Framing Camera Research Articles

Diagnosing X-pinch and cylindrical wire array Z-pinch with an all-optical framing camera on the “QiangGuang-I” facility

An all-optical framing camera has been established on the 1-MA “QiangGuang-I” facility for investigating the imploding dynamics of the X-pinch and the cylindrical wire array Z-pinch. This camera measures the photons flux distribution by detecting the photon-induced refractive index changes in the InP semiconductor with a probe laser beam. The InP-based imaging sensor with a 100 nm thick Al film and a 10 μm thick gold transmission grating of 20 μm spatial period was fabricated based on the lithography method. Two sequential frames with 1.5 ns time resolution and 13 ns inter frame separation time can be recorded in one single shot. Framing images indicating the plasma generation and dissipation of the X-pinch and the cylindrical wire array Z-pinch have been obtained, showing that this framing camera is a promising technique for investigating the mechanisms of the Z-pinch and other high energy density physics.

Basic principles and optical system design of 17.48keV high-throughput modified Wolter x-ray microscope.

High-precision x-ray imaging diagnostics of hotspot at the stagnation stage are essential for regulating implosion asymmetry and retrieving physical implosion parameters. With regard to 10-20keV energy band imaging, existing diagnostic instruments such as Kirkpatrick-Baez microscopes and pinhole cameras are insufficient in terms of spatial resolution and collection efficiency. The situation is even worse when high-speed, time-resolved imaging diagnostics are performed by coupling framing cameras or line-of-sight imagers. This article presents the basic principles and optical system design of a 17.48keV modified Wolter x-ray microscope, to resolve the problems encountered in high-energy imaging diagnostics. The proposed optical configuration offers a better spatial resolution, greater depth of field, and preliminary compliance with the requirements of high precision optical processing techniques. The spatial resolution is better than 1 µm in a field range ±150 µm, and is better than 3 µm in a total field of view ∼408 µm in diameter. The geometric solid angle is calculated as 3.0 × 10-5 sr and is estimated to be 1.2 × 10-6 sr, considering the reflectivity of the double mirrors. The proposed microscope is expected to effectively improve spatial resolution and signal-to-noise ratio for high-energy imaging diagnostics.

EVtracker: An Event-Driven Spatiotemporal Method for Dynamic Object Tracking.

An event camera is a novel bio-inspired sensor that effectively compensates for the shortcomings of current frame cameras, which include high latency, low dynamic range, motion blur, etc. Rather than capturing images at a fixed frame rate, an event camera produces an asynchronous signal by measuring the brightness change of each pixel. Consequently, an appropriate algorithm framework that can handle the unique data types of event-based vision is required. In this paper, we propose a dynamic object tracking framework using an event camera to achieve long-term stable tracking of event objects. One of the key novel features of our approach is to adopt an adaptive strategy that adjusts the spatiotemporal domain of event data. To achieve this, we reconstruct event images from high-speed asynchronous streaming data via online learning. Additionally, we apply the Siamese network to extract features from event data. In contrast to earlier models that only extract hand-crafted features, our method provides powerful feature description and a more flexible reconstruction strategy for event data. We assess our algorithm in three challenging scenarios: 6-DoF (six degrees of freedom), translation, and rotation. Unlike fixed cameras in traditional object tracking tasks, all three tracking scenarios involve the simultaneous violent rotation and shaking of both the camera and objects. Results from extensive experiments suggest that our proposed approach achieves superior accuracy and robustness compared to other state-of-the-art methods. Without reducing time efficiency, our novel method exhibits a increase in accuracy over other recent models. Furthermore, results indicate that event cameras are capable of robust object tracking, which is a task that conventional cameras cannot adequately perform, especially for super-fast motion tracking and challenging lighting situations.

Two hybrid model-based control strategies for a flexible parallel planar manipulator

Designers can reduce the inertia of moving parts of a robotic manipulator aiming for energy efficiency. Nevertheless, this design option could yield undesired vibrations due to its lighter components, requiring control strategies for their mitigation. In addition, other challenges arise when designing parallel manipulators since the control strategies cannot use a direct measurement of the manipulator end-effector’s pose and their coupling dynamics. This work proposes two hybrid model-based control design strategies for a 3RRR parallel manipulator with flexible links in this work. Both strategies have two feedback loops, one of which exploits the encoder’s measurements and the inverse kinematic model of the manipulator considering rigid links. The difference between the strategies is related to the feedback signal used in the second feedback loop that exploits an LQR/LQG strategy. The first strategy uses the end-effector’s pose measured by a limited frame camera, while the second uses the end-effector’s pose derived from a reduced-order finite element model. These strategies are experimentally compared with a PID strategy for assessing their assets and limitations. The steady-state error was lower when using the model-based control strategies. Moreover, the second strategy responds faster than the first for the evaluated tasks.

