Fast Camera Research Articles

Magnetic field imaging in a laboratory plasma

In laboratory plasmas, arrays of probes have typically been used to measure the evolution of the magnetic field topology. Here, we present initial image-based measurements of the magnetic topology in a low-temperature plasma using a purely optical diagnostic. Laser induced fluorescence measurements of neutral velocity distribution functions are made using a fast camera, imaging the Zeeman splitting of σ-peaks in neutral argon. The separation of σ-peaks provides spatially resolved magnetic field magnitude measurements with a detection threshold on the order of 10 G.

Visual Multi-Face Tracking Applied to Council Proceedings

Face recognition, face measurement, camera control, measurement of expressions and other tasks can benefit from online visual multi-face tracking. Given the availability of high quality general purpose detectors and tracking-by-detection frameworks, we provide guidance on how to develop a multi-face tracker out of standard components. In this paper, we train common object detectors specifically on faces to understand how well these detectors perform and evaluate different classifier loss functions. Our specific case study tracks faces in the context of council meetings and in parliamentary settings such as the Canadian House of Commons for which we create an annotated video set as a benchmark (see Fig. 1). These meetings in a parliamentary setting are often recorded from multiple cameras with participants and audiences walking around. Fast camera switching and zooming lead to significant scale changes of faces. Therefore, these settings can be characterized as tracking in unconstrained video. This will negatively impact the tracking accuracy and increase the likelihood of identity switches (IDS) between face labels. However, being able to track in unconstrained video enables a wider range of measurement applications. We find that while online tracking based on combining state-of-the-art methods can lead to high-quality tracking results, there is still a large gap between offline and online methods. The discussed method can be adapted to other tracking tasks for which large image databases are available.

Spoke behaviour in reactive HiPIMS

Plasma in high-power impulse magnetron sputtering discharge, similarly to other discharges utilising E × B field (Hall thrusters, homopolar devices), undergoes self-organisation into the ionisation zones predominantly rotating in the E × B direction, called spokes. Many studies were conducted focussing on the characterisation of their appearance, mode number, rotational velocity, merging and splitting events in different experimental conditions. Nevertheless, only very little research has been conducted in the case of reactive sputtering, where only the general spoke characteristics were evaluated. A dual-image fast camera screening was utilised to capture plasma emission on 3″ Nb target in a reactive mixture of nitrogen and argon. Spoke characteristics were evaluated while overall pressure and supplied power was kept constant and the content of nitrogen in N2/Ar mixture was varied. The shape, velocity and spoke mode number were significantly affected by the higher content of N2 in the mixture. To distinguish between the effects of the modified target surface state and reactive gas present in the plasma volume on spokes experiments with compound NbN target were also performed. Surprisingly, no real differences of spoke behaviour between Nb and NbN targets were observed.

Studies and Research on the Behavior of Polyethylene when Electrofusion Welding Fittings to High Density Polyethylene Pipes

This aim of this paper is to perform a study on the way the material of the fittings that can be welded through eletrofusion on polyethylene pipes withstands. The process is observed by means of the thermal and fast cameras. Also we intend to analyze the way the assembly consisting of the polyethylene fitting and pipe behaves during welding. The stresses caused by the welding process are observed, as well as the concurrent welding of the tapping tee and the branch saddle tee.

Continuous-wave planar laser induced fluorescence with a fast camera.

We present planar, laser induced fluorescence (PLIF) measurements of the velocity-resolved distribution function of ions in a plasma using a modulated, narrow linewidth, continuous-wave laser. Plasma emission is acquired with a high frame rate camera, and the laser light is spread into a thin sheet so that an entire plane of the plasma is imaged at each interrogation wavelength. Fourier analysis is conducted on each pixel of the images to separate the modulated fluorescent emission from the background light. Argon ion temperatures and bulk flow maps are reported in a helicon plasma source, and standard single-point LIF measurements provide validation of the PLIF measurement.

