In-line Holography Research Articles

Resolution Enhancement in Terahertz Digital In-line Holography by Sparsity-Based Extrapolation

We introduce a generalized extrapolation framework as a constrained optimization problem to enhance the resolution in terahertz digital in-line holography. The alternating minimization method is employed to iteratively extrapolate the hologram beyond the actually detecting area and to reconstruct the complex amplitude distribution of the object wavefront. Within this framework, we propose a sparsity-based extrapolation model based on L1-norm, where the object mask is not required and the generalized positive absorption constraint can be utilized. This work can achieve super-resolution reconstruction completely by numerical postprocessing without any modification to the imaging system. Both the simulation and experiments on the terahertz band demonstrate the feasibility of the proposed algorithms, and the limit of resolution can be extended by a factor of 1.67.

Reconstruction with In-Line Digital Holography Quantitative Phase Imaging for Tissue-Mimicking Phantom Samples

Optical imaging has attracted recent attention as a non-invasive medical imaging method in biomedical and clinical applications. In optical imaging, a light beam is transmitted through an under-test tissue by using an optical source. The beams which are gone through the tissue and/or reflected from the tissue surfaces are received by an array sensor. Based on the light intensity of these received beams on the sensor, sub-tissue maps are generated to scan large tissue areas so that any further biopsy is not required. Although the large tissue areas in pathological images can be scanned by using various methods, nonlinear deformations occur. To overcome this problem, the reconstruction process is frequently used. In this study, we propose an application of biomedical imaging based on performing the reconstruction of a phantom image via an in-line digital holography technique. Hence, many different sub-tissues can be imaged at the same time without the storage problem of the reconstructed image. To neglect the biopsy process required in medical imaging, the phantom image is obtained by using a linear array transducer for this study. We present the performance evaluation of the simulation results for the proposed technique by calculating the error metrics such as mean squared error (MSE), mean absolute error (MAE), and peak signal-to-noise ratio (PSNR). The obtained results reveal that the reconstructed images are well-matched to the original images, which are desired to be displayed by the holography technique.

High-speed digital off-axis holography to study atomization and evaporation of burning droplets

Droplet formation and evaporation are key processes of spray combustion, the mechanism of which is yet not fully understood. In this study, we develop a method based on high-speed (25 kHz) digital off-axis holography (DOH) to visualize and measure burning ethanol droplets in the atomization. Ligaments and fragments from a burning drop internally blown to disintegrate are studied after hologram processing. Reconstructed images are nearly free from influences of the flame with the measured droplets showing clear borders. Size and depth location uncertainties reduce to nearly 1/10 of those of commonly used with digital in-line holography. A time-resolved observation shows the dynamic formation of secondary droplets with stages classified as vapor penetration, sheet breakup, and ligament disintegration. Sizes and velocities of fragments are extracted and their distributions are analyzed at 1.12 ms, 3.12 ms, and 13 ms after the initiation of atomization. Results show that new secondary droplets have larger sizes and smaller velocities with time. Lagrangian measurements are performed with 3D droplet tracking algorithm. Evaporation rates of 31 droplets with different positions, sizes, and velocities are obtained and the results are in good accordance with previous studies. The DOH technique is promising to investigate the combustion of individual fuel droplets in a polydisperse droplet cloud as the real spray combustion.

Risk assessment and mitigation of airborne disease transmission in orchestral wind instrument performance

In collaboration with 16 musicians from the Minnesota Orchestra, we assess the airflow and particle concentration emitted from ten wind instruments under realistic performance conditions. Anemometer and schlieren measurement techniques are used to quantify the air flow, and aerodynamic particle sizer, laser sheet, and digital inline holography techniques are used to measure the particle concentration. The regions where the flow speed and particle concentrations are above the measurable background level vary among instruments depending on both air flow generation and particle production, but extend no farther than 30 cm from the instrument outlet for all instruments. Farther away, the upward-moving thermal plume generated by the temperature difference between the human body and ambient air is the dominant source of flow and aerosol transport. Brass instrument air flow increases with music amplitude and particle concentration exhibits an inverse response to note duration. Woodwinds emit more particles when note pitch increases. Covering the trumpet bell with one layer of acoustic fabric reduces the emitted particle concentration by ~60% with little impact on the sound quality. Adding more mask layers blocks more particles, but impedes performance and lowers the sound quality at higher frequencies (>1000 Hz). Computational fluid dynamics simulations initialized with experimental data show that placing an air cleaner above the instrument outlet can reduce the particle concentration by 90% due to the thermal plume driving aerosols upwards. Filtration efficiency further increases considerably (~10%) when lowering the ambient temperature from 25 °C to 20 °C to enhance the temperature difference with the human body.