A linear pushbroom satellite image epipolar resampling method for digital surface model generation

Epipolar resampling, also known as stereo rectification, is a prerequisite for optical satellite image stereo matching in digital surface model (DSM) generation. In epipolar resampling, image pairs are warped and thus can be matched pixel-to-pixel along the epipolar line. Different from the pinhole imaging model of frame cameras, the multi-center projection model of linear pushbroom sensors does not obey the epipolar geometric constraint, making it impossible to conjugate epipolar lines from stereo images. The epipolar resampling methods can resolve some of the inherent epipolar error, but not completely. The topographic relief brings high disparity values between epipolar image pairs, posing a challenge to the subsequent stereo matching to recover disparity map. In this paper, we introduce a novel epipolar resampling method for satellite imagery based on ortho rectification. This method takes full advantage of a rational function model to establish a connection between image space and object space coordinates, and thereby transform the two-dimensional disparity in ortho rectification products into one dimension. We propose a coarse-to-fine DSM generation pipeline to overcome the high-disparity-problem without additional time costs or precision decrement. We conducted experiments on various satellite stereo images, and validated both the extreme suppression on epipolar error as well as the feasibility of the proposed coarse-to-fine DSM generation pipeline. The experimental results show that the generated epipolar image pairs from our proposed method effectively solved the high disparity problem. Furthermore, the novel DSM generation method obtained an internal error below one pixel for a rectified stereo image pair in mountain area, and the lowest vertical error in the IARPA MVS dataset when compared with the common open-source programs and commercial software.

Study on the magneto-Rayleigh–Taylor instability of inverse exploding planar foils

Experiments of the explosion of thin planar foils have been carried on the QiangGuang-I facility (∼1.4 MA peak current, ∼110 ns rise time) to study the evolution of the plasma instability. An inverse-field configuration was utilized with the current return post placed in the center and two parallel planar aluminum foils of 20 μm thickness, 1 cm width built on each side. The foil was ablated into a plasma slab with a particular width, and the inner surface and outer surface expanded in opposite directions and suffered from unequal magnetic pressure, resulting in divergent instability evolution. To alter plasma acceleration, multiple load configurations, including symmetric and asymmetric cases, were used to change the distance between the foil and back-post. The diagnostic system was fielded to provide a side-on view of exploding foils, including laser shadowgraphy, laser interferometry, and an optical framing camera. The characteristic structure, wavelength spectrum, and growth rate of the instability were compared for both sides. During the early magnetohydrodynamics instability developing stage, the inner side had a similar feature to the outer side surface, but with a larger ratio of the long wavelength in the spectrum. The anti-correlated relationship between the correlation coefficients of two surfaces appeared to be a kink instability mode. When the interface was subjected to deceleration, a transition from an early instability mode to a Magneto-Rayleigh–Taylor (MRT) instability mode was observed in studies, with the growth rate and characteristic wavelength growing fast. Because of the higher magnetic pressure and earlier retardation, the inner edge was more severely affected by MRT instability than the outer edge. A theoretical model was developed to interpret the experimental data.

Geometric calibration of a hyperspectral frame camera with simultaneous determination of sensors misalignment

The recent development of lightweight and relatively low-cost hyperspectral sensors has created new perspectives for remote sensing applications. This study aimed to investigate the geometric calibration of a hyperspectral frame camera based on a tuneable Fabry–Pérot interferometer (FPI) and two sensors. The radiation passes through the optics and then through the FPI, where it is redirected to two sensors using a beam-splitting prism. Previous studies have shown significant variations between the interior orientation parameters for the different bands, both between bands of the same sensor and between different sensors, and that these variations are due to the principle of image acquisition. Discrepancies of tens of pixels were obtained by comparing image coordinates measured in different bands. In this research, it was proposed to calibrate this camera in a static mode with changes in the mathematical calibration model. The restriction of obtaining only one set of exterior orientation parameters by hypercube was applied, adding parameters related to the misalignment between the sensors and parameters of a linear function relating the camera principal distance to the wavelength values. The application of the parameters estimated with this approach reduced the discrepancies between image coordinates measured in different bands to values smaller than one pixel. Using the sensor calibration parameters in the mobile UAV operation in an aerial bundle adjustment reduced the root mean square error (RMSE) on checkpoints by approximately 20% compared to the traditional model in which the interior orientation parameters and lens distortions were calibrated for each band separately. Thus, it was possible to obtain accurate results that make the use of this camera more practical since only one set of calibration parameters for all bands is needed.