Digital holographic microscopy evaluation of dynamic cell response to electroporation.

Phase-derived parameters and time autocorrelation functions were used to analyze the behavior of murine B16 cells exposed to different amplitudes of electroporation pulses. Cells were observed using an off-axis digital holographic microscope equipped with a fast camera. Series of quantitative phase images of cells were reconstructed and further processed using MATLAB codes. Projected area, dry mass density, and entropy proved to be predictors for permeabilized cells that swell or collapse. Autocorrelation functions of phase fluctuations in different regions of the cell showed a good correlation with the local effectiveness of permeabilization.

Optical tomography of the plasma on the PROTO-SPHERA experiment

PROTO-SPHERA is a magnetic confinement device for controlled fusion experiments which forms a plasma spherical torus not around a metal centerpost (as in spherical tokamaks), but around a plasma centerpost. Thanks to a vacuum vessel in polymethylmethacrylate (PMMA) transparent to visible light, a system of six fast visible light cameras has been implemented around the PROTO-SPHERA experiment so as to perform an optical tomography. To do so, synchronous triggered frames acquisition and exposure time control have been performed using the MARTe2 software framework. Also, a proper calibration/reconstruction of each line of sight for all the cameras has been implemented, so as to compute a 3D representation of the vessel. A 2D tomography inversion of the plasma column equatorial plane has been performed under the consideration that the value of each pixel represents the integrated brightness along its line of sight. The camera acquisition system, the experimental setup and the obtained results will be presented.

A novel energy resolved neutron imaging detector based on a time stamping optical camera for the CSNS

The China Spallation Neutron Source (CSNS) operates in pulsed mode and has a high neutron flux. This provides opportunities for energy resolved neutron imaging by using the TOF (Time Of Flight) approach. An Energy resolved neutron imaging instrument (ERNI) is being built at the CSNS but significant challenges for the detector persist because it simultaneously requires a spatial resolution of less than 100 μm, as well as a microsecond-scale timing resolution. This study constructs a prototype of an energy resolved neutron imaging detector based on the fast optical camera, TPX3Cam coupled with an image intensifier. To evaluate its performance, a series of proof of principle experiments were performed in the BL20 at the CSNS to measure the spatial resolution and the neutron wavelength spectrum, and perform neutron imaging with sliced wavelengths and Bragg edge imaging of the steel sample. A spatial resolution of 57 μm was obtained for neutron imaging by using the centroiding algorithm, the timing resolution was on the microsecond scale and the measured wavelength spectrum was identical to that measured by a beam monitor. In addition, any wavelengths can be selected for the neutron imaging of the given object, and the detector can be used for Bragg edge imaging. The results show that our detector has good performances and can satisfy the requirements of ERNI for detectors at the CSNS.

Novel imaging technique for α-particles using a fast optical camera

A new imaging technique for α-particles using a fast optical camera focused on a thin scintillator is presented. As α-particles interact in a thin layer of LYSO fast scintillator, they produce a localized flash of light. The light is collected with a lens to an intensified optical camera, Tpx3Cam, with single photon sensitivity and excellent spatial & temporal resolutions. The interactions of photons with the camera is reconstructed by means of a custom algorithm, capable of discriminating single photons using time and spatial information.

