Time-of-flight Range Research Articles

A plano-convex thick-lens velocity map imaging apparatus for direct, high resolution 3D momentum measurements of photoelectrons with ion time-of-flight coincidence.

Since their inception, velocity map imaging (VMI) techniques have received continued interest in their expansion from 2D to 3D momentum measurements through either reconstructive or direct methods. Recently, much work has been devoted to the latter of these by relating electron time-of-flight (TOF) to the third momentum component. The challenge is having a timing resolution sufficient to resolve the structure in the narrow (<10ns) electron TOF spread. Here, we build upon the work in VMI lens design and 3D VMI measurement by using a plano-convex thick-lens (PCTL) VMI in conjunction with an event-driven camera (TPX3CAM) providing TOF information for high resolution 3D electron momentum measurements. We perform simulations to show that, with the addition of a mesh electrode to the thick-lens geometry, the resulting plano-convex electrostatic field extends the detectable electron cutoff energy range while retaining the high resolution. This design also extends the electron TOF range, allowing for a better momentum resolution along this axis. We experimentally demonstrate these capabilities by examining above-threshold ionization in xenon, where the apparatus is shown to collect electrons of energy up to ∼7eV with a TOF spread of ∼30ns, both of which are improved compared to a previous work by factors of ∼1.4 and ∼3.75, respectively. Finally, the PCTL-VMI is equipped with a coincident ion TOF spectrometer, which is shown to effectively extract unique 3D momentum distributions for different ionic species in a gas mixture. These techniques have the potential to lend themselves to more advanced measurements involving systems where the electron momentum distributions possess non-trivial symmetries.

Demonstration of Neutron Phase Imaging Based on Talbot–Lau Interferometer at Compact Neutron Source RANS

Neutron imaging based on a compact Talbot–Lau interferometer was demonstrated using the RIKEN accelerator-driven compact neutron source (RANS). A compact Talbot–Lau interferometer consisting of gadolinium absorption gratings and a silicon phase grating was constructed and connected to the RANS. Because of pulsed thermal neutrons from the RANS and a position-sensitive detector equipped with time-of-flight (TOF) analysis, moiré interference patterns generated using the interferometer were extracted at a TOF range around the design wavelength (2.37 Å) optimal for the interferometer. Differential phase and scattering images of the metal rod samples were obtained through phase-stepping measurements with the interferometer. This demonstrates the feasibility of neutron phase imaging using a compact neutron facility and the potential for flexible and unique applications for nondestructive evaluation.

A Simple and Cost-Effective Physical Distancing Violation Detector Using a Rotating Time of Flight Lidar

In this work, a simple and cost-effective physical distancing violation detector using a commercial lidar has been developed. Our system comprises time of flight (ToF) lidar, mounted a stepper motor to rotate ToF Lidar and range an object on the top. We control a rotation of the stepper motor, record the distance between the object and the ToF Lidar by using a microcontroller, and analyze the measuring data using a computer program. This system can also indirectly estimate the distance between two objects by applying a simple vector operation. This paper successfully detects and evaluates the distance between two dummy objects placed with various configurations. We obtained the estimated distances using our proposed method nearly equal to the actual distances measured manually. In addition, our system has been tested to measure the physical distances among people with three volunteers who stood 200 cm and 80 cm distances in an indoor environment. The experiment results show that the distance between volunteer 1 and volunteer 2 is 186.5 cm and the distance between volunteer 2 and volunteer 3 is 73.0 cm. These indicate our system could provide information whether a safe distance or a risk distance. This research work can help the authorities provide an instrument for reducing contagious diseases, especially COVID-19 pandemic outbreaks, by installing at a fixed location or in portable instrument services.

Range-precision improvement of a time-of-flight range sensor using dual reference plane sampling.

This study aimed to achieve high range precision in the sub-100 µm order with time-of-flight (TOF) range imaging for 3-D scanners. The precision of a TOF range imager was improved using dual reference plane sampling (DRPS). DRPS using two short-pulse lasers reduces driver jitter, which limits the range precision below sub-100 µm. A proof-of-concept measurement system implemented using a TOF range imager demonstrated the reduction in driver jitters, resulting in reduced column-to-column variation in range precision. The developed system also achieved a high precision of 52 µm using a single frame and 27 µm using a 10-frame average.

