Application of GRIN-lens-based in-line digital holographic microscopy to automatic detection and localization of...

Recently, we developed a compact and easy-to-implement in-line digital holographic microscope (DHM) using a GRIN rod lens, which provides better resolution (1.3 µm) compared with commonly used pinhole-based DHM setups. Here, we employ this microscope to acquire 3D holographically reconstructed images of silica microparticles, within the 10-300 µm size range, launched/released from a microelectro-mechanical systems (MEMS) device. To the best of our knowledge, this is the first time that a MEMS device is implemented to store and launch microparticles. The custom-designed MEMS device consists of a 50 µm thick flat circular silicon ultrasonic membrane mounted on an off-the-shelf piezoelectric transducer. Moreover, we propose and experimentally demonstrate a new automatic hybrid detection and localization method for particle field holography that benefits from a combination of well-known minimum intensity and variance of gray level distribution focus metrics. This robust method is fast and provides precise d e p t h/z position of particles. The proposed method is applied for particle testing of the MEMS device, reconstructing 3D visualization, and measuring the size and velocity of released particles. The obtained experimental results show that the velocity of released particles, previously dry-loaded onto the MEMS device, is of the order of a few tens of cm/s.