Filigree in the Surroundings of Polar Crown and High-Latitude Filaments

High-resolution observations of polar crown and high-latitude filaments are scarce. We present a unique sample of such filaments observed in high-resolution H$\alpha$ narrow-band filtergrams and broad-band images, which were obtained with a new fast camera system at the Vacuum Tower Telescope (VTT), Tenerife, Spain. The Chromospheric Telescope (ChroTel) provided full-disk context observations in H$\alpha$, Ca II K, and He I 10830 A. The Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provided line-of-sight magnetograms and ultraviolet (UV) 1700 A filtergrams, respectively. We study filigree in the vicinity of polar crown and high-latitude filaments and relate their locations to magnetic concentrations at the filaments' footpoints. Bright points are a well studied phenomenon in the photosphere at low latitudes, but they were not yet studied in the quiet network close to the poles. We examine size, area, and eccentricity of bright points and find that their morphology is very similar to their counterparts at lower latitudes, but their sizes and areas are larger. Bright points at the footpoints of polar crown filaments are preferentially located at stronger magnetic flux concentrations, which are related to bright regions at the border of supergranules as observed in UV filtergrams. Examining the evolution of bright points on three consecutive days reveals that their amount increases while the filament decays, which indicates they impact the equilibrium of the cool plasma contained in filaments.

Smart Artificial Markers for Accurate Visual Mapping and Localization.

Artificial marker mapping is a useful tool for fast camera localization estimation with a certain degree of accuracy in large indoor and outdoor environments. Nonetheless, the level of accuracy can still be enhanced to allow the creation of applications such as the new Visual Odometry and SLAM datasets, low-cost systems for robot detection and tracking, and pose estimation. In this work, we propose to improve the accuracy of map construction using artificial markers (mapping method) and camera localization within this map (localization method) by introducing a new type of artificial marker that we call the smart marker. A smart marker consists of a square fiducial planar marker and a pose measurement system (PMS) unit. With a set of smart markers distributed throughout the environment, the proposed mapping method estimates the markers’ poses from a set of calibrated images and orientation/distance measurements gathered from the PMS unit. After this, the proposed localization method can localize a monocular camera with the correct scale, directly benefiting from the improved accuracy of the mapping method. We conducted several experiments to evaluate the accuracy of the proposed methods. The results show that our approach decreases the Relative Positioning Error (RPE) by 85% in the mapping stage and Absolute Trajectory Error (ATE) by 50% for the camera localization stage in comparison with the state-of-the-art methods present in the literature.

Observation of asymmetric explosive density evolution in the deflagration-to-detonation transition for porous explosives

We report deflagration-to-detonation transition phenomena in cylindrical columns of porous explosives based on octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The HMX powder and LX-14 (95.5 wt. % HMX and 4.5 wt. % estane polyurethane) prills were observed with high-speed cameras and flash x rays under conditions that allow for precise scaling and registration of the recorded images. These experimental results test the conventional assumption of axisymmetric, effective one-dimensional burning. Polycarbonate confinement tubes allowed this direct imaging with steel spheres employed as radio-opaque fiducials. Spheres embedded in the explosive column revealed the displacement of the explosive prior to deflagration. X-ray and fast camera images show that a dense plug of compact explosive develops ahead of the deflagration front. The internal fiducials register both internal changes in the plug and changes in the position between successive x-ray images. Tantalum witness strips placed on the inner wall of the explosive channel revealed radial expansion of the tube bore at successive x-ray exposures, indicating the internal pressure of about 100 MPa just before detonation. Images indicate that the deflagration is not only asymmetric but also involves newly observed transient phenomena just before detonation. We typically see a brief but continuous increase in deflagration speed and the formation of a deflagration channel that bypasses the plug just prior to detonation. We hypothesize that these phenomena play an important role in the deflagration-to-detonation transition itself.

A novel algorithm to improve image reconstruction quality for 2D streak camera

The single shot compressed ultrafast photography (CUP) camera is the fastest camera available. This technique is based on random codes to encode and reconstruct the ultrafast dynamical scene by a compressed sensing algorithm. In this paper, a new 2D ultrafast camera with high reliability was introduced and a novel reconstruction algorithm named GAP-TV was proposed. Firstly, this camera was simulated with the TwIST and GAP-TV methods for a simple scene; the result showed that the GAP-TV method had better PSNR and SSIM performance than TwIST. Secondly, the optical propagation process data was performed to this proposed 2D camera, and the result showed that this novel method can reconstruct it well. In addition, the efficiency of the GAP-TV method is excellent than TwIST. Finally, an experiment was conducted to capture the transient event when a femtosecond laser pulse passed through a beam splitter. The result showed that the GAP-TV method can reconstruct the compressed 2D image well, and the CUP system can achieve 1011 frames per second. So, a novel 2D ultrafast camera with high temporal resolution (4 picosecond) was designed and verified. It has wide applications in inertial confined fusion, fluorescence lifetime imaging, and nuclear physics.

