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.

数字全息显微镜（digital holographic microscopy）可在无需主动重新聚焦以获取完整体积的前提下，对远大于其分辨率的三维体积区域开展全范围观测，从而实现对该体积内大量小型目标的同步观测。我们研发了一款专用显微镜，其可覆盖0.4×0.4×1.0 μm的观测体积，横向（xy方向）分辨率达亚微米级，轴向（z方向）分辨率为2 μm，可用于地球及潜在行星探测任务中液态环境内的微生物与矿物观测。由于环境样本通常混杂着尺寸接近仪器分辨率极限的无机物与微生物，因此难以直接区分活体与非活体目标。可运动微生物的主动运动可轻松将其与非运动目标（活体或无机物）区分开来，但要区分尺寸相近但成分与结构各异的非运动微生物与无机物，则需借助额外的识别手段。本研究通过评估视场内微米级尺寸的细胞、聚苯乙烯微球、氧化铝颗粒以及充气囊泡的扩散与浮力特性，验证了利用被动运动实现该类区分的可行性。