Abstract
In recent years, lensfree on-chip microscopy has developed into a promising and powerful computational optical microscopy technique that allows for wide-field, high-throughput microscopic imaging without using any lenses. However, due to the limited pixel size of the state-of-the-art image sensors, lens-free on-chip microscopy generally suffers from low imaging resolution, which is far from enough to meet the current demand for high-resolution microscopy. Many pixel super-resolution techniques have been developed to solve or at least partially solve this problem by acquiring a series of low-resolution holograms with multiple lateral sub-pixel shifting or axial distances. However, the prerequisite of these pixel super-resolution techniques is that the propagation distance of each low-resolution hologram can be obtained precisely, which faces two major challenges. On the one hand, the captured hologram is inherent pixelated and of low resolution, making it difficult to determine the focal plane by evaluating the image sharpness accurately. On the other hand, the twin-image is superimposed on the backpropagated raw hologram, further exacerbating the difficulties in accurate focal plane determination. In this study, we proposed a high-precision autofocusing algorithm for multi-height pixel-super-resolved lensfree on-chip microscopy. Our approach consists of two major steps: individual preliminary estimation and global precise estimation. First, an improved critical function that combines differential critical function and frequency domain critical function is proposed to obtain the preliminary focus distances of different holograms. Then, the precise focus distances can be determined by further evaluating the global offset of the averaged, low-noise reconstruction from all backpropagated holograms with preliminary focus distances. Simulations and experimental results verified the validity and effectiveness of the proposed algorithm.
Highlights
In recent years, high-resolution wide-field optical imaging has become a valuable tool in various biomedical applications such as cell counting [1], cell morphology measurement [2], and optical microscopy techniques that require low-cost and compact imaging systems [3,4,5]
We can more accurately determine the focus distances of different plane heights and improve the accuracy of super-resolution based on the multi-height pixel-super-resolved lensfree on-chip imaging
The plane in focus can be determined by searching for the minimum value of differential critical function for amplitude-contrast objects or the maximum value for amplitude-contrast objects
Summary
High-resolution wide-field optical imaging has become a valuable tool in various biomedical applications such as cell counting [1], cell morphology measurement [2], and optical microscopy techniques that require low-cost and compact imaging systems [3,4,5]. The above algorithms are difficult to directly apply to the autofocusing of multi-height pixel-super-resolved lensfree on-chip imaging. (3) In superresolution imaging, each hologram of different heights needs to be accurately reconstructed, which requires high accuracy of the focusing distance of each plane. A high-precision autofocusing method is proposed based on the multi-height pixel-super-resolved lensfree on-chip microscopy. We can more accurately determine the focus distances of different plane heights and improve the accuracy of super-resolution based on the multi-height pixel-super-resolved lensfree on-chip imaging.
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