Modelling the influence of strain and strain rate on the thermal softening during dynamic loading of ductile metals

Although the combined effect of strain rate and temperature on the behaviour of metals is widely recognized, no universally accepted viewpoints are available about the physical phenomena. Experiments on a highly ductile A2-70 steel, performed at moderate dynamic rates (10 s-1) and different initial temperatures (20 to 150 °C), are firstly aimed here at assessing whether the thermal softening previously verified at static rates on the same steel is also suitable for describing now the mixed effect of dynamic rates and consequent variable temperatures, or further contributions to the thermal softening are necessary for describing such mixed effects. A general multiplicative model of the dynamic hardening is proposed, based on a static flow curve at room temperature to be increased by the dynamic amplification and to be decreased by the thermal softening, the latter incorporating the known “static component” depending on both strain and constant temperatures, together with a new “dynamic component” incorporating the dependence on the temperature variation and promoted by fast straining. The dynamic amplification of the stress is then obtained from another series of dynamic tests ran at initial room temperature and four nominal strain rates between 1 and 1800 s-1. The trend obtained is compatible with the seizing of the strain rate effect beyond necking onset, already found for other metals in previous works. All the experiments are based on the acquisition of the current load (by load cells for the testing machine and by strain gauges for the Hopkinson bar) and of the current cross section through optical diameter measurements by a fast camera; then, the effective current values of true stress-true strain-true strain rate are measured on a semi-local basis over the neck section at different instants during the test.

Development of Prototype Simplified Neutron Scatter Camera for Nuclear Safeguards Applications

Neutron scatter cameras are a type of directional neutron detectors that rely on multiple neutron scatters to generate images that can reveal the direction and distribution of neutron sources. Fast neutron cameras which have recently been developed rely on multiple detector volumes and make use of neutron time-of-flight measurements. These designs, though effective in localizing the source direction, relies on a large amount of detection and electrical equipment, thus increasing size, cost, and complexity of the systems to unreasonable levels for some applications. This project seeks to develop a compact scatter camera that is less expensive than systems relying on multiple detector volumes. Crucially, two components and capabilities are needed to achieve this: fast scintillation detection materials and picosecond electrical pulse timing. Utilizing such electronics, distinguishing between scintillation light pulses generated by the same neutron within one detector volume is possible. An MCNPX-PoliMi model of such a system has been developed to guide prototype designs. A cube of EJ-230 fast plastic scintillator and six photomultiplier tubes (PMTs) were used to construct the prototype camera that localizes neutron sources based on the principle of cone back projection. Prototypes of the system in one, two, and three dimensions have shown promising initial results when coupled with a script that algorithmically identifies candidate neutron double scatter events and back projects probability cones in the direction of possible sources. Imaging resolution/quality, double scatter efficiency, and cost for the system are quantified. Paths forward for further improvement of a future system based on this camera’ operating principles are discussed.

Recognition of two-phase flow pattern inside evaporator tubes

The convective heat exchange mechanism in two-phase regime, can allow high exchange coefficients, as long as particular conditions are respected, in terms of vapor quality. In evaporator tubes this problem is particularly important as a deviation from the optimal flow conditions can drastically drop the efficiency of the heat exchange. This work describes an innovative methodology for identifying the different two-phase flow regimes in evaporator pipes, through the processing of the vibrational data acquired on the pipe itself. An experimental apparatus with transparent pipes has been built up to recognize flow patterns by a fast image acquisition camera. Images have been associated to vibration data acquired on pipes by accelerometer. The analysis of frequency spectrum, led to first interesting results thus it is possible to distinguish between “no bubbles” regimes and different “bubbles” regimes. A wide test campaign has been realized on vertical and horizontal pipes, simulating steam bubbles with nitrogen bubbles, reproducing the main flow patterns. The paper demonstrates the possibility to design smart and non-intrusive sensor to be applied on evaporated tubes for the recognizing of anomalous thermal flow condition and sets the stage for future engineering work.