Nonlinear characteristics and corrections of near-field underwater explosion shock waves

The shock wave characteristics within the near-field are one of the most challenging aspects of understanding an underwater explosion. The latest numerical and experimental techniques were utilized to investigate the near-field pressure distribution and decay features after a shock disturbance. The governing equations in the numerical simulation were discretized with a fifth-order weighted essentially non-oscillatory scheme in space and a third-order Runge–Kutta scheme in time, and multi-medium interactions were defined and resolved via the modified ghost fluid method. The test system consisted of a synchronized high-speed framing camera and polyvinylidene fluoride (PVDF) sensors. Three identical spherical composition B charges were examined under the same test conditions, and the raw data from the high-speed camera were processed with edge detection and circle fitting techniques. The comparison showed that the high-speed camera image data, the PVDF signals, and the numerical computation results were highly consistent with each other. Higher-order correction terms were added to the pressure peak distribution model and the pressure decay model as nonlinear corrections based on further comprehensive and insightful analysis of the verified results. The corrected models not only fit with the near-field data but had better accuracy under the far-field condition as well.

Application of an Event-Based Camera for Real-Time Velocity Resolved Kinetics.

We describe here the application of an inexpensive event-based/neuromorphic camera in an ion imaging experiment operated at 1 kHz detection rate to study real-time velocity-resolved kinetics of thermal desorption. Such measurements involve a single gas pulse to initiate a time-dependent desorption process and a high repetition rate laser, where each pulse of the laser is used to produce an ion image. The sequence of ion images allows the time dependence of the desorption flux to be followed in real time. In previous work where a conventional framing camera was used, the large number of megapixel-sized images required data transfer and storage rates of up to 16 GB/s. This necessitated a large onboard memory that was quickly filled and limited continuous measurement to only a few seconds. Read-out of the memory became the bottleneck to the rate of data acquisition. We show here that since most pixels in each ion image contain no data, the data rate can be dramatically reduced by using an event-based/neuromorphic camera. The data stream is thus reduced to the intensity and location information on the pixels that are lit up by each ion event together with a time-stamp indicating the arrival time of an ion at the detector. This dramatically increases the duty cycle of the method and provides insights for the execution of other high rep-rate ion imaging experiments.

Brine residues and organics in the Urvara basin on Ceres

Ceres is a partially differentiated dwarf planet, as confirmed by NASA’s Dawn mission. The Urvara basin (diameter ~170 km) is its third-largest impact feature, enabling insights into the cerean crust. Urvara’s geology and mineralogy suggest a potential brine layer at the crust-mantle transition. Here we report new findings that help in understanding the structure and composition of the cerean crust. These results were derived by using the highest-resolution Framing Camera images acquired by Dawn at Ceres. Unexpectedly, we found meter-scale concentrated exposures of bright material (salts) along the crater’s upper central ridge, which originate from an enormous depth, possibly from a deep-seated brine or salt reservoir. An extended resurfacing modified the southern floor ~100 Myr after crater formation (~250 Myr), long after the dissipation of the impact-generated heat. In this resurfaced area, one floor scarp shows a granular flow pattern of bright material, showing spectra consistent with the presence of organic material, the first such finding on Ceres beyond the vast Ernutet area. Our results strengthen the hypothesis that Ceres is and has been a geologically active world even in recent epochs, with salts and organic-rich material playing a major role in its evolution.

Image Deblurring Aided by Low-Resolution Events

Due to the limitation of event sensors, the spatial resolution of event data is relatively low compared to the spatial resolution of the conventional frame-based camera. However, low-spatial-resolution events recorded by event cameras are rich in temporal information which is helpful for image deblurring, while intensity images captured by frame cameras are in high resolution and have potential to promote the quality of events. Considering the complementarity between events and intensity images, an alternately performed model is proposed in this paper to deblur high-resolution images with the help of low-resolution events. This model is composed of two components: a DeblurNet and an EventSRNet. It first uses the DeblurNet to attain a preliminary sharp image aided by low-resolution events. Then, it enhances the quality of events with EventSRNet by extracting the structure information in the generated sharp image. Finally, the enhanced events are sent back into DeblurNet to attain a higher quality intensity image. Extensive evaluations on the synthetic GoPro dataset and real RGB-DAVIS dataset have shown the effectiveness of the proposed method.

