Low-speed Camera Research Articles

Crystal nucleation and growth kinetics of acoustically levitated liquid SCN-DC transparent alloys

As an important and promising experimental method for simulating the containerless state in outer space, acoustic levitation provides excellent contact-free condition to investigate solidification process. Meanwhile, the radiation pressure and acoustic streaming caused by nonlinear effects bring various kinds of novel phenomena to crystallization kinetics. In this work, high-speed CCD, low-speed camera and infrared thermal imager were used simultaneously to observe the crystallization process of acoustically levitated SCN-DC transparent alloys. The undercooling ability and solidification process of alloy droplets with different aspect ratios were explored under acoustic levitation state. For hypoeutectic SCN-10wt%DC, eutectic SCN-23.6wt%DC and hypereutectic SCN-40wt%DC alloys, the experimental maximum undercoolings reached 22.5(0.07<i>T</i><sub>L</sub>), 16(0.05<i>T</i><sub>E</sub>) and 32.5K(0.1<i>T</i><sub>L</sub>) respectively and the corresponding crystal growth velocities were 27.91, 0.21 and 0.45 mm/s. In SCN-10wt%DC hypoeutectic alloy, the nucleation mode of SCN dendrite changed from edge nucleation to random nucleation with the increase of undercooling. For SCN-23.6wt%DC eutectic alloy, when undercooling exceeded 12.6K, DC dendrite preferentially nucleated and grew, and then the (SCN+DC) eutectic grew attached to DC dendrite. Moreover, the growth interface of DC dendrite gradually changed from sharp to smooth within SCN-40wt%DC hypereutectic alloy as the undercooling degree rose. The undercooling distribution curve and nucleation probability variation trend were analyzed versus aspect ratio. It was found that as the aspect ratio increased, undercooling of alloy droplet increased firstly, then decreased, and finally remained almost unchanged. Further analysis showed that with the increase of aspect ratio, the cooling rate would rise and thus enhanced the undercooling. However, the increase in surface nucleation rate and the droplet oscillation inhibited deep undercooling of alloy droplet. Therefore, the coupled effects of cooling rate, surface nucleation rate and droplet oscillation determined the undercooling of the alloy. In the case of SCN-40wt%DC hypereutectic alloy, the acoustic streaming and surface oscillation arising from acoustic field were the principal factors intensifying surface nucleation.

Full-field measurements of high-frequency micro-vibration under operational conditions using sub-Nyquist-rate 3D-DIC and compressed sensing with order analysis

Full-field measurements of high-frequency micro-vibration under operational conditions using sub-Nyquist-rate 3D-DIC and compressed sensing with order analysis

Deep-learning-enabled temporally super-resolved multiplexed fringe projection profilometry: high-speed kHz 3D imaging with low-speed camera

AbstractRecent advances in imaging sensors and digital light projection technology have facilitated rapid progress in 3D optical sensing, enabling 3D surfaces of complex-shaped objects to be captured with high resolution and accuracy. Nevertheless, due to the inherent synchronous pattern projection and image acquisition mechanism, the temporal resolution of conventional structured light or fringe projection profilometry (FPP) based 3D imaging methods is still limited to the native detector frame rates. In this work, we demonstrate a new 3D imaging method, termed deep-learning-enabled multiplexed FPP (DLMFPP), that allows to achieve high-resolution and high-speed 3D imaging at near-one-order of magnitude-higher 3D frame rate with conventional low-speed cameras. By encoding temporal information in one multiplexed fringe pattern, DLMFPP harnesses deep neural networks embedded with Fourier transform, phase-shifting and ensemble learning to decompose the pattern and analyze separate fringes, furnishing a high signal-to-noise ratio and a ready-to-implement solution over conventional computational imaging techniques. We demonstrate this method by measuring different types of transient scenes, including rotating fan blades and bullet fired from a toy gun, at kHz using cameras of around 100 Hz. Experiential results establish that DLMFPP allows slow-scan cameras with their known advantages in terms of cost and spatial resolution to be used for high-speed 3D imaging tasks.

