Absorption Imaging Research Articles

Temperature measurement in methane-ammonia co-fired laminar premixed flame using mid-infrared laser absorption tomography

Temperature measurement in methane-ammonia co-fired laminar premixed flame using mid-infrared laser absorption tomography

Invited Paper] A High SNR Global Shutter CMOS Image Sensor Technology for High Precision Absorption Imaging Applications

Invited Paper] A High SNR Global Shutter CMOS Image Sensor Technology for High Precision Absorption Imaging Applications

Combined flow, temperature and soot investigation in oxy-fuel biomass combustion under varying oxygen concentrations using laser-optical diagnostics

In pulverized oxy-fuel biomass combustion, intricate interaction of complex fluid-mechanical, particle-dynamical, and chemical processes manifests even at intermediate scales. This study performs a comprehensive analysis aimed to understand the processes of flame stabilization, pollutant formation, and combustion behavior of biomass particles, while evaluating the impact of varying oxygen concentration on combustion. The investigation is conducted with a comprehensive data set, encompassing various oxy-fuel operation conditions ranging from 27vol.% to 36vol.% in O2 concentration, along a comparable air flame. These conditions are realized in an optically accessible gas-assisted solid fuel combustor designed to generate flames up to 70kWth. Advanced laser-optical diagnostics are employed to capture the combustion process. These techniques include the use of two-phase PIV/PTV for simultaneous measurements of flow and particle velocities, the application of tomographic absorption spectroscopy to elucidate three-dimensional gas temperature distributions, and the establishment of a quasi-simultaneous LII and LIF experimental setup to capture both PAH and soot occurrences. Additionally, chemiluminescence imaging of CH∗ is incorporated. The analysis extends to both single-phase and two-phase combustion, revealing a pronounced influence of oxygen concentration on both regimes. The inner recirculation zone, characteristic of a swirl flame, decreases with increasing oxygen content, coupled with an acceleration of the main flow downwards due to heightened heat release proximate to the nozzle. In two-phase combustion, a significant combustion delay is observed compared to single-phase combustion. Furthermore, the occurrences of PAHs and soot are clearly separated, and a pronounced influence of local gas composition on soot formation is evident. Comparative examination between oxy-fuel and air combustion show that an oxy-fuel condition with an oxygen concentration just below 33vol.% closely mirrors the flame characteristics of air combustion.

Attitude Determination of Photovoltaic Device by Means of Differential Absorption Imaging

Future wireless power transmission will cover power levels up to kilowatts or more and transmission distances up to the scale of kilometers. With its narrow beam divergence angle, optical wireless power transmission (OWPT) is a promising candidate for such system implementations. In the operation of OWPT, it is necessary to estimate the position, direction (azimuth, elevation), and attitude of the target photovoltaic device before the power supply. The authors have proposed the detection of targets using differential absorption imaging and positioning with a combination of stereo imagery. In the positioning by stereo imagery, a condition regarding the consistency of the left and right images can be defined. This corresponds to the certain value of the exposure time of the image sensor, and this depends on the target’s attitude angle. In this paper, we discuss target attitude estimation using this minimum exposure time at which the integrity measure converges. A physical model was derived under general conditions of target position and experimental configuration. Target attitudes were estimated within an error range of 10 to 15 degrees in approximately 60 degrees range. On the other hand, there is an attitude estimation method based on the apparent size of the target. When using this method to estimate the attitude angle, errors are significantly large for specular and diffuse mixed targets like the PV. The method proposed in this paper is a robust attitude estimation method for the photovoltaic device in OWPT.

Imaging Ultrafast Dynamics of Pressure-Driven Phase Transitions in Black Phosphorus and Anomalous Coherent Phonon Softening.

