Frame Exposure Time Research Articles

Parameters of the Lightning Attachment Processes in a Negative Cloud‐To‐Ground Stroke Observed on a Microsecond Timescale

AbstractWe present time‐correlated ultra‐high‐speed video camera and electromagnetic field measurements of the attachment processes in a natural negative cloud‐to‐ground stroke. The video camera frame exposure time and pixel resolution were 740 ns and 0.91 m/pixel, respectively. The common streamer zone (CSZ) was first observed 2.52 µs preceding the first frame showing the return stroke (RS) in progress, when the upward and downward leader‐tips were 9.8 m apart. In the next frame, the two leaders were observed to have propagated toward each other within the CSZ, with their tips being 0.91 m apart. Our observations show with unprecedented precision/clarity that (a) the slow front in the field waveform is associated with the CSZ, and (b) the “proper” start of the RS is marked by the onset of the fast transition in the field waveform which occurs at the completion of the attachment processes (when the upward and downward leaders have merged).

Foil explosion in megagauss magnetic fields: Non-uniform expansion and instabilities

This paper presents an analysis of the results of an experiment on the explosion of copper foils, which was carried out on the MIG generator that produced a current through the foil at a level of 2 MA with a current pulse rise time of 100 ns. The foil length (along the z axis) was 20 mm, width (along the x axis) 5 mm, and thickness (along the y axis) 250 μm. It was observed that when the foils were exploded in megagauss magnetic fields with the foil current flowing along the z axis, the expansion of the plasma was extremely nonisotropic. The expansion of the foil material along the x axis was suppressed, and it expanded only along the y axis. An analysis of the experimental data carried out using a radiation magnetohydrodynamic code has shown that the nonisotropy of the expansion was associated with an enhancement of the magnetic field at the narrow side of the foil. As a result of the field enhancement, the magnetic pressure at the narrow foil side significantly exceeded the magnetic pressure in the center of the foil surface. Using an HSFC Pro optical camera (providing a frame exposure time of 3 ns), large-scale instabilities with wavelengths of 0.2–0.5 mm were detected on the foil surface. In appearance, these instabilities were similar to those developing in an exploding rod. It was suggested that the most likely cause of the appearance of the plasma structure observed in the experiment was the growth of flute instabilities.

Acousto-optically driven lensless single-shot ultrafast optical imaging

Driven by many applications in a wide span of scientific fields, a myriad of advanced ultrafast imaging techniques have emerged in the last decade, featuring record-high imaging speeds above a trillion-frame-per-second with long sequence depths. Although bringing remarkable insights into various ultrafast phenomena, their application out of a laboratory environment is however limited in most cases, either by the cost, complexity of the operation or by heavy data processing. We then report a versatile single-shot imaging technique combining sequentially timed all-optical mapping photography (STAMP) with acousto-optics programmable dispersive filtering (AOPDF) and digital in-line holography (DIH). On the one hand, a high degree of simplicity is reached through the AOPDF, which enables full control over the acquisition parameters via an electrically driven phase and amplitude spectro-temporal tailoring of the imaging pulses. Here, contrary to most single-shot techniques, the frame rate, exposure time, and frame intensities can be independently adjusted in a wide range of pulse durations and chirp values without resorting to complex shaping stages, making the system remarkably agile and user-friendly. On the other hand, the use of DIH, which does not require any reference beam, allows to achieve an even higher technical simplicity by allowing its lensless operation but also for reconstructing the object on a wide depth of field, contrary to classical techniques that only provide images in a single plane. The imaging speed of the system as well as its flexibility are demonstrated by visualizing ultrashort events on both the picosecond and nanosecond timescales. The virtues and limitations as well as the potential improvements of this on-demand ultrafast imaging method are critically discussed.