Kilometer-scale crater size-frequency distributions on Ceres

This paper presents an investigation of the size-frequency distribution (SFD) of sub-kilometer-sized projectiles in the Main Belt based on the crater distribution on Ceres. Using image data of 35 m/pixel obtained by the Dawn mission with an onboard Framing Camera during the Low Altitude Mapping Orbit (LAMO), we counted craters having diameter larger than 1 km on the entire surface of Ceres. Based on the crater counting, we investigated the crater size-frequency distribution (CSFD) for the entire surface of Ceres and specifically for floors of 15 named impact craters of various ages. Distinct secondary craters were excluded for derivation of the CSFDs for the 15 crater floors. The CSFDs for the entire surface and those for 8 out of the 15 crater floors show good agreement with the crater production function (PF) for the Lunar Derived Model (LDM), which scales the well-investigated lunar cratering record to the impact environment of Ceres. Although the CSFDs for the other 7 crater floors show somewhat deviations from the PF, the deviations are less likely to show projectile information but are more likely to show the contamination with unidentified secondary craters. Our results indicate that the projectile SFDs on Ceres during 220 Ma – 1.9 Ga are invariably consistent with that found for the Moon. On the other hand, the derived SFDs of sub-kilometer-sized projectiles on Ceres are apparently different from those for the Main Belt Asteroids that have been observed telescopically, which might result from subsurface discontinuity or the Yarkovsky effect.

High-resolution dual-energy sixteen-channel Kirkpatrick–Baez microscope for ultrafast laser plasma diagnostics

High-resolution x-ray imaging diagnostics play a crucial role in fundamental research, such as inertial confinement fusion (ICF) and high-energy density physics (HEDP). Plasma signals are typically characterized by small scales, rapid evolution, and spectral complexity. These characteristics make it essential to develop x-ray diagnostics optics with high spatial resolution, collection efficiency, and spectral resolution. These requirements can be met using a combination of a high-resolution multi-channel Kirkpatrick–Baez (KB) microscope with spectrum-resolved multilayers and a time-resolved framing camera. This study describes the optical and multilayer design of a dual-energy sixteen-channel KB microscope. The calibrated results of online and offline imaging are shown. By utilizing a dual-energy multi-channel KB microscope, high-resolution backlighting and self-emission x-ray imaging can be realized and detailed information related to plasma density and temperature can be simultaneously obtained.

Research on Spatial Resolution Characteristics of Pulse Dilation X-Ray Framing Camera

Research on Spatial Resolution Characteristics of Pulse Dilation X-Ray Framing Camera

Current channel in plasma of inverse exploding planar foils

In the research of <i>Z</i>-pinch, the initial establishment of current in load, current density distribution mode and current evolution characteristics in different stages are of great significance in studying the development of plasma dynamics, analyzing plasma instability mode, and carrying out the MHD numerical simulation. Thin planar foil is an ideal configuration with continuous two-dimensional structure to study the problem. Based on an ideal model that consists of foil-backpost configuration under the assumption of infinite length in reverse exploding condition, the establishment, distribution and evolution of the current channel, magnetic field distribution characteristics and the force state of foil plasma by the inductive mode are calculated and analyzed theoretically. Relevant experiments are carried out on the QG-1 facility with about 1.4 MA peak current and 100 ns rise time to verify the calculated results. The self-emission graphs of exploding foils are recorded by visible-light frame cameras from side-on view and end-on view. Different load configurations are utilized including symmetric and asymmetric condition both assembled with two 20-μm-thick aluminum foils. The results show that the establishment of the initial current in foil (0–70 ns) accord with the expectation of the inductance model. The inductance feature determines the distribution of the current in this stage, which causes the current to aggregate in foil edges and then the edges will suffer much higher magnetic field and <i> <b>J</b> </i> × <i> <b>B</b> </i> force than the center. This results in the formation of a clamp plasma morphology. The later images (70–120 ns) show that the intense emission region will transfer from the foil edges to the center, which proves a transfer of current channel. So the magnetic field and <i> <b>J</b> </i> × <i> <b>B</b> </i> force in the foil center increase and a peak convex plasma morphology forms then. It shows that the current distribution will be affected by the plasma evolution process. The convergence process of ablated plasma with current finally leads to rapidly switch the current channel (in 60–80 ns).

