Pixel super-resolution in digital holography by regularized reconstruction

Digital in-line holography is commonly used to reconstruct 3D images from 2D holograms for microscopic objects. One of the technical challenges that arise in the signal processing stage is removing the twin image that is caused by the phase-conjugate wavefront from the recorded holograms. Twin image removal is typically formulated as a non-linear inverse problem due to the irreversible scattering process when generating the hologram. Recently, end-to-end deep learning-based methods have been utilized to reconstruct the object wavefront (as a surrogate for the 3D structure of the object) directly from a single-shot in-line digital hologram. However, massive data pairs are required to train deep learning models for acceptable reconstruction precision. In contrast to typical image processing problems, well-curated datasets for in-line digital holography does not exist. Also, the trained model highly influenced by the morphological properties of the object and hence can vary for different applications. Therefore, data collection can be prohibitively cumbersome in practice as a major hindrance to using deep learning for digital holography. In this paper, we proposed a novel implementation of autoencoder-based deep learning architecture for single-shot hologram reconstruction solely based on the current sample without the need for massive datasets to train the model. The simulations results demonstrate the superior performance of the proposed method compared to the state of the art single-shot compressive digital in-line hologram reconstruction method.

This paper presents the Pixel Super Resolution with Axial Scanning (PSR-AS) technique to enhance the resolution of Lensless Digital In-line Holographic Microscopy (LDHM). By utilizing multiple holograms captured at different axial distances, PSR allows for the recovery of high-resolution information, improving the image clarity and information content. Experimental results demonstrate a twofold lateral resolution enhancement, achieving the resolution of 1.23 μm in amplitude imaging and significantly augmenting phase reconstruction quality. The integration of PSR-AS into LDHM systems leads to notable reductions in noise and artifacts, offering a promising approach for high-resolution imaging in applications such as biomedical diagnostics and environmental monitoring. Full Text: PDF References Y. Park, C. Depeursinge, and G. Popescu, "Quantitative phase imaging in biomedicine," Nature Photon, 12(10), 578 (2018). CrossRef D. Gabor, "A New Microscopic Principle," Nature, 161(4098), 777 (1948). CrossRef K. Matsushima and T. Shimobaba, "Band-Limited Angular Spectrum Method for Numerical Simulation of Free-Space Propagation in Far and Near Fields," Opt. Express, 17(22), 19662 (2009). CrossRef Y. Wu and A. Ozcan, "Lensless digital holographic microscopy and its applications in biomedicine and environmental monitoring," Methods, 136, 4 (2018). CrossRef J. Zhang, J. Sun, Q. Chen, and C. Zuo, "Resolution Analysis in a Lens-Free On-Chip Digital Holographic Microscope," IEEE Transactions on Computational Imaging, 6, 697 (2020). CrossRef Gerchberg, R. W.. “A practical algorithm for the determination of phase from image and diffraction plane pictures.” Optik 35, 237 (1972). CrossRef H. Lee, J. Kim, J. Kim, P. Jeon, S. A. Lee, and D. Kim, "Noniterative sub-pixel shifting super-resolution lensless digital holography," Opt. Express,29(19), 29996 (2021). CrossRef A. Greenbaum and A. Ozcan, "Maskless imaging of dense samples using pixel super-resolution based multi-height lensfree on-chip microscopy," Opt. Express, 20(3), 3129 (2012). CrossRef J. Zhang, J. Sun, Q. Chen, J. Li, and C. Zuo, "Adaptive pixel-super-resolved lensfree in-line digital holography for wide-field on-chip microscopy," Sci Rep, 7(1), 11777 (2017). CrossRef L. I. Rudin, S. Osher, and E. Fatemi, "Nonlinear total variation based noise removal algorithms," Physica D: Nonlinear Phenomena, 60(1), 259 (1992). CrossRef