Applying high pressure to effectively modulate the electronic and lattice structures of materials could unravel various physical properties associated with phase transitions. In this work, high-pressure-compatible femtosecond pump-probe microscopy was constructed to study the pressure-dependent ultrafast dynamics in black phosphorus (BP) thin films. We observed pressure-driven evolution of the electronic topological transition and three structural phases as the pressure reached ∼22 GPa, which could be clearly differentiated in the transient absorption images containing spatially resolved ultrafast carrier and coherent phonon dynamics. Surprisingly, an anomalous coherent acoustic phonon mode with pressure softening behavior was observed within the range of ∼3-8 GPa, showing distinct laser power and time dependences. Density functional theory calculations show that this mode, identified as the shear mode along the armchair orientation, gains significant electron-phonon coupling strength from out-of-plane compression that leads to decreased phonon frequency. Our results provide insights into the structure evolution of BP with pressure and hold potential for applications in microelectromechanical devices.

One-dimensional direct absorption sensor for flat flame characterization

Laser absorption techniques are widely used in combustion research to provide a detailed characterization of reactive media. Recently, the improvements of high-speed infrared camera have enabled the development of laser absorption imaging (LAI) techniques with application to combustion environment. In this paper, we present the development and implementation of the first one-dimensional direct absorption (1D-DA) speciation sensor in the near-infrared, absorption transition near 1391.7 nm, for measuring water in a flat flame. Our 1D-DA sensor relies on a simple structure based on standard optical components, and a novel post-processing method was developed to correct diffraction-like signal deformation. With our 1D-DA sensor, we characterized two C2H4-O2-Ar flames with different equivalence ratios, and validated the results using both the traditional DA method based on a single pixel detector, and Cantera flame simulations using USC-Mech II. Our sensor achieves a spatial resolution better than 70μm along along the vertical direction, with an uncertainty estimated to be around 13%.Novelty and Significance Statement1. First one-dimensional direct absorption (1D-DA) speciation sensor applied to a near-IR line-scan camera.2. Novel approach to remove diffraction pattern related to laser light coherence.3. High spatial resolution along along the vertical direction.4. Strong potential for future tomographic applications

Four-dimensional laser absorption cinematography of species and temperature dynamics at 2 kHz in reacting flows.

A four-dimensional (4D) mid-infrared laser absorption imaging technique has been developed and demonstrated for quantitative, time-resolved, volumetric measurements of temperature and species concentration in dynamic combustion flows. This technique employs a dual high-speed infrared camera setup to capture turnable radiation from a quantum cascade laser near 4.85 µm to resolve rovibrational absorption transitions of carbon monoxide at two orthogonal projection angles. The laser is modulated with a customized waveform to adaptively resolve two target transitions with an increased density of data samples in proximity to the transition peaks, therefore ensuring accurate and quantitative spectral interpretation while minimizing the required frame rate. A 3D masked Tikhonov regularized inversion was performed to reconstruct spectrally resolved absorbance at every grid point of each frame, which enables subsequent interpretation of local gas properties in time. These methods are applied to achieve quantitative 4D cinematography of temperature and carbon monoxide in a propagating C2H4/O2 flame with a spatial pixel resolution of ∼70 µm and a temporal resolution of 2 kHz.

High-resolution spectroscopic imaging of atoms and nanoparticles in thin film vaporization

We introduce a spectroscopic absorption imaging method in the UV regime (225–400 nm) to study tin vapor created by irradiating a thin film with a low intensity 108 W cm−2 nanosecond laser pulse, a case inspired by current developments around “advanced target shaping” in industrial laser-produced plasma sources for extreme ultraviolet light. The 4-ns-time-resolved, 10-μm-spatial-resolution images contain a 10-cm−1-resolution spectrum of the vapor in each pixel 100 ns after the vaporization. The images allow us to reveal a homogeneous temperature profile throughout the vapor of around 3000 K. We obtain a density map of the atoms (with a peak density of 5 × 1018 cm−3) and nanoparticles (1012 cm−3 for the best fitting 20 nm radius case), which both are shown to be present in the vapor. For each free atom, approximately three appear to be clustered in nanoparticles, and this composition is invariant over space and density. The density and temperature maps of the free atoms are combined to estimate the electron density (peaking at 1013 cm−3) in the vapor.