Dual-Tap Computational Photography Image Sensor With Per-Pixel Pipelined Digital Memory for Intra-Frame Coded Multi-Exposure

A coded-exposure-pixel image sensor for computational imaging applications is presented. Each frame exposure time is divided into ${N}$ subframes. Within each subframe, each pixel sorts photo-generated charge into two charge taps depending on that pixel’s 1-bit binary code. ${N}$ global updates of arbitrary pixel-wise codes are implemented in each frame to enable ${N}$ short global pixel-specific subexposures within one frame. To make these subexposures global, two latches per pixel are utilized in a pipelined fashion. The code for the next subframe is loaded into latch 1 in a row parallel fashion, while the code for the current subframe is being applied by latch 2 globally for photo-generated charge sorting during the current subexposure. A $280^{H}\times 176^{V}$ image sensor prototype with $11.2{\text{-}}\mu \text{m}$ pixel pitch has been fabricated in a $0.11{\text{-}}\mu \text{m}$ CMOS image sensor (CIS) technology. The image sensor has been demonstrated in two computational photography applications, each using only a single frame of a video: 1) computing both albedo (a measure of reflectivity) and 3-D depth maps by means of structured-light imaging and 2) computing surface normals (3-D orientations) map by means of photometric stereo imaging. These demonstrations experimentally validate some of the unique capabilities of this computational image sensor, such as accurate 3-D visual scene reconstruction using only one camera, while maintaining its native specifications: the full spatial resolution and the maximum frame rate.

Delay in plasma generation on the surface of coper and duralumin conductors coated with dielectrics

The skin explosion was investigated for homogeneous and double-layer cylindrical copper and duralumin conductors coated with zirconium dioxide. The MIG generator used in the experiment produced a current of amplitude up to 2.5 MA and rise time 100 ns. To detect the plasma generated on the surface of a conductor due to its electrical explosion, the light emitted by the plasma was imaged using a four-frame optical camera with a frame exposure time of 3 ns. In addition, vacuum x–ray diodes were used to detect the instant at which the temperature of the surface plasma, assumed to be a blackbody, became greater than 1 eV. The internal structure and density of the surface plasma were examined based on its transmission images. The images were taken by exposing the plasma to the x-radiation with hν > 1 keV generated by an X-pinch for 2–3 ns. The QUINTA facility was used to carry out plasma-assisted deposition of multilayer coatings composed of alternating 0.8-μm-thick zirconium dioxide layers and (more durable) 0.2-μm-thick zirconium layers. The total number of layers in a coating could be up to sixteen. At magnetic fields lower than 300 T, the onset of plasma generation on the dielectric-coated load part occurred with a delay of up to 70 ns relative to that on a bare copper or duralumin conductor. Increasing the field to 350–400 T decreased the delay in plasma generation to about 30 ns.

Delay in plasma generation on copper and duralumin conductors coated with titanium or zirconium

Skin explosion investigations have been performed for homogeneous and double-layer cylindrical conductors made of copper and duralumin and coated with titanium or zirconium. Titanium and zirconium have close values of electrical conductivity and sublimation energy density, whereas the ionic mass of zirconium is twice that of titanium. The experiment was carried out on the MIG high current generator at a current of amplitude up to 2.5 MA and rise time 100 ns. The plasma generated on the conductor surface was imaged using a four-frame optical camera with a frame exposure time of 3 ns. The internal structure of the surface plasma was examined using transmission images of the plasma exposed during 2-3 ns to the x radiation with hv>0.8 keV generated by an X-pinch. It has been demonstrated that the time delay to the plasma formation on the surface of double-layer copper and duralumin conductors with a deposited zirconium or titanium layer is comparable for Zr and Ti (with the conductivity ratio equal to 26-27 for a homogeneous copper conductor and a deposited layer and to 8.2-8.5 for a homogeneous duralumin conductor and a deposited layer), and it is shorter for double-layer duralumin conductors than for double-layer copper conductors. In a magnetic field lower than 300 T, plasma formation on the surface of a double-layer copper conductor started about 400 ns later than on the surface of a homogeneous copper conductor and 250 ns later than on the surface of a duralumin conductor. When the field was increased to 350-400 T, the delay time to plasma formation decreased to about 35 ns. For double-layer conductors, the difference in expansion velocities between titanium and zirconium surface plasmas was insignificant.

