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Visualization of skin vibration induced by a bone-conduction device using a high speed digital holographic recording

This paper proposes a first attempt to visualize and analyze the vibrations propagating at the surface of a human face's skin induced by a bone conduction device. The proposed method allows the qualitative visualization and quantitative measurement of the surface movements illuminated by a coherent laser beam. To do this, we developed a new approach in a so-called "quasi-time-averaging regime" allowing the retrieval of the vibration amplitude and phase from a sequence of digital Fresnel holograms recorded with a high image rate. The experimental set-up is based on off-axis digital Fresnel holography and a high power continuous wave laser. The sensor is a high speed CMOS camera permitting recordings with a high spatial resolution (1024×1024 pixels) up to 2.4kHz. The set-up is able to provide full field measurements in the frequency bandwidth 100Hz-600Hz. Recording in the quasi-time-averaging regime leaded to the development of a dedicated algorithm able to extract the vibration using only three holograms from the sequence. The design of the algorithm depends on the ratio between exposure time and vibration period. Results exhibit propagation of vibrations at the skin surface, amplitudes being at most at 200nm, and speed velocity can be estimated at each frequency.