Despite the rapid progress in optical imaging, most of the advanced microscopy modalities still require complex and costly set-ups that unfortunately limit their use beyond well equipped laboratories. In the meantime, microscopy in resource-limited settings has requirements significantly different from those encountered in advanced laboratories, and such imaging devices should be costeffective, compact, light-weight and appropriately accurate and simple to be usable by minimally trained personnel. Furthermore, these portable microscopes should ideally be digitally integrated as part of a telemedicine network that connects various mobile health-care providers to a central laboratory or hospital. Toward this end, here we demonstrate a lensless on-chip microscope weighing 46 grams with dimensions smaller than 4.2cm × 4.2cm × 5.8cm that achieves sub-cellular resolution over a large field of view of ~24 mm 2 . This compact and light-weight microscope is based on digital in-line holography and does not need any lenses, bulky optical/mechanical components or coherent sources such as lasers. Instead, it utilizes a simple light-emitting-diode (LED) and a compact optoelectronic sensor-array to record lensless holograms of the objects, which then permits rapid digital reconstruction of regular transmission or differential interference contrast (DIC) images of the objects. Because this lensless incoherent holographic microscope has orders-of-magnitude improved light collection efficiency and is very robust to mechanical misalignments it may offer a cost-effective tool especially for telemedicine applications involving various global health problems in resource limited settings.

We experimentally illustrate a lensfree holographic imaging platform to perform on-chip cytometry. By controlling the spatial coherence of the illumination source, we record a 2D holographic diffraction pattern of each cell or micro-particle on a chip using a high resolution sensor array that has ~2 µm pixel size. The recorded holographic image is then processed by using a custom developed decision algorithm for matching the detected hologram texture to existing library images for on-chip characterization and counting of a heterogeneous solution of interest. The holographic diffraction signature of any microscopic object is significantly different from the classical diffraction pattern of the same object. It improves the signal to noise ratio and the signature uniformity of the cell patterns; and also exhibits much better sensitivity for on-chip imaging of weakly scattering phase objects such as small bacteria or cells. We verify significantly improved performance of this holographic on-chip cytometry approach by automatically characterizing heterogeneous solutions of red blood cells, yeast cells, E. coli and various sized micro-particles without the use of any lenses or microscope objectives. This lensless on-chip holography platform will especially be useful for point-of-care cytometry and diagnostics applications involving e.g., infectious diseases such as HIV or malaria.

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We present a detailed investigation of the performance of lens-free holographic microscopy toward high-throughput on-chip blood analysis. Using a spatially incoherent source that is emanating from a large aperture, automated counting of red blood cells with minimal sample preparation steps at densities reaching up to ~0.4 × 10 6 cells/μL is presented. Using the same lens-free holographic microscopy platform, we also characterize the volume of the red blood cells at the single-cell level through recovery of the optical phase information of each cell. We further demonstrate the measurement of the hemoglobin concentration of whole blood samples as well as automated counting of white blood cells, also yielding spatial resolution at the subcellular level sufficient to differentiate granulocytes, monocytes, and lymphocytes from each other. These results uncover the prospects of lens-free holographic on-chip imaging to provide a useful tool for global health problems, especially by facilitating whole blood analysis in resource-poor environments.Together with several recent contributions, lens-free imaging 1-11 is now becoming more and more important, especially for cytometry and microscopy applications. Among others, one field that would enormously benefit from lens-free on-chip digital imagers is the field of microfluidics. 12,13 Over the past decade, microfluidics has revolutionized the available toolset to handle cells by significantly reducing the required device and reagent volumes, as well as the associated costs. Lens-free on-chip imaging could provide further reduction in the size and the cost of these microfluidic systems, while, at the same time, improving the throughput of imaging and/or characterization. To complement these advances in lens-free imaging and microfluidics, recently, we have introduced an on-chip holographic microscopy platform with subcellular resolution over a large field of view (~24 mm 2 ). [8][9][10][11] In this technique, by controlling the spatial coherence of the illumination source (which can be a simple lightemitting diode), we record the two-dimensional (2D) holographic diffraction signature of each cell or microparticle on an opto-electronic sensor array without the use of any lenses or lasers (see Figure 1). This recorded holographic pattern is then rapidly processed using a customdeveloped algorithm 9,10 to perform wide field-of-view (FOV) microscopy of semitransparent objects such as blood cells on the chip.Providing a spatial resolution of ~1.4-1.5 μm, 9,11 which is comparable to a 10× objective-lens with a numerical aperture of ~0.1-0.2 at ~500-600 nm, but with an order-of-magnitude-larger * Author to whom correspondence should be addressed. FOV (e.g., 24 mm 2 vs ~2 mm 2 ), our lens-free microscopy platform is capable of imaging a large number of cells simultaneously. As a result, this system meets the requirements and needs of whole blood analysis applications, which inevitably necessitate high-throughput imaging and characterization. Toward this end, here, we presen...