Lensfree on-chip microscopy over a wide field-of-view using pixel super-resolution

Abstract

We demonstrate lensfree holographic microscopy on a chip to achieve ~0.6 µm spatial resolution corresponding to a numerical aperture of ~0.5 over a large field-of-view of ~24 mm2. By using partially coherent illumination from a large aperture (~50 µm), we acquire lower resolution lensfree in-line holograms of the objects with unit fringe magnification. For each lensfree hologram, the pixel size at the sensor chip limits the spatial resolution of the reconstructed image. To circumvent this limitation, we implement a sub-pixel shifting based super-resolution algorithm to effectively recover much higher resolution digital holograms of the objects, permitting sub-micron spatial resolution to be achieved across the entire sensor chip active area, which is also equivalent to the imaging field-of-view (24 mm2) due to unit magnification. We demonstrate the success of this pixel super-resolution approach by imaging patterned transparent substrates, blood smear samples, as well as Caenoharbditis Elegans.

Pixel super-resolution in digital holography by regularized reconstruction

“…In state-of-the-art approaches, a hologram is registered to another hologram [13,23], whereas in the proposed approach, registration is between a noiseless model and the noisy data, as discussed in section A. Assuming a perfect reconstruction and ignoring boundary effects, a theoretical gain of √ 2 in precision for the estimated shifts is expected.…”

“…Its cost-effectiveness associated with the democratization of high resolution, and high definition imaging sensors made it possible to develop on-chip wide field holographic microscopes [8] suited for the detection of bacteria and viruses [9,10], cytometry [11], or the characterization of protein aggregates [12]. In this case, the resolution of the lensless microscope is driven by the pixel pitch of the chosen sensor, but can be enhanced using pixel super-resolution strategies [13]. Simulation of light back-propagation is still often used for hologram reconstruction because it is simple, although it is prone to border effects and twin image noise, which dramatically reduce the reconstruction signal to noise ratio (SNR) and consequently the accuracy of the reconstruction.…”

“…These approaches are sometimes referred to as compressive sensing methods [19][20][21]. To overcome the limitation caused by the pixel pitch of the hologram, in 2010, Ozcan's team introduced pixel superresolution (SR) [13], which was already used in other modalities [22,23]. These authors suggested it was possible to recover a higher resolution hologram using a stack of rigidly shifted lower resolution holograms and to reconstruct them by back-propagation.…”

Optofluidic Tomography on a Chip

“…7 In the meantime, there has been a significant interest in lensfree imaging modalities toward the development of microscopes that can be integrated on a chip. 8,9 These lensfree imaging modalities can potentially allow replacing conventional bulky light microscopes with their compact and onchip counterparts for use in microfluidic systems. For the same purpose, conventional optofluidic microscopy 10 utilizes microfluidic channels to deliver the sample onto the imaging system, enabling integration of lensfree imaging with microfluidics.…”

“…The exact value of this tilt angle is not critical and need not be known a priori; it simply ensures that the flow of the object along the microchannel generates a shift component in both x and y, enabling digital synthesis of higher resolution holograms through pixel super-resolution ͑SR͒. Owing to its unique hologram recording geometry with unit fringe magnification, 8,9 our holographic optofluidic microscopy platform permits imaging of the flowing objects using multiple illumination angles as shown in Fig. 1, which is the key to achieve optical computed tomography.…”