Astigmatic dual-beam interferometric particle imaging for metal droplet 3D position and size measurement.

We propose astigmatic dual-beam interferometric particle imaging (ADIPI) to simultaneously measure the three-dimensional (3D) position and size of spherical metal droplets. A theoretical model reveals that the orientation and spacing of the ADIPI fringes generated from the two reflections propagating through an astigmatic imaging system relate to the depth position and size, respectively. Proof-of-concept experiments on micron-sized gallium droplets are performed, and the tilted fringes in elliptical patterns are observed in the ADIPI interferogram, confirming theoretical predictions. Droplet 3D position and size are determined with ADIPI, and the relative discrepancies are within 5% and 2% compared to those with a dual-view digital inline holography system, demonstrating the feasibility and high accuracy of ADIPI.

Quantification of the size, 3D location and velocity of burning iron particles in premixed methane flames using high-speed digital in-line holography

Due to its low emission and high energy density, iron powder has been proposed as a promising recyclable metal fuel for a future low-carbon society. The comprehensive understanding of combustion behavior of iron particles is crucial for studying fundamental mechanisms, developing suitable combustion technologies, and designing efficient iron powder combustor. In this work, iron particles are combusted in a modified Bunsen burner with a stable metal powder supplying system. As a versatile three-dimensional (3D) imaging technique, high-speed digital in-line holography (DIH) is employed to reconstruct the 3D particle field and simultaneously quantify the size, 3D location and velocity of burning iron particles. The statistical results of three cases with oxygen volume fraction varying from 24% to 40% are obtained and compared at different heights above the burner. Along the height, some typical features of the burning iron particles were observed. The violent combustion of iron particles accelerates the ejection of the particles radially outward from the central region of the flame, resulting in non-uniform spatial distribution of the particles and reducing the particle number density in the measurement volume. Such trend is enhanced with increased oxygen concentration. Besides, the observed particle size enlarged as the height increases, which validates the swelling phenomenon of iron particle oxidation. The results demonstrate that DIH is a powerful tool for in-situ, quantitative characterization of particle dynamics in flames.

3D characterisation of debris clouds under hypervelocity impact with large-field pulsed digital in-line holography

To expand the field of view and obtain a full-scale image of a debris cloud, a large-field pulsed digital in-line holography (DIH) system is developed to study the three-dimensional (3D) positions and shapes of debris clouds generated by hypervelocity impact. A general model for the large-field pulsed DIH system is introduced. Derived from strict theoretical analysis, it suggests that the hologram recorded by a large-field pulsed DIH system can be described by an equivalent lensless system. At the Hypervelocity Impact Research Centre of the China Aerodynamics Research and Development Centre, based on hypervelocity impact equipment with a 7.6 mm bore, experiments on a 3 mm aluminium sphere impacting a 1 mm thick aluminium target plate with a velocity of 3.58 km/s were carried out. Ensuring the successful capture of the transient state of debris clouds, the large-field pulsed DIH system is synchronised with the impact event and the combination of the neutral density filter and the bandpass filter is proposed, to eliminate the plasma radiation and enhance the signal-to-noise ratio of the hologram. The experimental results show that the holographic fringes are clearly recorded and the detailed shapes and structures of both large and small aluminium fragments are observed after reconstruction. The structure of debris clouds can be divided into three parts: front, core and shell, agreeing well with results measured by laser shadowgraph. The study demonstrates the feasibility of a large-field pulsed DIH system for accurate measurements of ultrafast debris clouds and shows great potential in the diagnostics of hypervelocity impacts.

Backward interferometric speckle imaging for evaluating size and morphology of irregular coal particles

The size and morphology of irregular particles are essential parameters for a variety of industrial processes. The backward interferometric speckle imaging technique can be applied to measure the relevant parameters of opaque irregular particles. To verify the accuracy of the backward interferometric speckle imaging technique, an elaborately designed experimental system was used to combine a backward interferometric speckle imaging system with a digital inline holography (DIH) system to characterise the same pulverised coal particles for quantitative comparison. The results of the backward interferometric speckle imaging were in good agreement with those of DIH in terms of particle size and morphology. In the backward interferometric speckle imaging system, the laser illumination and recording system are configured on the same side, and therefore only one window is required during measurement. This facilitates its instrument integration and provides significant potential for industrial applications involving powder processing.

