcester, MA, USA Mechanical Engineering Department, Worcester Polytechnic Institute, Worcester, MA, USA e-mail: htang3@wpi.edu P. Psota Faculty of Mechatronics, Informatics and Interdisciplinary Studies, Technical University of Liberec, Liberec, Czech Republic J. J. Rosowski · J. T. Cheng Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, MA, USA Department of Otology and Laryngology, Harvard Medical School, Boston, MA, USA e-mail: John_Rosowski@meei.harvard.edu; Tao_Cheng@meei.harvard.edu C. Furlong Mechanical Engineering Department, Worcester Polytechnic Institute, Worcester, MA, USA e-mail: cfurlong@wpi.edu © The Society for Experimental Mechanics, Inc. 2021 M.-T. Lin et al. (eds.), Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59773-3_10 79
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