Development of a light-field fluorescence microscope for in situ life searches in the solar system

With oceans and energy sources present on several outer solar system moons, it is natural ask whether life might also be present on such bodies. To search for microbial life, 3-d imaging microscopes are needed that can efficiently search through sizeable liquid volumes for particles with signs of cellular structure and morphology, as well as to examine motilities. Our 3-d digital holographic microscopy approaches have been described earlier; that technique can provide sample amplitude and phase information across a volume. On the other hand, fluorescence imaging can provide complementary chemical information, and fluorescence light-field microscopy (FLFM) is an emerging approach to 3-d fluorescence imaging. Here we describe a 3-d FLFM imager concept, with the ultimate goal of combining both microscope types into a multi-mode microscope for lander instrument packages. Light-field imaging is a technique that can be used to refocus to different depths without mechanical refocusing. This technique generally results in lower resolution than standard imaging, but compensates with the ability to refocus to planes well beyond the normal depth of field. However, when imaging sparse, point-like fluorescent signals, or when simultaneous higher-resolution 3-d images of the entire field are available with a complementary 3-d microscope, such as, e.g., a digital holographic microscope, the finest spatial resolution may not be necessary for the light-field fluorescence microscope. In this paper, we first consider the parameters describing a light-field microscope, in order to be able to optimize them to our application. Refocusing to different sample planes is then modeled with a simple ray-trace algorithm. Our goal is to reach a suitable compromise between spatial resolution, field of view, and depth of field within a coincident volume of view for the fluorescence microscope and the digital holographic microscope. Even without the use of super-resolution techniques, lateral and axial resolutions on the order of several microns are possible, across a field of view on the order of a millimeter and a depth of field of a few tenths of a millimeter, both of which are comparable to the values obtained with our digital holographic microscopes. Finally, we describe a prototype fluorescent light-field microscope benchtop setup with which we have carried out initial laboratory demonstrations.