The anisotropic nature of singlet fission in single crystalline organic semiconductors

The escalating global energy predicament implores for a revolutionary resolution—one that converts sunlight into electricity—holding the key to supreme conversion efficiency. This comprehensive review embarks on the exploration of the principle of generating multiple excitons per absorbed photon, a captivating concept that possesses the potential to redefine the fundamental confines of conversion efficiency, albeit its application remains limited in photovoltaic devices. At the nucleus of this phenomenon are two principal processes: multiple exciton generation (MEG) within quantum-confined environments, and singlet fission (SF) inside molecular crystals. The process of SF, characterized by the cleavage of a single photogenerated singlet exciton into two triplet excitons, holds promise to potentially amplify photon-to-electron conversion efficiency twofold, thereby laying the groundwork to challenge the detailed balance limit of solar cell efficiency. Our discourse primarily dissects the complex nature of SF in crystalline organic semiconductors, laying special emphasis on the anisotropic behavior of SF and the diffusion of the subsequent triplet excitons in single-crystalline polyacene organic semiconductors. We initiate this journey of discovery by elucidating the principles of MEG and SF, tracing their historical genesis, and scrutinizing the anisotropy of SF and the impact of quantum decoherence within the purview of functional mode electron transfer theory. We present an overview of prominent techniques deployed in investigating anisotropic SF in organic semiconductors, including femtosecond transient absorption microscopy and imaging as well as stimulated Raman scattering microscopies, and highlight recent breakthroughs linked with the anisotropic dimensions of Davydov splitting, Herzberg–Teller effects, SF, and triplet transport operations in single-crystalline polyacenes. Through this comprehensive analysis, our objective is to interweave the fundamental principles of anisotropic SF and triplet transport with the current frontiers of scientific discovery, providing inspiration and facilitating future ventures to harness the anisotropic attributes of organic semiconductor crystals in the design of pioneering photovoltaic and photonic devices.

Phase retrieval for X-ray differential phase contrast radiography with knowledge transfer learning from virtual differential absorption model

Grating-based X-ray phase contrast radiography and computed tomography (CT) are promising modalities for future medical applications. However, the ill-posed phase retrieval problem in X-ray phase contrast imaging has hindered its use for quantitative analysis in biomedical imaging. Deep learning has been proved as an effective tool for image retrieval. However, in practical grating-based X-ray phase contrast imaging system, acquiring the ground truth of phase to form image pairs is challenging, which poses a great obstacle for using deep leaning methods. Transfer learning is widely used to address the problem with knowledge inheritance from similar tasks. In the present research, we propose a virtual differential absorption model and generate a training dataset with differential absorption images and absorption images. The knowledge learned from the training is transferred to phase retrieval with transfer learning techniques. Numerical simulations and experiments both demonstrate its feasibility. Image quality of retrieved phase radiograph and phase CT slices is improved when compared with representative phase retrieval methods. We conclude that this method is helpful in both X-ray 2D and 3D imaging and may find its applications in X-ray phase contrast radiography and X-ray phase CT.

Efficient denoising of cold atom images using the optimized eigenface recognition algorithm.

Absorption imaging is a widely employed technique for detecting cold atom clouds and Bose-Einstein condensates (BECs). There are situations where such images may suffer from unwanted interference fringes, resulting in uncertainties in determining crucial parameters such as the atom number, temperatures, or even dynamics in small timescales. Reducing the acoustic vibrations and recording image frames synchronized with the source of such vibrations can largely reduce these fringes; however, some residual fringes still need to be taken care of for precision measurements. In this study, we propose an efficient image post-processing technique for noise reduction that effectively mitigates such interference patterns. Our approach makes use of the well-known eigenface recognition algorithm, combined with an optimized masking strategy applied to the image of the atomic cloud using a small number of basis sets. The use of a limited basis set ensures minimal computational time, allowing this method to be readily incorporated into every experimental run. Through the application of our technique, we successfully reduce interference fringes and improve the accuracy of parameter estimation by 50% in the absorption imaging of cold atoms. The temperature uncertainties of cold 87 R b atoms are reduced by more than 50% after the algorithm is applied. This approach holds significant promise for enhancing the reliability and precision of experimental measurements in diverse research fields where absorption imaging is employed.