Imaging Performance of Double-Lens Pulse-Dilation Framing Camera

Imaging Performance of Double-Lens Pulse-Dilation Framing Camera

Simultaneous high-resolution x-ray backlighting and self-emission imaging for laser-produced plasma diagnostics using a two-energy multilayer Kirkpatrick–Baez microscope

A simultaneous high-resolution x-ray backlighting and self-emission imaging method for laser-produced plasma diagnostics is developed in which two Kirkpatrick–Baez imaging channels for high-energy and low-energy diagnostics are constructed using a combination of multilayer mirrors in near-coaxial form. By using a streak or framing camera placed on the image plane, both backlit and self-emission images of a laser-produced plasma with high spatial and temporal resolution can be obtained simultaneously in a single shot. This paper describes the details of the method with regard to its optical and multilayer design, assembly, and alignment method. In addition, x-ray imaging results with a spatial resolution better than 5 µm in the laboratory and experimental results with imploding capsules in the SG-III prototype laser facility are presented.

Digital processing of shadowgraph images taking into account the diffraction of light at a shock front

The aim of the study is to determine the shock wave position in experimental shadowgraph images and to evaluate the accuracy by digital image processing. The experimental images were obtained with the shock tube with a rectangular channel. The shadowgraph optical system formed a parallel light beam. It passed through the plane-parallel quartz glasses of the shock tube test section. The process synchronization system at the facility allows registering the shadowgraph images of unsteady flows with shock waves with a high-speed camera or with a single frame camera. The obtained spatial intensity profiles were used to determine the coordinates of gas-dynamic discontinuities at different stages of the flow evolution. shadowgraph patterns were analysed taking into account diffraction at the shock front in case of a laser light source.

System Calibration Including Time Delay Estimation for GNSS/INS-Assisted Pushbroom Scanners Onboard UAV Platforms

Unmanned aerial vehicles (UAVs ) equipped with imaging sensors and integrated global navigation satellite system/inertial navigation system (GNSS/INS ) units are used for numerous applications. Deriving reliable 3D coordinates from such UAVs is contingent on accurate geometric calibration, which encompasses the estimation of mounting parameters and synchronization errors. Through a rigorous impact analysis of such systematic errors, this article proposes a direct approach for spatial and temporal calibration (estimating system parameters through a bundle adjustment procedure) of a GNSS/INS -assisted pushbroom scanner onboard a UAV platform. The calibration results show that the horizontal and vertical accuracies are within the ground sampling distance of the sensor. Unlike for frame camera systems, this article also shows that the indirect approach is not a feasible solution for pushbroom scanners due to their limited ability for decoupling system parameters. This finding provides further support that the direct approach is recommended for spatial and temporal calibration of UAV pushbroom scanner systems.

Multichannel Operation of a Multi-Gap, Multichannel Spark Gap Switch Using a Low-Magnitude Trigger Pulse and Its Performance Evaluation

This article describes the performance of a multi-gap, multichannel (MGMC) spark gap switch, evaluated numerically and experimentally, to achieve multichanneling using a relatively low-magnitude trigger pulse. First, an electrostatic analysis of the switch geometry was carried out using COMSOL Multiphysics 5.5 to determine the electric field distribution during high-voltage hold-off before triggering and to determine the coupling parameters between trigger and other intermediate electrodes of the switch. Second, a circuit model was developed in PSpice Lite 16.6 for predicting the process of channel formation and its influence on adjacent channels. Based on the simulation results obtained, a six-gap, eight-channel, planar MGMC switch operating in air has been designed and developed. The developed switch was, then, incorporated into a compact experimental setup to experimentally evaluate various switch parameters such as inductance, switching delay, jitter, and closing time when triggered using a negative-polarity trigger pulse of magnitude −40 kV, generated using a compact, in-house developed trigger generator. In contrast to most of the reported MGMC switches that use complicated Marx-based generators to produce ≥120-kV trigger pulses for multichannel formation, the experimental results, thus obtained, report comparable switch performance using a low-magnitude (−40 kV) trigger pulse. In addition, framing and streak camera images taken during the current discharge indicate the presence of ~6 luminous channels. In addition, short-circuit experiments for estimating switch inductance by consecutively shorting one or more channels also indicate multichanneling in the switch using a low-magnitude trigger pulse.