Gantry rotational motion-induced blur in cone-beam computed tomography.

As neuro-endovascular image-guided interventions (EIGIs) make use of higher resolution detectors, gantry rotational motion-induced blur becomes more noticeable in acquired projections as well as reconstructed images by reducing the visibility of vascular and device features whose visualization could be critical in the treatment of vascular pathology. Motion-induced blur in projections views is a function of an object's position in the field-of-view (FOV), gantry rotational speed, and frame capture or exposure time. In this work different frame rates were used to investigate the effects of blurring from individual projections on the reconstructed image. To test the effects of these parameters on reconstructed images, a regular pattern phantom of small objects was simulated and projection views were generated at various different frame rates for a given simulated rotational velocity. The reconstruction was made using a linear interpolation of filtered backprojections. Images reconstructed from lower frame rates showed significant blurring in the azimuthal direction, increasingly worse towards the periphery of the image. However, those reconstructed from higher frame rates showed significantly less blur throughout the entire FOV. While lower frame rates could be used with slower gantry speeds this would increase the risk of voluntary or involuntary patient motion contributing to blur over the entire FOV. A high frame rate used with high gantry speeds could reliable provide images without gantry-motion blur while reducing the risk of patient-motion blur. Frame rates exceeding 2000 fps available with photon counting detectors such as the X-counter Actaeon1 are available.

Selection of the tagged photons by off axis heterodyne holography in ultrasound-modulated optical tomography

Ultrasound-modulated optical tomography (UOT) is a technique that images optical contrast deep inside scattering media. Heterodyne holography is a promising tool that is able to detect UOT-tagged photons with high efficiency. In this work, we describe theoretically the detection of the tagged photon in heterodyne holography-based UOT, show how to filter the untagged photon, and discuss the effect of shot noise. The discussion also considers speckle decorrelation. We show that optimal detection sensitivity can be reached, if the frame exposure time of the camera used to perform the holographic detection is on the order of the decorrelation time.

Paper: Expert Viewers' Preferences for Higher Frame Rate 3D Film

Recently the movie industry has been advocating the use of frame rates significantly higher than the traditional 24 frames per second. This higher frame rate theoretically improves the quality of motion portrayed in movies, and helps avoid motion blur, judder and other undesirable artifacts. Previously we reported that young adult audiences showed a clear preference for higher frame rates, particularly when contrasting 24 fps with 48 or 60 fps. We found little impact of shutter angle (frame exposure time) on viewers' choices. In the current study we replicated this experiment with an audience composed of imaging professionals who work in the film and display industry who assess image quality as an aspect of their everyday occupation. These viewers were also on average older and thus could be expected to have attachments to the "film look" both through experience and training. We used stereoscopic 3D content, filmed and projected at multiple frame rates (24, 48 and 60 fps), with shutter angles ranging from 90° to 358°, to evaluate viewer preferences. In paired-comparison experiments we assessed preferences along a set of five attributes (realism, motion smoothness, blur/clarity, quality of depth and overall preference). As with the young adults in the earlier study, the expert viewers showed a clear preference for higher frame rates, particularly when contrasting 24 fps with 48 or 60 fps. We found little impact of shutter angle on viewers' choices, with the exception of one clip at 48 fps where there was a preference for larger shutter angle. However, this preference was found for the most dynamic "warrior" clip in the experts but in the slower moving "picnic" clip for the naïve viewers. These data confirm the advantages afforded by high-frame rate capture and presentation in a cinema context in both naïve audiences and experienced film professionals. © 2016 Society for Imaging Science and Technology.

Single-frame electro-optical cameras with nanosecond exposures

This paper presents the results of developing a family of single-frame electro-optical cameras with nanosecond exposures. The devices thus developed cover a broad spectral range—from near-IR radiation to hard x-rays. The technical characteristics of the cameras are presented, and the results of measuring their parameters are given. The design features of the devices and possible ways to use them are given. A block diagram of the cameras is discussed, and a method is proposed for measuring the frame-exposure time.