Sensor synchronized DIC: A robust approach to linear and nonlinear modal analysis using low frame rate cameras

Sensor synchronized DIC: A robust approach to linear and nonlinear modal analysis using low frame rate cameras

Temporal compressive holographic microscopy based on alternating deep denoisers and total variation prior

We present temporal compressive holographic microscopy to achieve high-speed phase imaging with a low-speed camera. In this method, alternating deep denoising network and total variation denoiser are combined into a plug-and-play framework to reconstruct ten high-speed holographic frames from a single-shot measurement. Therefore, we can observe a 500 frame per second (fps) holographic scene when the camera works at 50 fps. This technique provides an effective tool to detect dynamic phase change scenes.

Snapshot compressive structured illumination microscopy.

We propose a snapshot compressive structured illumination microscopy (SoSIM) system to increase the number of reconstructed resolution-enhanced (RE) images per second and reduce the data bandwidth by capturing compressed measurements. In this system, multiple low-resolution images are encoded by a high-speed digital micro-mirror device with random binary masks. These images are then captured by a low-speed camera as a snapshot compressed measurement. Following this, we adopt an efficient deep neural network to reconstruct nine images with different structured illumination patterns from a single measurement. The reconstructed images are then combined into a single-frame RE image using the method of spectral synthesis in the frequency domain. When the camera operates at 100 frames per second (fps), we can eventually recover dynamic RE videos at the same speed with 100 fps.

Reinforcement Learning for Adaptive Video Compressive Sensing

We apply reinforcement learning to video compressive sensing to adapt the compression ratio. Specifically, video snapshot compressive imaging (SCI), which captures high-speed video using a low-speed camera is considered in this work, in which multiple ( B ) video frames can be reconstructed from a snapshot measurement. One research gap in previous studies is how to adapt B in the video SCI system for different scenes. In this article, we fill this gap utilizing reinforcement learning (RL). An RL model, as well as various convolutional neural networks for reconstruction, are learned to achieve adaptive sensing of video SCI systems. Furthermore, the performance of an object detection network using directly the video SCI measurements without reconstruction is also used to perform RL-based adaptive video compressive sensing. Our proposed adaptive SCI method can thus be implemented in low cost and real time. Our work takes the technology one step further towards real applications of video SCI.

Coded aperture compression temporal imaging based on a dual-mask and deep denoiser.

Coded aperture compressive temporal imaging (CACTI) is the mapping of multiple frames using different encoding patterns into a single measurement and then using an algorithm to reconstruct the required high-dimensional signals, thus enabling high-speed photography on low-speed cameras. An encoding pattern and a reconstruction algorithm both play a critical role for CACTI. To improve the quality of the reconstruction, in terms of encoding, we took advantage of the reflective properties of the digital micromirror device and used a complementary dual-mask pattern to obtain more projection information. In terms of decoding, we developed what we believe, to the best of our knowledge, is a new model combining the weighted Landweber regularization with the relaxation strategy and a deep denoiser. The experimental results show the superiority of our proposed encoding-decoding combination, which achieves better performance in terms of the peak SNR, structural similarity index measure, and visual effects.

Vibration mode identification method for structures using image correlation and compressed sensing

All objects are subject to deformations of their shapes that correspond to their natural vibration modes. The analysis of these modes is extremely useful for structural design and structural health monitoring. Traditionally, expensive measurement equipment and many vibration sensors have been required to analyze vibration modes. Recently, a method that measures displacement from images (digital image correlation) has enabled the identification of vibration modes with a single camera. However, the identification of high-frequency vibration modes requires a high-speed camera, which increases costs and reduces spatial resolution. In this study, a low-cost and highly accurate method for identifying vibration modes and frequencies, randomized single-exposure sampling (RSES), was developed by applying compressed sensing to images captured with a strobe flash and a low-speed camera. This method extracts vibration modes using proper orthogonal decomposition and applies compressed sensing to the time function to recover high-speed vibrations from low-speed measurement results. The performance of RSES was evaluated by conducting vibration experiments on beams and comparing the results with the theoretical values using acceleration sensor measurements as boundary conditions. The results demonstrated that images captured at a shutter speed of 10 fps could measure the vibration modes and amplitudes of beams vibrating at a single frequency (ranging 170–3210 Hz). From 40 images, the amplitude of vibration could be measured with errors of less than 1 %, 2 %, and 26 % for vibrational frequencies of 170, 1130, and 3210 Hz, respectively. The larger error at 3210 Hz was attributed to the amplitude of vibration being only a few micrometers owing to the shaker limitations, a value close to the resolution of digital image correlation. This method can be used to accurately analyze structures that vibrate at high speeds, which is useful for vibration control design and other applications.

