Point sources for digital lensless holographic microscopy: comparative assessment

The light source in digital lensless holographic microscopy (DLHM) plays a key role in the optical performance of this microscopy methodology. The underlying physics that support DLHM rely on having an outward propagating spherical wavefront illuminating the sample to record on the surface of a digital camera the intensity of the diffracted wavefield that is magnified by free-space propagation; this first stage constitutes the recording. The information of the sample is numerically retrieved by computing the diffraction that an inward propagating spherical wavefront undergoes in the recorded intensity in a stage known as reconstruction. In both stages of this lensless microscopy method, it is assumed that perfect spherical wavefronts are utilized; hence, the closer the illumination light sources to exp[±ikr]/|r| are, the better the performance of the microscope. For the recovery of the information of the sample, the accuracy of the inward propagating spherical wavefront exp[−ikr]/|r| is guaranteed by its correct sampling in the numerical computation of the diffraction process; thus, the main challenge is the physical production of light sources to illuminate the sample whose amplitude profile is accurately described by exp[ikr]/|r|. In this work, the different approaches that our research group has explored to produce illuminating point sources for DLHM are evaluated. Point sources based on pinholes, coned-shaped optical fibers, optical pick-up units, and aspheric lenses are evaluated by rating their robustness, compactness, reliability, cost and easiness of production, and their performance on imaging test targets and biosamples. Some additional DLHM light sources reported in the literature are also briefly reviewed based on their available data.