11.3 Experimental Setup A portable setup with remotely controlled capabilities was realized at Worcester Polytechnic Institute (WPI) and moved inside a double-walled sound booth located in the facilities of the Massachusetts Eye and Ear Infirmary (MEEI). The HDHM setup includes a high-speed camera (Photron SA-5), a CW laser (532 nm, 50 mW), variable beam splitter, beam combiner wedge, mirrors, beam expanders, piezoelectric phase shifter, laser to fiber coupler and a holographic otoscope, as shown in Fig. 11.3. The laser beam is split into reference and illumination beams by use of a beam splitter. The illumination beam, after being coupled into a fiber optic waveguide, irradiates the TM through the earcanal. The object beam and the reference beam are combined at the CCD camera by use of the wedge. The sample is excited by a speaker (placed within 15 cm of the TM surface) with 50μs square pulses that produce an acoustic click. The sound pressure variations at the surface of the TM are measured by a probe microphone. During the measurements, cooling fans associated with the laser and camera systems are switched off. All the measurements were repeated thrice consecutively with 100 ms intervals to investigate the repeatability of the motions under various environmental conditions. All animal procedures were approved by the Massachusetts Eye and Ear Animal Care Committee. Chinchillas were anesthetized and remained so throughout the in-vivo measurements. The cartilaginous and boney part of ear canals were resected to expose the majority of the TM surface and the middle ear cavity was vented, but otherwise intact. The TM was painted with a thin layer of ZiO solution for improved reflectivity. After a series of in-vivo measurements, the animal was euthanized and measurements repeated without repositioning the animal. Fig. 11.2 Schematic representation of the HDHS and the acquisition timing diagrams: (a) timing diagram demonstrating the principle of operation of the high-speed 2 + N frame acquisition method [7]; and (b) schematic representation of the developed high-speed holographic system equipped with a custom-made otoscope. Illumination and observation inside the ear canal is performed by fiber optics and a GRadient INdex (GRIN) rod lens, respectively. The otoscope has a relatively large depth of focus, which is sufficient to measure motions of the coneshaped TM 11 High-Speed Holography for In-Vivo Measurement of Acoustically Induced Motions of Mammalian Tympanic Membrane 77
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