Pixelation, blur and additional noise of imaging system limit the resolution of final images acquired. Many pixel superresolution algorithms have been applied to enhance the resolution of imaging system by merging a sequence of lowresolution holograms with different type of imaging system, for example by shifting illumination source or using wavelength scanning. Most of these pixel super-resolution imaging systems can only be implemented to single-layer sample. For multi-layer imaging system and volumetric imaging scenarios, the relative displacement of various sample at different layers will disturb each other. Herein, we report a portable, cost-effective, lensless wide-filed digital in-line holographic microscopy imaging system based on in-line hologram segmentation and pixel super-resolution algorithm, which can separate target sample from the background and improve the resolution of the sample. We demonstrated the effectiveness of our system with numerical simulation and experiment with volumetric sample. In numerical simulation, we applied a very simple two-layer sample model that samples in two layers have various moving speed and directions and also did the volumetric imaging experiment with cuvette containing algae floating in. In our system, the sensor captured a sequence of low-resolution diffraction patterns. The target sample mix with background disturbance, which will invalidate the direct pixel super-resolution technique. We applied segmentation algorithm to the reconstructed images from holograms, separating target sample from background and generating a sequence of sub-images containing only target sample with same resolution and numerical aperture as original holograms. Finally the enhanced resolution reconstructed image of target sample was obtained with pixel super-resolution algorithm, which can go beyond pixel limitation and get sub-pixel perspective microscopy. This imaging system has the advantages of wide-field, portable and lensless.
In a compact digital lensless inline holographic microscope (LIHM), where the sample-to-sensor distance is short, the imaging resolution is often limited by sensor pixel size instead of the system numerical aperture. We propose to solve this problem by applying data interpolation with an iterative holographic reconstruction method while using grating illumination in the LIHM system. In the system setup, the Talbot self-image of a Ronchi grating was used to illuminate the sample, and the inline hologram was recorded by a CMOS imaging sensor located behind the sample. The hologram was then upsampled by data interpolation before the reconstruction process. In the iterative holographic reconstruction, the sample support was defined by the bright areas of the grating illumination pattern and was used as constraint. A wide-field image can also be obtained by shifting the grating illumination pattern. Furthermore, we assumed that the sample was amplitude object, i.e., no obvious phase change was caused by the sample, which provided additional constraint to refine the interpolated data values. Besides improved resolution, the iterative holographic reconstruction also helped to reduce the twin-image background. We demonstrated the effectiveness of our method with simulation and imaging experiment by using the USAF target and polystyrene microspheres with 1 μm diameter as the sample.
KEYWORDS: Optical coherence tomography, Endoscopy, Biomedical optics, Compressed sensing, Data acquisition, 3D image processing, Mirrors, 3D acquisition, Single mode fibers, Signal detection
Conventionally endoscopic OCT images are reconstructed by uniform sampling in both axial and transverse direction of the image. To shorten the scanning time and reduce the acquisition data, we propose to sparsely sample the axial scans and use compressive sensing (CS) to reconstruct the full endoscopic OCT image with randomly chosen axial scans that are much less than conventional Nyquist criteria requirement. And we study the ratio of sparse sampling numerically and experimentally that are required to reconstruct acceptable OCT images. We demonstrate that the OCT image acquisition time can be significantly reduced because of much less acquired data.
KEYWORDS: Image resolution, 3D image reconstruction, Holography, Super resolution, Digital holography, Holograms, Reconstruction algorithms, Microscopes, Sensors, Detection and tracking algorithms
We report a new holographic microscope using pixel super-resolution algorithm. In our method, a sequence of low resolution images are acquired by a complementary metal oxide semiconductor (CMOS) sensor in digital inline holography system and the resolution is limited by the sensor pixel size. Then the super-resolution algorithm is applied to the low resolution images to get the image with much higher resolution that beyond the Nyquist criteria. We perform both numerical simulation and experiments to demonstrate our method with US Air Force Target used as the sample. The sample is randomly moved in the sample plane and a set of holograms are captured by the camera in inline holographic system. We use two methods to reconstruct the sample image. In the first method, super-resolution algorithm is applied with the low resolution holograms to get the high resolution hologram. Then the high resolution hologram is reconstructed using auto-focusing algorithm to get the high resolution sample image. In the second method, the raw holograms are directly reconstructed to get a set of low resolution sample images, then the super-resolution algorithm is applied to get the high resolution sample image. We observed that the above mentioned two methods can get similar results in both numerical stimulation and experiments. We believe that the combination of pixel super-resolution algorithm and digital in-line holography can be very useful to implement a compact low-cost microscope with high resolution.
High resolution is always a pursuing target in the imaging field, as a new prospective technique in imaging applications, digital in-line holography has become a very active field for compactness, more information and low-cost. However, for compact system, the resolution is often limited by sensor pixel size. To overcome this problem, we propose an iterative reconstruction method with data interpolation based on the grating illumination. In our method, the Talbot self-image of a Ronchi grating is exerted in the sample plane as a priori constraint which lead to the convergence of the iteration, the iteration between the sample plane and the sensor plane can provide some extra information with interpolation in the sensor plane based on the a priori constraint, furthermore, the iteration reconstruction can also eliminates the twin-image to improve the image quality. Numerical simulation has been conducted to show the effectiveness of this method. In order to make a further verification, we have developed a lensless in-line holographic microscope with a compact and wide field-of-view design. In our setup, the sample was under the Talbot image illumination of the Ronchi grating, which was illuminated by a collimated laser beam, and holograms were recorded by a digital imaging sensor. We can shift the grating laterally to get a wide-field image. We demonstrated the resolution of our imaging system by using the USAF resolution target as a sample, and the results shown the resolution improvement of the image.
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