Digital holographic microscope recordings of passive motion of small particles

Digital holographic microscopy provides the ability to observe throughout
a volume that is large compared to its resolution without the need to
actively refocus to capture the entire volume. This enables simultaneous
observations of large numbers of small objects within such a volume. We
have constructed a microscope that can observe a volume of 0.4 x 0.4 x 1.0
µm with submicrometer resolution (in xy) and 2 µm resolution (in z) for
observation of microorganisms and minerals in liquid environments on Earth
and on potential planetary missions. Because environmental samples are
likely to contain mixtures of inorganics and microorganisms of comparable
sizes near the resolution limit of the instrument, discrimination between
living and non-living objects may be difficult. The active motion of
motile organisms can be used to readily distinguish them from non-motile
objects (live or inorganic), but additional methods are required to
distinguish non-motile organisms and inorganic objects that are of
comparable size but different composition and structure. We demonstrate
the use of passive motion to make this discrimination by evaluating
diffusion and buoyancy characteristics of cells, styrene beads, alumina
particles, and gas-filled vesicles of micron scale in the field of view.