Direct Measurement of Three-Dimensional Velocity by a Doppler-Phase-Shifting Holography

In order to understand the details of the flow field in the microand nanofluidic devices, it is necessary to measure the three-dimensional velocity field under a microscopy. Thus, the development of a new measuring technique for the three-dimensional velocity by a single camera is strongly needed. One solution is the use of holography, but it has been known that the resolution in the depth direction is very poor for the commonly used in-line holography. Presently, the Doppler-phase-shifting holography has been used for the three-dimensional measurement of an object. This method extracts the signal of a fixed frequency caused by the Doppler howling between the object light and the reference light, either or both of which travels at a constant speed. Applying the shifting to the holography, the threedimensional shape of an object can be measured very precisely. Here, the frequency of the Doppler howling is determined by the velocity difference between the object light and the reference light. This implies that the velocity of an object can be measured by the Doppler frequency. In this study, a concave mirror was traversed at a constant speed and its holography has been observed by a high-speed camera. By extracting only the first order refraction signal at the Doppler frequency, the 0th order object light can be removed from the hologram and thus a precise measurement of the surface profile has been achieved. At the same time, the lateral velocity of the mirror can be measured by the Doppler frequency. Furthermore, a 5 yen coin has been traversed by different velocities at different angles and its shapes and the three-dimensional velocities have been measured. The transverse velocities are measured by the PIV algorithm and at the same time the longitudinal velocity is measured by the Doppler frequency. The results reveal that the precise measurement of the three-dimensional velocity by a single camera is possible by the use of the Doppler-phase-shifting holography. This method can be applied to the particles motions in the microor nanodevices, and thus the threedimensional velocities will be measured under a microscopy.