Imaging overpressurised fracture networks and geological barriers hindering fluid migrations across a slow-deformation seismic gap

There is an ongoing debate on the processes producing background seismicity and deformation transients across seismic gaps, i.e., regions that lack historical large-magnitude earthquakes. Essential missing elements are geophysical images that resolve sources of geophysical unrest. Here, we apply seismic scattering and absorption tomography to data recorded during the 2010–2014 seismic sequence within the Mt. Pollino seismic gap region (Southern Italy). The tomographic models show high sensitivity to fluid content, deformed fractured structures, and impermeable layers stopping fluid migrations. They bridge the gaps between geological and geophysical models and provide a highly-resolved image of the source of seismic and deformation unrest within this seismic gap. High absorption topping the western Pollino seismic volume appears pressurized between the low-Vp/Vs and low-scattering San Donato metamorphic core and a deep basement. Absorbing fluids can only migrate laterally to the east, blocked in the west and southwest by deep low-scattering barriers associated with east-dipping faults and to the north and southeast by saturated overpressurized low-scattering basins. This eastern migration is only partially effective, producing seismicity across the lowest boundary of the high-absorption volume. Our results showcase the potential of seismic scattering and absorption when imaging structures causing geophysical unrest processes across fault networks.

Laser absorption tomography based on unstructured meshing

Laser absorption tomography is an imaging technique used to monitor gas flow and combustion processes. The measurement plane is typically discretized using square pixels in which the flow field is assumed to be uniform. This approach can introduce large discretization errors compared to a finite element mesh with linear interpolation, especially when the domain has an irregular and non-rectangular boundary. In this work, we assess the influence of triangular mesh and quadrilateral mesh schemes on reconstruction accuracy. Our theoretical analysis and numerical studies demonstrate that linear interpolation yields superior performance compared to the conventional uniform-pixel method, and the error can be reduced from 0.250 to 0.097 and 0.160 to 0.105 for triangular and quadrilateral meshes, respectively. Non-body-fitted mesh can increase error by 70 % –99 % . The experiment also proves the superiority of body-fitted meshes with linear interpolation. Moreover, triangular mesh exhibits better noise immunity than quadrilateral mesh. The reconstruction quality appears to be insensitive to the mesh quality for the quadrilateral mesh.

Visualization and Analysis of Temporal and Steady-State Gas Concentration in Process Chamber Using 70-dB SNR 1000 fps Absorption Imaging System

We are developing a 70-dB SNR 1,000 frame-per-second absorption imaging system, and it enables us to visualize gas concentration distribution in a vacuum chamber for semiconductor manufacturing. This article presents the measurement and analysis of temporal and steady-state NO2 gas concentration distribution using this system. In the steady-state gas flow condition which supplied from a shower plate, the spectrometry of NO2 gas shows a good linear relationship between the absorbance and the partial pressure with the system. The system enables the visualization of steady-state of NO2 gas concentration distribution which supplied from one hole of shower plate, and the results are compared with simulation to discuss the reasonability of the measurement. The system also enables to visualize the temporal NO2 gas concentration distribution supplied from a single nozzle, and the advantages of the system’s features of 70-dB and 1,000 fps are discussed.

Laser absorption tomography of complex combustion fields based on finite element node strategy and adaptive edge optimization algorithm

Laser absorption spectroscopy tomography is a technique for combustion diagnosis that provides two-dimensional temperature and species concentration measurements. However, its current implementation assumes a homogeneous distribution of discrete grids, introducing errors and uncertainties in the reconstruction results due to large gradients between adjacent grids and information loss within a grid. This work aims to develop a finite element node strategy (FES) to address these issues. To enhance the reconstruction performance, we propose an adaptive edge optimization algorithm (AEO) that can effectively reduce artifacts and preserve edges. We validate the effectiveness of the algorithm through comprehensive simulations and experiments. The simulation results show that the error of integrated projection value exhibits a trend of rise with increasing resolution, and it is also true that the more complex the reconstruction distribution is, the larger the error becomes. In the simulation, the maximum relative error by using the discrete grids strategy (DGS) reached 1.3 % for complex distribution, whilst it was only 0.1 % when the FES was used. Two representative flame phantoms were used in the simulation. The reconstruction error of 4.51 % was achieved in one phantom by using the FES over 5.11 % for using DGS, in the other phantom, the error was 3.14 % for the FES compared with 3.88 % for the DGS. With the addition of the adaptive edge optimization algorithm (FES-AEO), the temperature reconstruction errors were reduced to 1.49 % and 1.77 % respectively, and the characteristic peaks and edge values were closer to the true values. The experimental results show that when the resolution is 1.67 mm, the mean relative error of the DGS measurements is 1.26 %, while the FES-AEO is only 0.65 %.