Snapshot temporal compressive light-sheet fluorescence microscopy via deep denoising and total variation priors.

We present a snapshot temporal compressive light-sheet fluorescence microscopy system to capture high-speed microscopic scenes with a low-speed camera. A deep denoising network and total variation denoiser are incorporated into a plug-and-play framework to quickly reconstruct 20 high-speed video frames from a short-time measurement. Specifically, we can observe 1,000-frames-per-second (fps) microscopic scenes when the camera works at 50 fps to capture the measurement. The proposed method can potentially be applied to observe cell and tissue motions in thick living biological specimens.

A Dual DIC System for Analysis of Dynamic Mechanical Properties of Large Sandstone under Uniaxial Compression Load

In this paper, an experiment is carried out to acquire the dynamic mechanical properties of a simulated sandstone tunnel by a dual DIC system. The sandstone tunnel is simulated by large sandstone with a prefabricated hole in the center. The speckle size required by DIC system was evaluated, and the results showed that for large specimens a marker pen could be used to spot speckles and make sure that the diameters of speckle points in an image should be ranged from three to five pixels. The dual DIC system is composed of a low-speed camera and a high-speed camera. The low-speed camera is used to record the speckle patterns of the sandstone in one side during the whole process of compression load, and the high-speed camera is placed in the other side to record speckle patterns for 11.5 seconds before and after failure. It is realized that monitoring whole process of deformation and instantaneous failure in two directions is required. Measurement results are effectively analyzed. The results are shown as follows: At the initial stage of loading the sandstone is in an elastic stage without macroscopic cracks. With the increase in compression load the sandstone has several small stress releases and several obvious macroscopic cracks. In the final stage of loading, the distribution of normal stress and shear stress are almost the same, and cracks are subjected to the coupling effect of normal stress and shear stress. The two ends of the prefabricated hole perpendicular to the applied load direction are prone to cracks parallel to the applied load direction.

A combined velocity and temperature measurement with an LED and a low-speed camera

Microfluidic devices are governed by three-dimensional velocity and temperature fields, and their boundary conditions are often unknown. Therefore, a measurement technique is often desired to measure both fields in a volume. With astigmatism particle tracking velocimetry (APTV) combined with luminescence lifetime imaging, the temperature and all velocity components in a volume can be measured with one optical access. While the three-dimensional particle position is determined by evaluating the shape of the corresponding particle image, the temperature measurement relies on estimating the temperature-dependent luminescence lifetime derived from particle images on two subsequent image captures shortly after the photoexcitation. For this, typically a high-energetic pulsed laser is required to ensure a high signal-to-noise ratio. However, it can also cause additional heating of the fluid. We show that this problem is solved by replacing the pulsed laser with an LED. To compensate for the lower power provided by the LED, we adapted the timing schedule and vastly extended the illumination time and the exposure time for both image captures. In addition, we were able to replace the typically used high-speed camera with an ordinary double-frame camera. In this way, very low measurement uncertainties on all measured quantities can be achieved while keeping the temperature of the fluid unaffected. Random errors dominate within the two focal planes of APTV, yielding a standard deviation of the temperature of individual particles of about 1 only. The measurement error caused by the movement of tracer particles during the much longer illumination and exposure time were found to be acceptable when the measured velocity is low. With the circumvention of light-source induced heating and the lower cost of hardware devices, the adapted approach is a suitable measurement technique for microfluidic related research.