Megahertz-rate digital holography system

Given the need for a simple and fieldable diagnostic that can enable the study of ultrahigh-speed phenomena in three dimensions, this work presents experiments and simulations to optimize and demonstrate a digital in-line holography system with an acquisition rate of 1 million frames per second. The system is used to study the collision of a supersonic projectile and its bow shock with 1 mm diameter water droplets. The holograms acquired are three-dimensionally reconstructed and de-twinned to analyze shock wave phenomena in these supersonic impact events and demonstrate the potential of ultrahigh-speed digital holography as a robust technique to probe complex shock wave dynamics and droplet breakup.

Improved minimum intensity projection in holographic reconstruction via SNR-enhanced holography

In digital in-line particle holography (DIPH), minimum intensity projection (MIP) is a crucial step in the reconstruction procedure, the MIP image quality is significantly degraded by nonuniform-illumination, speckle noise and the artifacts from off-focal planes. Here, we report a method that improves the MIP image quality by fully exploiting the advantages of SNR-enhanced holography, along with a nouniform illumination correction procedure without requiring a-priori assumptions about particle structures. Experimental results for nonspherical particles show that the proposed method can correct nonuniform-illumination and mitigate the artifacts from off-focal planes, allowing one to extract the particle’s size, shape and transverse location from a 2D single image.

On-line measurement of pulverized coal fineness on a 300 MWe power plant with pulsed digital inline holography

Pulverized coal fineness (PCF) is a key parameter in coal-fired power plants, demanding accurate and on-line monitoring. This study presents on-line measurement of PCF on a 300 MWe coal-fired power plant with pulsed digital inline holography (DIH). A prototypal instrumentation system, comprised of sampling, measurement, collection, and output, is established and tested on the two mills of the plant. Fineness characterized by the size-area distribution (SAD) and various size-volume distributions (SVD) obtained by DIH are compared to the size-mass distribution (SMD) by the off-line sieving method. The absolute R90 deviations of SAD are 3.60% and 3.72% for the two mills, respectively, with the equivalent area circle diameter (Deac) to estimate the sieve size (Ds). A SVD for which Ds is estimated by the maximum inscribed circle diameter (Dmic) and the particle volume (VSeac) by the equivalent sphere with diameter of Deac yields the deviations of 2.18% and − 0.25%.DIH is applied to the on-line coal fineness monitoring on a 300 MWe power plant. An on-line holographic instrumentation system is established. Fineness results indicate the operation conditions of the tested mills. Different fineness characterizations are compared and two optimums are recommended.

Abstract PO-080: Deep learning-based analysis of heterogeneity of breast cancer cells using lens-free digital in-line holography

Abstract Lens-free digital in-line holography (LDIH) produces cellular diffraction patterns from a large field of view that lens-based microscopes cannot achieve. It is a promising diagnostic tool allowing comprehensive cellular analysis with high-throughput capability. Since these diffraction images are far more complicated to discern, conventionally computational algorithms are used to reconstruct cellular images for the diffraction patterns. However, it is inefficient and prone to errors. Here, we developed a deep learning architecture, HoloNet to directly analyze the diffraction patterns from LDIH. In addition to the standard CNN (Convolutional Neural Network), the HoloNet includes a holo-branch that extracts large features from holograms and integrate them with the small features from the standard CNN. Using HoloNet, we accurately predicted the intensity values of ER/PR and HER2 in breast cancer cells and classified four known subtypes of breast cancer cells. Moreover, we applied a HoloNet dual embedding, which learns high-level diffraction features related to breast cancer cell types and the intensities of ER/PR and HER2. This allowed us to identify potential subtypes of breast cancer cells from the cell lines and the fine needle aspiration samples from breast cancer patients. We demonstrate that our HoloNet can enable LDIH to perform more detailed and rapid analyses of subtypes of breast cancer cells. Citation Format: Tzu-Hsi Song, Mengzhi Cao, Jouha Min, Hyungsoon Im, Hakho Lee, Kwonmoo Lee. Deep learning-based analysis of heterogeneity of breast cancer cells using lens-free digital in-line holography [abstract]. In: Proceedings of the AACR Virtual Special Conference on Artificial Intelligence, Diagnosis, and Imaging; 2021 Jan 13-14. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(5_Suppl):Abstract nr PO-080.