Innovative Shoe-Integrated System Based on Time-of-Flight Range Sensors for Fall Detection on Various Terrains

This letter presents a shoe-integrated fall detection system that is based on seven time-of-flight (ToF) range sensors to identify falls on different terrains. The ToF range sensors, which measure the real-time distance between the sensors and the ground surface, can be used to detect simultaneously the variation of the gait cycle and the geometry of the terrain. Using the statistical features of the ToF range signals and an error-correcting output codes-based model, 17 experimental activities, executed by nine participants, are classified into three classes—activities of daily life, near falls, and falls. The experimental F1-scores for these three classes are 100, 98.17, and 99.07%, respectively. The arrangement of ToF range sensors is simplified to develop a compact shoe-integrated system architecture for fall detection.

How proton pulse characteristics influence protoacoustic determination of proton-beam range: simulation studies

The unique dose deposition of proton beams generates a distinctive thermoacoustic (protoacoustic) signal, which can be used to calculate the proton range. To identify the expected protoacoustic amplitude, frequency, and arrival time for different proton pulse characteristics encountered at hospital-based proton sources, the protoacoustic pressure emissions generated by 150 MeV, pencil-beam proton pulses were simulated in a homogeneous water medium. Proton pulses with Gaussian widths ranging up to 200 μs were considered. The protoacoustic amplitude, frequency, and time-of-flight (TOF) range accuracy were assessed. For TOF calculations, the acoustic pulse arrival time was determined based on multiple features of the wave.Based on the simulations, Gaussian proton pulses can be categorized as Dirac-delta-function-like (FWHM < 4 μs) and longer. For the δ-function-like irradiation, the protoacoustic spectrum peaks at 44.5 kHz and the systematic error in determining the Bragg peak range is <2.6 mm. For longer proton pulses, the spectrum shifts to lower frequencies, and the range calculation systematic error increases (⩽23 mm for FWHM of 56 μs). By mapping the protoacoustic peak arrival time to range with simulations, the residual error can be reduced. Using a proton pulse with FWHM = 2 μs results in a maximum signal-to-noise ratio per total dose. Simulations predict that a 300 nA, 150 MeV, FWHM = 4 μs Gaussian proton pulse (8.0 × 106 protons, 3.1 cGy dose at the Bragg peak) will generate a 146 mPa pressure wave at 5 cm beyond the Bragg peak.There is an angle dependent systematic error in the protoacoustic TOF range calculations. Placing detectors along the proton beam axis and beyond the Bragg peak minimizes this error. For clinical proton beams, protoacoustic detectors should be sensitive to <400 kHz (for −20 dB). Hospital-based synchrocyclotrons and cyclotrons are promising sources of proton pulses for generating clinically measurable protoacoustic emissions.

A Time-of-Flight CMOS Range Image Sensor Using 4-Tap Output Pixels with Lateral-Electric-Field Control

This paper presents a 160&#xD7;240-pixel CMOS Time-of-Flight (TOF) range imager with a 4-tap charge modulator using pinned-photodiode high-speed lock-in pixels in a 0.11&#x3BC;m CIS process. The proposed lock-in pixel structure using lateral electric field (LEF) control is suitable for implementing a multiple-tap charge modulator while achieving high-speed charge transfer for high time resolution. A CMOS TOF range imager with the multiple-tap charge modulators is expected to have background light cancelling capability in one frame and to improve range resolution with shifted short double light pulses. In this paper, we report the design, implementation and evaluation results of the TOF range imager using the 4-tap LEF modulator (LEFM). The TOF range imager has a range resolution of less than 3.2mm for the range from 1.0m to 2.0m at the frame rate of 15.7fps.

Calibration of time-of-flight cameras for accurate intraoperative surface reconstruction

In image-guided surgery (IGS) intraoperative image acquisition of tissue shape, motion, and morphology is one of the main challenges. Recently, time-of-flight (ToF) cameras have emerged as a new means for fast range image acquisition that can be used for multimodal registration of the patient anatomy during surgery. The major drawbacks of ToF cameras are systematic errors in the image acquisition technique that compromise the quality of the measured range images. In this paper, we propose a calibration concept that, for the first time, accounts for all known systematic errors affecting the quality of ToF range images. Laboratory and in vitro experiments assess its performance in the context of IGS. For calibration the camera-related error sources depending on the sensor, the sensor temperature and the set integration time are corrected first, followed by the scene-specific errors, which are modeled as function of the measured distance, the amplitude and the radial distance to the principal point of the camera. Accounting for the high accuracy demands in IGS, we use a custom-made calibration device to provide reference distance data, the cameras are calibrated too. To evaluate the mitigation of the error, the remaining residual error after ToF depth calibration was compared with that arising from using the manufacturer routines for several state-of-the-art ToF cameras. The accuracy of reconstructed ToF surfaces was investigated after multimodal registration with computed tomography (CT) data of liver models by assessment of the target registration error (TRE) of markers introduced in the livers. For the inspected distance range of up to 2 m, our calibration approach yielded a mean residual error to reference data ranging from 1.5±4.3 mm for the best camera to 7.2±11.0 mm. When compared to the data obtained from the manufacturer routines, the residual error was reduced by at least 78% from worst calibration result to most accurate manufacturer data. After registration of the CT data with the ToF data, the mean TRE ranged from 3.7±2.1 mm for point-based and 5.7±1.9 mm for surface-based registration for the best camera to 6.2±3.4 and 11.1±2.8 mm, respectively. Compared to data provided by the manufacturer, the mean TRE decreased by 8%-60% for point-based and by 18%-74% for surface-based registration. Using the proposed calibration approach improved the measurement accuracy of all investigated ToF cameras. Although evaluated in the context of intraoperative image acquisition, the proposed calibration procedure can easily be applied to other medical applications using ToF cameras, such as patient positioning or respiratory motion tracking in radiotherapy.