Vibration Mode Identification Method for Structures Using Image Correlation and Compressed Sensing

All objects have deformed shapes (vibration modes) that tend to vibrate, and measuring these modes is extremely useful in structural design and failure diagnosis. Traditionally, expensive measurement equipment and many vibration sensors were commonly used to measure vibration modes, but a method that measures displacement from images (Digital Image Correlation) has made it possible to identify vibration modes with a single camera. However, the identification of high-frequency vibration modes requires a high-speed camera, which increases the cost and reduces the resolution. In this study, a low-cost and highly accurate method for identifying vibration modes and frequencies (RSES: Randomized Single Exposure Sampling) was developed by applying compressed sensing to images taken with a strobe flash and a low-speed camera. This method successfully identified vibration modes and frequencies of structures vibrating at frequencies more than 300 times the shooting speed.

Vision vibration measurement based on coded illumination in a single frame

A vision-based mechanical vibration measurement method is presented and verified by experiments in this paper. The coded illumination is projected on the objects by a digital light processing projector with a digital micromirror device in it. The projection patterns are designed to be concentric. In one integration time of the camera, the projector is exposed several times, which embeds temporal information in the images. A single frame can be divided into subframes by separating the coded concentric patterns. The centroids of the coded concentric patterns are fitted, and the centroids are treated as virtual feature points with vibration information. The acquisition devices are common low-speed cameras, and they record the vibration whose frequency exceeds the camera frame rate. The temporal resolution is increased by 10 times, corresponding to the 400 Hz sampling frequency in the experiment. We can measure the vibration of multiple points with different sampling frequencies. The frequency measurement accuracy is in the subhertz level in low-frequency measurement, relative error is always slightly greater than 0.01 in high-frequency measurement, and the amplitude resolution is 130 µm.

Rotary LED Transmitter for Improving Data Transmission Rate of Image Sensor Communication

This study focuses on image sensor communication (ISC), which is a form of visible light communication, using a light-emitting diode (LED) and a camera as the transmitting and receiving device, respectively. In an ISC system, the receiver captures optical signals transmitted by blinking LEDs as an image and demodulates the data using the captured image. The ISC system can spatially separate signals and noises in the image, thus providing superior anti-interference ability. However, because the speed of ISC depends on the shooting speed of the camera, the receiver suffers from a low data transmission rate when a low-speed camera is used. To improve the data rate of ISC, we have developed a rotary LED transmitter. This transmitter cylindrically rotates the blinking LEDs during the exposure time of the camera. The camera captures multiple blinking states of LEDs as afterimages in a single image, thereby increasing the amount of data received per image. In this paper, we propose an ISC system using a rotary LED transmitter and present an experiment for the evaluation of the data rate. The result indicates that the data rate of the rotary LED transmitter is 60 times that of the conventional method.

Full-field motion and deformation measurement of high speed rotation based on temporal phase-locking and 3D-DIC

• A systematic solution for full-field 3D motion and deformation of high speed rotation object is presented, which combines high-speed rotation image acquisition using low-speed camera with 3D digital image correlation, both low-cost, flexible and accurate. • The process of phase-locked signal generation is explained, which is used to trigger the camera in a “phase-stepping” way so as to allow low-speed camera to achieve the entire circle images acquisition of high-speed rotation asynchronously. • A layer-by-layer matching method based on closest point is proposed to search uncoded targets between two rotated images, futherly the 3D motion parameters can be obtained after 3D reconstruction and singular value decomposition. • A binocular rotation correlating strategy is proposed to correlate the speckle subsets between a referece image and a rotated image, which is accurate and robust for the decorrelation problem in DIC. • In addition to the 3D motion and deformation measurement of high-speed rotating objects, our method can also be used for full-field 3D deformation measurement of high-speed vibration, and online fault diagnosis under various cyclic alternating load conditions, etc. The full-field and non-contact measurement of three-dimensional motion and deformation of high-speed rotating machinery is a very challenging task in the engineering field. Two issues, the cost of high-speed camera and the decorrelation between rotated images, are hard to avoid for investigators. In this paper, a systematic solution is presented, which combines high-speed rotation image acquisition using low-speed camera with 3D digital image correlation, both low-cost, flexible and accurate. Firstly, the frequency and initial phase of high-speed rotation are calculated by FFT, and temporal phase-locked signal is generated to trigger the dual-camera asynchronously to collect the circumferential images with equal phase interval. Then a layer-by-layer matching method is proposed to search uncoded targets between two rotated images, while a binocular rotation correlating strategy to correlate the speckle subsets. Finally, the corresponded uncoded targets and speckle subsets are reconstructed in three dimension via the camera calibration results, furtherly the 3D motion and deformation fields can be obtained. Five experiments, including two measurement repeatability tests, an accuracy test, a rigid motion measurement of rotating table and a dynamic deformation measurement of ventilation fan, prove the accuracy and effectiveness of the proposed method.