Analysis of Local Charges at Hetero-interfaces by Electron Holography – A Comparative Study of Different Techniques

Interface charges confined within a few nanometers of hetero-interface can be characterized by measuring the phase shift of the transmitted beam using different electron holography techniques. However, reliable measurement of the electrostatic potential arising from the interface charges is challenging as the mean inner potential difference (ΔV0) between two adjoining materials as well as local variation of the sample thickness affect the phase shift. In the present study, we show how electron holography can be used to characterize the confined charges at an oxide hetero-interface and evaluate the applicability of different techniques for this purpose. The model system chosen for this study is a LaAlO3/SrTiO3 (LAO/STO) (111) hetero-interface featuring a two-dimensional electron gas (2DEG), where the ΔV0 between LAO and STO is about 2 eV, which is unignorably large and dominates the net potential variation across the interface. For transmission electron microscopy specimens prepared by focused ion beam we applied three different variants of electron holography techniques: off-axis, inline and hybrid electron holography; and compare the results obtained by these approaches in terms of the information transfer in the spatial frequency domain, and the signal-to-noise ratio of the electric field and charge density maps. To correctly assess the information pertinent to the interface-confined charges, we calculate the electrostatic potential and electric field distribution based on a charge model with taking account of the ΔV0 between LAO and STO and compared the calculated profiles with the experimental results after calibrating the local thickness variation across the LAO/STO interface. The results show that hybrid electron holography recovers the information across a wide range of spatial frequencies, and as a result, delivers the most reliable charge density information, albeit convoluted with the unavoidable effects arising from ΔV0.

Investigation of particle density on dust cloud dynamics in a minimum ignition energy apparatus using digital in-line holography

Solids industries have inherent dust explosion hazards. Process risk assessment parameters include overpressure, deflagration index, and minimum ignition energy. These parameters are influenced by dust dynamics, such as turbulence and concentration. Cloud dynamics are affected by particle morphology, density, size, and polydispersity, which affect explosion parameters. Influence of properties such as density during explosion parameter measurement is often overlooked. This study examines particle density influence on dust clouds in the Kühner MIKE3. Two clouds of particles with different density, but similar particle size and morphology were examined with high-speed digital in-line holography, providing velocity and concentration measurements prior to ignition. Results show velocity and concentration of denser dust is lower compared to lighter dust after ignition delay. The results highlight the density effect on cloud dynamics and make a case for particle density to be considered in setting ignition delay in order to get consistent cloud dynamics and test results.

Application of magnified digital in-line holography (MDIH) to the measurement of the evaporation process of desulfurization wastewater droplets in a high-temperature gas flow

A magnified digital in-line holography was applied to measure the evaporation process of deionized water droplets and desulfurization wastewater droplets in the temperature range of 298–423 K. A quartz-glass evaporation chamber was designed to conduct experiments. The total length in the measurement zone was 450 mm. The droplets were generated by a piezoelectric jetting device (Microfab Inc., Plano, Texas, USA). The nozzle (MJ-AT-01-50) consisted of lead zirconate titanate piezoelectric ceramic (PZT) material and a glass capillary with a 50 μm orifice diameter. The relationships between the evaporation process and operating conditions, such as temperature, the concentration of desulfurization wastewater, added solutes, and velocity of carrier gas, were discussed. The evaporation rates of deionized water at 298 K, 323 K, 373 K, and 423 K were 0.1156 µm2/ms, 0.3398 m2/ms, 0.5204 m2/ms, 0.6381 µm2/ms, respectively. The evaporation rates of desulfurization wastewater at 298 K, 323 K, 373 K, and 423 K were 0.05201 µm2/ms, 0.1673 µm2/ms, 0.2912 µm2/ms, 0.3606 µm2/ms, respectively. The evaporation rate of deionized water is approximately twice that of desulfurization wastewater. The progress hereby presented enables us to study the evaporation of droplets in three aspects: First, the evaporation process of a monodisperse droplet stream injected into a high-temperature gas flow can be investigated. Second, the equivalent pixel was calculated to be 2.4299 µm/pixel and the magnification factor was 5.76. Third, factors affecting the evaporation rate of desulfurization wastewater are investigated. The data are valuable for achieving zero discharge of wastewater and optimizing operating conditions.