Development of fast video recording of plasma interaction with a lithium limiter on T-11M tokamak

A new high-speed color camera with interference filters was installed for fast video recording of plasma-surface interaction with а Lithium limiter on the base of capillary-porous system (CPS) in T-11M tokamak vessel. The paper presents the results of the study of tokamak plasma interaction (frame exposure time up to 4μs) with CPS Lithium limiter in a stable stationary phase, unstable regimes with internal disruption and results of processing of the image of the light emission around the probe, i.e. e-folding length for neutral Lithium penetration and e-folding length for Lithium ion flux in SOL region.

Sub-μs time resolution in wide-field time-correlated single photon counting microscopy obtained from the photon event phosphor decay

Fast frame rate complementary metal–oxide–semiconductor cameras in combination with photon counting image intensifiers can be used for microsecond resolution wide-field fluorescence lifetime imaging with single photon sensitivity, but the time resolution is limited by the camera exposure time. We show here how the image intensifier's P20 phosphor afterglow can be exploited for accurate timing of photon arrival well below the camera exposure time. By taking ratios of the intensity of the photon events in two subsequent frames, photon arrival times were determined with 300 ns precision with 18.5 μs frame exposure time (54 kHz camera frame rate). Decays of ruthenium and iridium-containing compounds with around 1 μs lifetimes were mapped with this technique, including in living HeLa cells, using excitation powers below 0.5 μW. Details of the implementation to calculate the arrival time from the photon event intensity ratio are discussed, and we speculate that by using an image intensifier with a faster phosphor decay to match a higher camera frame rate, photon arrival time measurements on the nanosecond time scale could be possible.

Wide-field time-correlated single photon counting (TCSPC) microscopy with time resolution below the frame exposure time

Fast frame rate CMOS cameras in combination with photon counting intensifiers can be used for fluorescence imaging with single photon sensitivity at kHz frame rates. We show here how the phosphor decay of the image intensifier can be exploited for accurate timing of photon arrival well below the camera exposure time. This is achieved by taking ratios of the intensity of the photon events in two subsequent frames, and effectively allows wide-field TCSPC. This technique was used for measuring decays of ruthenium compound Ru(dpp) with lifetimes as low as 1μs with 18.5μs frame exposure time, including in living HeLa cells, using around 0.1μW excitation power. We speculate that by using an image intensifier with a faster phosphor decay to match a higher camera frame rate, photon arrival time measurements on the nanosecond time scale could well be possible.

Camera-Based Lock-in and Heterodyne Carrierographic Photoluminescence Imaging of Crystalline Silicon Wafers

Carrierographic (spectrally gated photoluminescence) imaging of a crystalline silicon wafer using an InGaAs camera and two spread super-bandgap illumination laser beams is introduced in both low-frequency lock-in and high-frequency heterodyne modes. Lock-in carrierographic images of the wafer up to 400 Hz modulation frequency are presented. To overcome the frame rate and exposure time limitations of the camera, a heterodyne method is employed for high-frequency carrierographic imaging which results in high-resolution near-subsurface information. The feasibility of the method is guaranteed by the typical superlinearity behavior of photoluminescence, which allows one to construct a slow enough beat frequency component from nonlinear mixing of two high frequencies. Intensity-scan measurements were carried out with a conventional single-element InGaAs detector photocarrier radiometry system, and the nonlinearity exponent of the wafer was found to be around 1.7. Heterodyne images of the wafer up to 4 kHz have been obtained and qualitatively analyzed. With the help of the complementary lock-in and heterodyne modes, camera-based carrierographic imaging in a wide frequency range has been realized for fundamental research and industrial applications toward in-line nondestructive testing of semiconductor materials and devices.

基于大功率LED的闪光成像速度测量系统

A non-contacted speed-measurement system for high speed moving objects is designed.During one single frame exposure time of CCD,the flashlight flares to a moving object twice and the two corresponding images of the object are captured by CCD in sequence.The movement speed of the object can be determined from the displacement of the images versus their interval time.This system has the advantage of real-time measurement,thus it breaks through the limitation of single point measurement technology in space.Moreover,it can acquire not only the spacial information of one point at different moments,but also the information of different space points at the same moment.