Stereo-DIC Measurements of a Vibrating Bladed Disk: In-Depth Analysis of Full-Field Deformed Shapes

Vibration measurements of turbomachinery components are of utmost importance to characterize the dynamic behavior of rotating machines, thus preventing undesired operating conditions. Local techniques such as strain gauges or laser Doppler vibrometers are usually adopted to collect vibration data. However, these approaches provide single-point and generally 1D measurements. The present work proposes an optical technique, which uses two low-speed cameras, a multimedia projector, and three-dimensional digital image correlation (3D-DIC) to provide full-field measurements of a bladed disk undergoing harmonic response analysis (i.e., pure sinusoidal excitation) in the kHz range. The proposed approach exploits a downsampling strategy to overcome the limitations introduced by low-speed cameras. The developed experimental setup was used to measure the response of a bladed disk subjected to an excitation frequency above 6 kHz, providing a deep insight in the deformed shapes, in terms of amplitude and phase distributions, which could not be feasible with single-point sensors. Results demonstrated the system’s effectiveness in measuring amplitudes of few microns, also evidencing blade mistuning effects. A deeper insight into the deformed shape analysis was provided by considering the phase maps on the entire blisk geometry, and phase variation lines were observed on the blades for high excitation frequency.

Low-speed cameras system for 3D-DIC vibration measurements in the kHz range

The digital image correlation (DIC) was used in this paper to obtain full-field measurements of a target vibrating at a frequency higher than the maximum cameras’ frame rate. The down-sampling technique was implemented to compensate for the cameras’ moderate frame rate, thus getting an accurate displacement acquisition even at 6.5 kHz. Two innovative methods to support the DIC application were introduced. The use of fringe projection (or structured light), initially applied on the sample at rest, reduced the effort and time required for the stereo matching task’s solution and improved this setting’s accuracy and reliability. Additionally, a new time-domain image filtering was proposed and tested to improve the quality of the obtained DIC maps. In combination with the down-sampling, the effect of this filtering technique was tested in this work at (approx.) 2500 and 6500 Hz by measuring the response of a bladed disk to sinusoidal excitation. Evidence of improved results was observed for both frequencies for amplitudes in the range of 10 µm.

Snapshot temporal compressive microscopy using an iterative algorithm with untrained neural networks.

We report a snapshot temporal compressive microscopy imaging system, using the idea of video compressive sensing, to capture high-speed microscopic scenes with a low-speed camera. An untrained deep neural network is used in our iterative inversion algorithm to reconstruct 20 high-speed video frames from a single compressed measurement. Specifically, using a camera working at 50 frames per second (fps) to capture the measurement, we can recover videos at 1000 fps. Our deep neural network is embedded in the inversion algorithm, and its parameters are learned simultaneously with the reconstruction.

Concentration-based velocity reconstruction in convective Hele\u2013Shaw flows

We examine the process of convective dissolution in a Hele–Shaw cell. We consider a one-sided configuration and we propose a newly developed method to reconstruct the velocity field from concentration measurements. The great advantage of this Concentration-based Velocity Reconstruction (CVR) method consists of providing both concentration and velocity fields with a single snapshot of the experiment recorded in high resolution. We benchmark our method vis–à–vis against numerical simulations in the instance of Darcy flows, and we also include dispersive effects to the reconstruction process of non-Darcy flows. The absence of laser sources and the presence of one low-speed camera make this method a safe, accurate, and cost-effective alternative to classical PIV/PTV velocimetry processes. Finally, as an example of possible application, we employ the CVR method to analyse the tip splitting phenomena.Graphic abstract