Characteristics and neutron imaging of capillary water absorption for metakaolin and steel fiber reinforced slag based-geopolymer mortars

The manufacturing of cement consumes energy and natural resources of raw materials, and emits large amounts of CO2. Geopolymers are eco-friendly building materials that have the potential to replace cement-based building materials. We used neutron radiography to report new data on water absorption and distribution in metakaolin (MK) and steel fiber reinforced slag-based mortars for the first time. Due to contradictory studies on the influence of adding steel fibers to slag-based geopolymers on compressive strength and the limited research on water permeability, workability and sorptivity, new results were presented. Alkaline-activated MK and steel fiber reinforced slag -based geopolymer mortars were prepared. The neutron radiography (NR) technique was used to investigate capillary water absorption in the mortars. The fresh and mechanical strengths of the mortars, as well as their permeability, workability and sorptivity, were determined. The compressive, flexural, and splitting tensile strengths, as well as the water permeabilities, increased, whereas the workability decreased as the steel fiber percentage in slag-based mortars increased. The results showed that the addition of steel fiber at volume fractions of 1%, 2%, and 3% increased the −7 and 28 days compressive strengths for the slag-based mortars by 44.7%, 50.7%, and 57.1% and 41.9%, 50.2%, and 55.1%, respectively. The NR results revealed that the resistance to water penetration in reinforced slag-based mortars is greater than that of MK. The addition of steel fiber introduced inhomogeneity in the matrix of the reinforced samples and created water-accumulating regions. Water proceeded regularly in the Mk samples but irregularly in the slag-based mortars. The capillary absorption factors k for Mk and slag-based mortars with steel fiber at volume fractions of 1%, 2%, and 3% were found to be 0.37 cm min−1/2, 0.2 cm min−1/2, 0.17 cm.min−−1/2, 0.19 cm min−1/2, and 0.2 cm min−1/2, respectively. According to NR, the optimal percentage of steel fibers in the slag-based mortar impeding water flow is 1%–2%. Steel fiber used at a high proportion to slag-based mortars increased sorptivity. To best fit the results at high absorption times, the second Fick's law of diffusion can be used.

Enhancing Contrast of Spatial Details in X-ray Phase-Contrast Imaging through Modified Fourier Filtering

In-line X-ray phase contrast imaging, which is simple to experiment with, provides significantly higher sensitivity, compared to conventional X-ray absorption imaging. The inversion of the relationship between recorded Fresnel diffraction intensity and the phase shift induced by the object is called phase retrieval. The transport of intensity equation (TIE), a simple method of phase retrieval, which is solved by the fast Fourier transform algorithm proposed by Paganin et al., has been widely adopted. However, the existing method suffers from excessive suppression of high-frequency information, resulting in loss of image details after phase retrieval, or insufficient detail contrast, leading to blurry images. Here, we present a straightforward extension of the two-distance FFT-TIE method by modifying the Fourier filter through the use of a five-point approximation to calculate the inverse Laplacian in a discrete manner. Additionally, we utilize a combination of continuous Fourier transform and a four-point approximation to compute the gradient operator. The method is evaluated by simulating samples with a shape similar to the resolution test map and by using a photograph of a dog for further evaluation. The algorithm that incorporates the modified gradient operator and the algorithm that solely utilizes the continuous Fourier transform for gradient computation were compared with the results obtained using the two-distance FFT-TIE method. The comparisons were conducted using the results obtained from two distances from the sample to the detector. The results show that this method improves the contrast of spatial details and reduces the suppression of high spatial frequencies compared to the two-distance FFT-TIE method. Furthermore, in the low-frequency domain, our algorithm does not lose much information compared to the original method, yielding consistent results. Furthermore, we conducted our experiments using carbon rods. The results show that both our method and the FFT-TIE method exhibit low-frequency distortion due to the requirement of close proximity between the absorption maps and the detector. However, upon closer inspection, our proposed method demonstrates superior accuracy in reproducing the finer details of the carbon rod fibers.