Digital in-line holography and plenoptic photography are two techniques for single-shot, volumetric measurement of 3D particle fields. Here we present a preliminary comparison of the two methods by applying plenoptic imaging to experimental configurations that have been previously investigated with digital in-line holography. These experiments include the tracking of secondary droplets from the impact of a water drop on a thin film of water and tracking of pellets from a shotgun. Both plenoptic imaging and digital in-line holography successfully quantify the 3D nature of these particle fields. This includes measurement of the 3D particle position, individual particle sizes, and three-component velocity vectors. For the initial processing methods presented here, both techniques give out-of-plane positional accuracy of approximately 1-2 particle diameters. For a fixed image sensor, digital holography achieves higher effective in-plane spatial resolutions. However, collimated and coherent illumination makes holography susceptible to image distortion through index of refraction gradients, as demonstrated in the shotgun experiments. On the other hand, plenotpic imaging allows for a simpler experimental configuration. Furthermore, due to the use of diffuse, white-light illumination, plenoptic imaging is less susceptible to image distortion in the shotgun experiments. Additional work is needed to better quantify sources of uncertainty, particularly inmore » the plenoptic experiments, as well as develop data processing methodologies optimized for the plenoptic measurement.« less

3D boundary line measurement of irregular particle with digital holography

The optical extinction caused by a small particle, such as an aerosol particle, is an important measurable quantity. Understanding the influence of atmospheric aerosols on the climate, assessing visibility in urban environments, and remote sensing applications such as lidar all need accurate measurements of particle extinction. While multiple methods are known to measure extinction, digital in-line holography (DIH) features the unique ability to provide contact-free images of particles simultaneously with estimates for the extinction cross section. This is achieved through an integration of a measured hologram followed by an extrapolation. By means of a supercontinuum laser, we investigate the measurement of the cross section via DIH for stationary particles across a broad spectrum, from 440nm to 1040nm. The particles considered include a 50µm glass microsphere, a volcanic ash particle, and an iron(III) oxide particle. The results show the ability to estimate a particle's cross section to within 10% error across portions of the spectrum and approximately 20% error otherwise. An examination of the accompanying hologram-derived particle images reveals details in the images that evolve with wavelength. The behavior suggests a basic means to resolve whether absorption or scattering dominates a particle's extinction.

The authors propose an optical-path-length-shifting digital holography as a technique capable of single-shot recording of three-dimensional information of objects. With a single image sensor, the proposed technique can simultaneously record all of the holograms required for the in-line digital holography that reconstruct the image of an object from two intensity measurements at different planes. The technique can be optically implemented by using an optical-path-length-shifting array device located in the common path of the reference and object waves. The array device has periodic structure of two-step optical-path difference. The configuration of the array device of the proposed technique is simpler than the phase-shifting array device required for parallel phase-shifting digital holographies. Therefore, the optical system of the proposed technique is more suitable for the realization of a single-shot in-line digital holography system that removes the conjugate image from the reconstructed image. The authors conducted both a numerical simulation and a preliminary experiment of the proposed technique. The reconstructed images were quantitatively evaluated by using root mean squared error. In comparison to single-shot digital holography using the Fresnel transform alone, with the proposed technique the root mean squared errors of the technique were reduced to less than 1/6 in amplitude and 1/3 in phase. Also the results of the simulation and experiment agreed well with the images of an object. Thus the effectiveness of the proposed technique is verified.

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.

Pixel super-resolution (PSR) techniques have been developed to overcome the sampling limit in lensless digital holographic imaging. However, the inherent non-convexity of the PSR phase retrieval problem can potentially degrade reconstruction quality by causing the iterations to tend toward a false local minimum. Furthermore, the ill posedness of the up-sampling procedure renders PSR algorithms highly susceptible to noise. In this Letter, we propose a heuristic PSR algorithm with adaptive smoothing (AS-PSR) to achieve high-fidelity reconstruction. By automatically adjusting the intensity constraints on the estimated field, the algorithm can effectively locate the optimal solution and converge with high reconstruction quality, pushing the resolution toward the diffraction limit. The proposed method is verified experimentally within a coherent modulation phase retrieval framework, achieving a twofold improvement in resolution. The AS-PSR algorithm can be further applied to other phase retrieval methods based on alternating projection.