Fast Foveating Cameras for Dense Adaptive Resolution.

Traditional cameras field of view (FOV) and resolution predetermine computer vision algorithm performance. These trade-offs decide the range and performance in computer vision algorithms. We present a novel foveating camera whose viewpoint is dynamically modulated by a programmable micro-electromechanical (MEMS) mirror, resulting in a natively high-angular resolution wide-FOV camera capable of densely and simultaneously imaging multiple regions of interest in a scene. We present calibrations, novel MEMS control algorithms, a real-time prototype, and comparisons in remote eye-tracking performance against a traditional smartphone, where high-angular resolution and wide-FOV are necessary, but traditionally unavailable.

Design and Image Research of Tennis Line Examination Based on Machine Vision Analysis

In view of the intelligent demand of tennis line examination, this paper performs a systematic analysis on the intelligent recognition of tennis line examination. Then, a tennis line recognition method based on machine vision is proposed. In this paper, the color region of the image recognition region is divided based on the region growth, and the rough estimation of the court boundary is realized. In order to achieve the effect of camera calibration, a fast camera calibration method which can be used for a variety of court types is proposed. On the basis of camera calibration, a tennis line examination and segmentation system based on machine vision analysis is constructed, and the experimental results are verified by design experiments. The results show that the machine vision analysis-based intelligent segmentation system of tennis line examination has high recognition accuracy and can meet the actual needs of tennis line examination.

Optimizing of Experimental Load of PFZ-200 Plasma Focus

This article presents a study of neutron emission on the PFZ-200 plasma focus at the Department of physics on FEE CTU in Prague, Czech Republic. In order to achieve the highest and most stable neutron yields, the deuterium working gas pressure and the anode shape were systematically varied. We observed the plasma time to the pinch and the discharge current by the Rogowski coil and neutron emission by the silver activation detector and scintillation time-of-flight detectors. The imploded plasma was visualized using a fast X-ray pinhole camera with a gated microchannel plate detector. The experiment presents the z-pinch discharges with the current maximum above 200 kA and the average neutron yields of ${3 \times 10^{8}}$ neutrons/shot. Measured pinch times were in the range from 1.65 to 1.85 $\mu \text{s}$ . The hollow round anode configuration performed the most stable neutron yields with a deviation under 20%.

OnionNet: Single-View Depth Prediction and Camera Pose Estimation for Unlabeled Video

In real scenes, humans can easily infer their positions and distances from other objects with their own eyes. To make the robots have the same visual ability, this paper presents an unsupervised OnionNet framework, including LeafNet and ParachuteNet, for single-view depth prediction and camera pose estimation. In OnionNet, for speeding up OnionNet’s convergence and concretizing objects against the gradient locality and moving objects in videos, LeafNet adopts two decoders and enhanced upconvolution modules. Meanwhile, for improving the robustness of fast camera movement and rotation, ParachuteNet uses and integrates three pose networks to estimate multi-view camera pose parameters by combining with the modified image preprocess. Different from existing methods, single-view depth prediction and camera pose estimation are trained view-by-view, where the variations between views is gradual reduction of view range and outer pixels disappear in next view, similar to onion peeling. Moreover, the LeafNet is optimized with pose parameter from each pose network in turn. Experimental results on the KITTI dataset show the outstanding effectiveness of our method: single-view depth performs better than most supervised and unsupervised methods which contain two same subtasks, and pose estimation gets the state-of-the-art performance compared with existing methods under the comparable input settings.