High-speed imaging of sound using
parallel phase-shifting interferometry
parallel phase-shifting interferometry
K. Ishikawa et al., ``High-speed imaging of sound using parallel phase-shifting interferometry,'' Optics Express, vol.24, no.12, pp.12922－12932, 2016. [Paper]
Sound-field imaging, the visualization of spatial and temporal distribution of acoustical properties such as sound pressure, is useful for understanding acoustical phenomena. This study investigated the use of parallel phase-shifting interferometry (PPSI) with a high-speed polarization camera for imaging a sound field, particularly high-speed imaging of propagating sound waves. The experimental results showed that the instantaneous sound field, which was generated by ultrasonic transducers driven by a pure tone of 40 kHz, was quantitatively imaged. Hence, PPSI can be used in acoustical applications requiring spatial information of sound pressure.
Measurement system
PPSI is an optical interferometer equiped with a high-speed polarized camera (HPC). The HPC captures four phase-shifted interferograms in a single image, thereby enabling quantitatively obtain a 2D optical phase map using phase-shifting algorithm. Since the HPC can operate up to 1.5 million frames per second, a slow-motion video of 2D optical phase is recorded. We proposed to utilize this instrument for sound-field imaging. Because the PPSI has high sensitivity across all frequencies from audible to ultrasonic range, it can be very a powerful tool for observing acoustic phenomena.
Measurement principle
PPSI detects four phase-shifted interference fringe by single-shot. The key component is HPC, a high-speed imaging device equipped with pixelated polarizer mask. Micro fabrication of tiny linear polarizers allows us to detect four 90 degree phase-shifted fringes, which enables to calculate 2D optical phase map from a single image. This unique feature provides ability to image 2D sound field in real-time with audio or ultrasound frame rate. We have also developed signal processing pipeline to extract sound field from a high-speed optical phase video based on temporal or spatiotemporal filters.