Three-dimensional object recognition using joint fractional Fourier transform correlators with the help of digital Fresnel holography

The development of methods to measure the size of nanoparticles is a challenging topic of research. The proposed method is based on the metrology of the stable vapor bubble created by thermal coupling between a laser pulse and the nanoparticle in a droplet. The measurement is realized by digital in-line holography. The size of the nanoparticle is deduced from numerical simulations computed with a photo-thermal finite element method.

Digital in-line holography (DIH) with a divergent beam is used to measure size and concentration of cavitation bubbles (6-100 μm) in hydrodynamic facilities. A sampling probe is directly inserted in the cavitation tunnel, and the holograms of the bubbles are recorded through a transparent test section specially designed for DIH measurements. The recording beam coming from a fiber-coupled laser diode illuminates the sample volume, and holograms are recorded by a CMOS camera. From each hologram, the sampling volume can be reconstructed slice by slice by applying a wavelet-based reconstruction method. Because of the geometry of the recording beam, a magnification ratio must be introduced for recovering the 3D location and size of each bubble. The method used for processing holograms recorded in such a configuration is presented. Then, statistical results obtained from 5000 holograms recorded under different pressures in the cavitation tunnel are compared and discussed.

High-speed (20 kHz) digital in-line holography (DIH) is applied for 3D quantification of the size and velocity of fragments formed from the impact of a single water drop onto a thin film of water and burning aluminum particles from the combustion of a solid rocket propellant. To address the depth-of-focus problem in DIH, a regression-based multiframe tracking algorithm is employed, and out-of-plane experimental displacement accuracy is shown to be improved by an order-of-magnitude. Comparison of the results with previous DIH measurements using low-speed recording shows improved positional accuracy with the added advantage of detailed resolution of transient dynamics from single experimental realizations. The method is shown to be particularly advantageous for quantification of particle mass flow rates. For the investigated particle fields, the mass flows rates, which have been automatically measured from single experimental realizations, are found to be within 8% of the expected values.

Chapter 14 - Imaging aerosol particles with digital in-line holography

Structural vibrations can be measured with optical digital holography. Such a method provides measurements with a very high spatial resolution and is a nonintrusive technique. This method is based on the interference between a reference laser beam and the field diffracted by the studied object. Using a high speed camera, it can also be implemented in the time domain to investigate non-stationary problems. In this paper, we present a recent investigation which shows that the high-speed digital holography is comparable with classical laser vibrometry. Furthermore, an experimental investigation of the vibratory field inside the Acoustic Black Hole is discussed. The principle of digital holography is explained and it is used here to provide a full field measurement of the velocity field at the extremity of the structure.

Pulverized coal fineness is one of the key parameters in the operation of coal-fired power plant, which is closely related to the combustion efficiency of boiler, power consumption of pulverizing system and equipment wear. In this paper, an experimental study on on-line monitoring of pulverized coal fineness by digital holography was carried out. An on-line measurement system based on pulse digital in-line holography was established. The system included four main parts: sampling, measuring, collecting and data processing. The measurement system was applied to the pulverized coal fineness monitoring in two primary air pipes of a 300 MW unit. The pulverized coal flow in the two pipes were from two different mills. The self-developed reconstruction software was used to process the holograms in real time and output the fineness results. The pulverized coal samples collected by the system were sieved in the laboratory, and the sieving results were compared with the holographic results to verify the feasibility and accuracy of the measurement system. Results show that the holographic results of area distribution are basically consistent with the sieving results, and the average absolute deviation is less than 4%. The test also diagnose that the coal fineness of one mill deviates greatly from the designed value, proving that the measurement system has great application potential for on-line monitoring of pulverized coal fineness in coal-fired power plants.