A Bi-Co Corridor Construction Effectively Improving the Selectivity of Electrocatalytic Nitrate Reduction toward Ammonia by Nearly 100.

Improving the selective ammonia production capacity of electrocatalytic nitrate reduction reaction (NO3 RR) at ambient conditions is critical to the future development and industrial application of electrosynthesis of ammonia. However, the reaction involves multi-proton and electron transfer as well as the desorption and underutilization of intermediates, posing a challenge to the selectivity of NO3 RR. Here the electrodeposition site of Co is modulated by depositing Bi at the bottom of the catalyst, thus obtaining the Co+Bi@Cu NW catalyst with a Bi-Co corridor structure. In 50mm NO3 - , Co+Bi@Cu NW exhibits a highest Faraday efficiency of ≈100% (99.51%), an ammonia yield rate of 1858.2µgh-1 cm-2 and high repeatability at -0.6V versus the reversible hydrogen electrode. Moreover, the change of NO2 - concentration on the catalyst surface observed by in situ reflection absorption imaging and the intermediates of the NO3 RR process detected by electrochemical in situ Raman spectroscopy together verify the NO2 - trapping effect of the Bi-Co corridor structure. It is believed that the measure of modulating the deposition site of Co by loading Bi element is an easy-to-implement general method for improving the selectivity of NH3 production as well as the corresponding scientific research and applications.

Nonlinear multispectral tomographic absorption deflection spectroscopy based on Bayesian estimation for spatially resolved multiparameter measurement in methane flame exhaust

In aerospace propulsion systems with high temperature, pressure, and distortion, inhomogeneous thermophysical gradients can cause line-of-sight deflection and spectral integral deviation. This introduces nonlinearity and measurement error into spectral models, impacting multiparameter field measurements. To address this, we developed a nonlinear multispectral tomographic absorption deflection spectroscopy (MTADS) technique, based on Bayesian inference, for temperature and gas concentration prediction. Our forward model considers inhomogeneous refractive index gradient and laser deflection, synthesising multispectral laser absorption signal through a gas mixing equation and multipoint ray tracing. Simulation results reveal inhomogeneous parameter distribution can cause non-Gaussian and non-zero mean deviations of the spectral absorption signal. The inverse model increases spectral lines to expand measurement signals under sparse optical arrangements. Combining the measured laser absorption signal with Bayesian statistical information of the estimated parameter field improves measurement robustness and accuracy. The maximum a posteriori (MAP) estimation introduces priors such as experiment samples, computational fluid dynamics data, and diffusive transport-induced spatial smoothing. A proof-of-concept based on methane diffusion flame data demonstrates the effectiveness of MTADS, enabling quantitative flame topology and parameter distribution, more accurate 2D temperature, and concentration imaging. These measurements are crucial for revealing chemical reaction dynamics and monitoring combustion systems.

A cadaveric breast cancer tissue phantom for phase-contrast X-ray imaging applications.

As mammography X-ray imaging technologies advance and provide elevated contrast in soft tissues, a need has developed for reliable imaging phantoms for use in system design and component calibration. In advanced imaging modalities such as refraction-based methods, it is critical that developed phantoms capture the biological details seen in clinical precancerous and cancerous cases while minimizing artifacts that may be caused due to phantom production. This work presents the fabrication of a breast tissue imaging phantom from cadaveric breast tissue suitable for use in both transmission and refraction-enhanced imaging systems. Human cancer cell tumors were grown orthotopically in nude athymic mice and implanted into the fixed tissue while maintaining the native tumor/adipose tissue interface. The resulting human-murine tissue hybrid phantom was mounted on a clear acrylic housing for absorption and refraction X-ray imaging. Digital breast tomosynthesis was also performed. Both attenuation-based imaging and refraction-based imaging of the phantom are presented to confirm the suitability of this phantom's use in both imaging modalities.