Experimental characterization of secondary atomization at high Ohnesorge numbers

A droplet subjected to external aerodynamic perturbations and fragmenting into smaller droplets is defined as secondary atomization. In the past decades, droplet breakup at low Ohnesorge numbers (Oh < 0.1) had been characterized with unanimity in the community regarding its physics. However, droplet breakup at high Oh number, i.e. atomization where the influence of viscosity is significant, has not been studied in detail, and there is a poor agreement in conclusions by various researchers. This work presents a thorough analysis of the modes of deformation and breakup exhibited by a high viscous (Oh = 0.5, Oh =1) droplet subjected to continuous airflow (We = 30-120). Several modes of droplet breakup are presented and transition case with no droplet fragmentation has been discovered. Size-velocity pdfs of droplets at different Ohnesorge numbers for varying Weber numbers were obtained by processing the images acquired using Digital in-line holography (DIH). Particle image velocimetry (PIV) was employed to characterize the airflow in cases where the droplets exhibited no breakup and cases with multiple bag formation. The percentage of minimum backflow velocity required for droplet breakup is also presented.

Low-light phase imaging using in-line digital holography and the transport of intensity equation

In one of our recent studies, we have shown that a large defocusing distance in the transport of intensity equation (TIE) is required to retrieve the phase information in low-light conditions (OSA Continuum 3 (2020) 236). It has been found that image misalignment issues come into the picture due to translation of the camera for large distances, which creates artifacts in phase recovery. The issue of misalignment can be overcome if, instead of the object, we use the digital hologram of the object for the required defocused intensities and then apply the TIE. In this paper, we demonstrate low-light phase imaging by combining digital holography with the TIE. We reconstruct the required multiple intensity distributions from the captured in-line digital hologram. To implement the low-level light illumination, a variable neutral density filter has been used while recording the in-line digital hologram. Simulation and experimental results with different objects (United States air-force chart and glue drop) are presented. To simulate the low-light conditions, a Poisson distribution based photon-counting imaging technique has been applied.

Twin-image suppression in digital in-line holography based on wave-front filtering

Digital holography is an imaging process able to recreate three-dimensional representations of objects from recording pattern interference among distinct waves. The in-line configuration setup is a variant considered the simplest physical implementation, providing a feasible manner for acquisition and the same time higher resolution for free-living microscopy imaging using a single illumination system. However, the well-known twin-image problem is bounded to the technique, since there is no separation among reference and objects beams in this configuration. As a result, computational numerical diffraction routines present the twin-image effect intrinsically, imposing several difficulties in terms of post-processing requirements. In this context, this paper aims to present a numerical approach able to provide consistent suppression of twin-image problem for in-line holography, during the numerical diffraction procedure for phase retrieval, combining image subtraction and edge detection techniques. The proposed solution was implemented in Python language, and metrics defined to assess it were both qualitative and quantitative, based on edge detection and some image comparison metrics. The obtained results of the proposed approach present a significant reduction in the twin-image artifacts in the reconstructions of both experimental and simulated holograms, considering a spherical reference wave, regardless of the shapes and sizes of objects.

Experimental study on on-line monitoring pulverized coal fineness of 300 MW coal-fired plant by digital holography

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.

Dual-sheet interferometric particle imaging for opaque particle size and 2D location measurement

Dual-beam interferometric particle imaging (DIPI) has been proposed to measure opaque spherical particle size in the single-point region of interest. This work extends DIPI to measure the size and two-dimensional position of opaque spherical particle in a plane with dual-sheet illumination. A general DIPI model based on geometric optics is established to formulate the interference signal formation of DIPI by modeling the phase difference of two reflected light at far-field. Proof-of-concept experiments, incorporating digital inline holography (DIH), are performed for validation. An interferogram processing algorithm which adopts the sum-of-squared-difference image registration method and Fourier spectrum analysis has been proposed to locate the centroid of each out-of-focus pattern and extract the spatial frequency of fringes, and subsequently particle position and size. The average relative deviation values of DIPI in size measuring and two-dimensional locating are 2.4% and 1.9%, respectively, compared to those values by DIH, which demonstrates the practicability of DIPI in both respects. The extension of DIPI from point-probe to planar measurement promotes its applications to real scenarios, e.g., metal droplet in solid rocket propulsion.