Depth-resolved holographic optical coherence imaging using a high-sensitivity photorefractive polymer device

We present coherence-gated holographic imaging using a highly sensitive photorefractive (PR) polymer composite as the recording medium. Due to the high sensitivity of the composite holographic recording at intensities as low as 5 mW/cm2 allowed for a frame exposure time of only 500ms. Motivated by regenerative medical applications, we demonstrate optical depth sectioning of a polymer foam for use as a cell culture matrix. An axial resolution of 18 μm and a transverse resolution of 30 μm up to a depth of 600 μm was obtained using an off-axis recording geometry.

The particle velocity field inside a two-dimensional aerated hopper

A two-dimensional lab-scale hopper with a flat bottom used for aeration and transparent glass front wall was used to visualize the flow pattern of fine particles (FCC particles of 64 μm and glass beads of 147 μm) near the hopper outlet at different airflow rates. Particle traces near the hopper outlet are recorded with a digital video camera and analyzed with a semiautomated image analysis procedure to yield the particle velocity field. This procedure has been verified and set up with a preliminary analysis in which the effects of several variables and in particular the frame exposure time have been addressed. Results indicate that the quality of the solids flow changes considerably with the aeration rate. At zero or low gas rates, the flow is of the funnel flow type with the formation of a well-defined channel around the hopper axis which is of the same size of the outlet. The solids on the sides are stationary. This channel widens with increasing aeration rate and solids away from the hopper axis also move. Results have been discussed and compared with pressure measurements carried out in previous work [3rd World Congress on Particle Technology (1998)].

Vacuum arc cathode spot parameters from high-resolution luminosity measurements

Cathode spots on arc-cleaned copper and molybdenum electrodes in vacuum were studied by fast image converter framing and streak camera photography with high temporal and spatial resolution. The frame exposure time was 20 ns and the interframe period was 200 ns; the streak sweep time was between 200 ns and 1 μs. Spatial structures with a resolution of 5 μm could be determined by observing the spot movement with a small slit at the streak camera and a high sweep speed. Strong fluctuations of the light emission of the spot were found with characteristic times of 50–100 ns. When the spot moved out of the slit field of view a stepwise decrease occurred in the measured light, indicating an internal substructure of the spot with distances between fragments less than 10 μm and even smaller fragment diameters. The current per fragment was estimated to be 20–40 A. The frames confirm the short time constants of the spot. From frame to frame a spot motion was observed in most cases, yielding spot residence times <200 ns.

An Adaptive (Tracking) Procedure for Measuring Visual Search

PEST, an adaptive (tracking) procedure originally developed for sensory research, was modified for cognitive studies with the multiple sequential frames paradigm and a letter-search task. The program which controlled stimulus presentation was designed to track the frame duration required for successful search on 75% of trials (the duration threshold). For each of the three subjects used, the threshold increased as a linear function of the number of sequentially presented frames, which varied between 1 and 10. Each tracking run was immediately followed by 40 trials with frame-exposure time fixed at the computer-selected threshold. Performance on these fixed level trials was close to the expected 75% correct. The Discussion addresses issues related to the use of the threshold measure in basic and applied cognitive research, explores the present findings about performance on fixed level trials at the computer-selected threshold, and examines the increase in duration threshold with the number of sequentially presented frames.

Image-Smearing Errors in Framing Camera Photographs

In framing camera studies of self-luminous gas discharge columns where the boundaries move appreciably during the exposure time of a single frame, image smearing results. The effective exposure thus produced is derived and provides an estimate of timing errors to be ascribed to dimensions measured from the cine films. From the assumption of logarithmic dependence of image density on exposure, it is found that the timing error ΔT is normally only a small fraction of the frame exposure time for good quality images. In the particular case where the intensity and spectral composition of the emitted light are constant, ΔT is also constant and simply results in a shift in the time origin. A detailed analysis is presented for the Barr and Stroud CP5 framing camera, and the end result is quoted for the Beckman and Whitley camera Model 189.