High-Speed Imaging of Gas Flow by Parallel Phase-Shifting Digital Holography

We succeeded in high-speed imaging of gas flow by means of parallel phase-shifting digital holography. This technique is capable of capturing three-dimensional (3-D) information of object and carrying out phase-shifting interferometry with a single-shot exposure because the interference fringe images in which the information of multiple phase-shifted holograms is spatially multiplexed are simultaneously recorded. We constructed a high-speed phase-shifting digital holography system by employing a quarter-wave plate and a high-speed camera. The image sensor of the camera has an anisotropic polarization-detecting function pixel by pixel. Each pixel of the polarization-detecting function corresponds to each pixel of the image sensor. The phase retardation of the reference wave is determined by the direction of the polarization axis of the each pixel. A compressed gas flow sprayed from a nozzle was set as an object. We attained the reconstructed images of phase variation caused by the gas flow. We also succeeded in phase imaging at the rate of 180,000 frames per second when the number of pixels of the captured image was 128 × 128. Additionally, we also obtained temporal subtraction images of the reconstructed images. The achieved frame rate was the fastest among not only phase-shifting digital holography but also digital holography and phase-shifting interferometry which have been ever reported, for our knowledge. It is expected that parallel phase-shifting digital holography and the constructed system can contribute to 3-D moving picture measurement of dynamically moving objects such as particle flows, shock waves, mechanical vibration, and so on.