Digital holographic microscope for measuring three-dimensional particle distributions and motions

TLDR

Better understanding of particle‑particle and particle‑fluid interactions requires accurate 3D measurements of particle distributions and motions. The study introduces in‑line digital holographic microscopy as a viable tool for measuring dense micrometer and submicrometer particle distributions in liquid solutions with depths of 1–10 mm. The method records a magnified hologram to achieve a depth of field about 1000× the particle diameter and a depth of focus of ~10 diameters, then uses cinematographic holography to resolve thousands of particles and track their motions. The approach yields a depth of field roughly 1000× the particle diameter and a depth of focus of about 10 diameters, enabling quantitative measurement of axial resolution and the tracking of thousands of particles, as demonstrated by detailed trajectories and morphology of a free‑swimming copepod nauplius.

Abstract

Better understanding of particle-particle and particle-fluid interactions requires accurate 3D measurements of particle distributions and motions. We introduce the application of in-line digital holographic microscopy as a viable tool for measuring distributions of dense micrometer (3.2 microm) and submicrometer (0.75 microm) particles in a liquid solution with large depths of 1-10 mm. By recording a magnified hologram, we obtain a depth of field of approximately 1000 times the object diameter and a reduced depth of focus of approximately 10 particle diameters, both representing substantial improvements compared to a conventional microscope and in-line holography. Quantitative information on depth of field, depth of focus, and axial resolution is provided. We demonstrate that digital holographic microscopy can resolve the locations of several thousand particles and can measure their motions and trajectories using cinematographic holography. A sample trajectory and detailed morphological information of a free-swimming copepod nauplius are presented.