Pose-insensitive nose detection in TOF-scans

One of the fundamental benefits of 3D range data, as opposed to 2D images, is the pose invariance of 3D geometrical information. Time-of-flight (TOF) cameras provide such range data at a high temporal, but so far only at a medium spatial resolution. We present an algorithm to detect the tip of the nose in TOF range data, where the subjects’ head pose varies between −90° and 90° around the vertical axis (yaw) and between −30° and 30° in pitch rotation. The main idea is to rotate the head around a virtual head axis and extract the silhouettes for all rotations in 5° steps. To find the correct rotation and the tip of the nose, we compare these extracted silhouettes to an average head profile template, which was calculated from 200 high resolution laser-scans. In a test set of 120 TOF scans of 10 different persons, the detected nose tip was within 10 mm from the correct position in 89% and within 25 mm in 97.5% of the cases.

Image Sensing Technology and Their Applications: Empirical Verification of Range Resolution for a TOF Range Image Sensor with Periodical Charge Draining Operation Under Influence of Ambient Light

The range resolution of a time-of-flight (TOF) range image sensor with periodical charge-draining operation is analyzed under the influence of ambient light. The analysis explains the effectiveness of a small duty light pulse and a periodical charge-draining structure to reduce the influence of ambient light. A theoretical model of range resolution taking into consideration factors of the duty ratio, the ambient light component, and the diffusion component is presented. The theoretical model under the influence of ambient light well agrees with the experimental results. The offset-charge-canceling technique using two sub-frames effectively enables accurate range measurements under ambient light.

Robust Optical Time-of-Flight Range Imaging Based on Smart Pixel Structures

The reliable detection of the three-dimensional position of arbitrary objects in a scene is a key capability of most animals and one of the most important tasks in machine vision. Today's preferred technical solution is optical time-of-flight (TOF) range imaging, due to its simplicity, its distance resolution, its large and adaptable measurement range, as well as the absence of shadowing problems. In order significantly to extend the application areas of TOF 3-D cameras, in particular for outdoor use, we show how their performance can be improved in all relevant respects: background light suppression is improved by an order of magnitude by the minimum charge transfer method. Multicamera operation is achieved by a binary pseudo-noise modulation/demodulation technique. This method also avoids all practical ambiguity problems typically encountered with harmonic modulation. Higher temporal demodulation resolution becomes possible with a pixel structure employing lateral electric fields. We have realized such pixels with a commercially available CCD/CMOS process, and our measurement results confirm that gigahertz demodulation imaging is possible. The practicality of all theoretical concepts is demonstrated with a miniaturized TOF 3-D camera platform whose LED array light source is modulated at a typical rate of 20 MHz. Our work contributes, therefore, to opening up new application domains of the soaring optical TOF range imaging techniques.

A CMOS Time-of-Flight Range Image Sensor With Gates-on-Field-Oxide Structure

This paper presents a new type of CMOS time-of-flight (TOF) range image sensor using single-layer gates on field oxide structure for photo conversion and charge transfer. This simple structure allows the realization of a dense TOF range imaging array with 1515 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pixels in a standard CMOS process. Only an additional process step to create an n-type buried layer which is necessary for high-speed charge transfer is added to the fabrication process. The sensor operates based on time-delay dependent modulation of photocharge induced by back reflected infrared light pulses from an active illumination light source. To reduce the influence of background light, a small duty cycle light pulse is used and charge draining structures are included in the pixel. The TOF sensor chip fabricated measures a range resolution of 2.35 cm at 30 frames per second and an improvement to 0.74 cm at three frames per second with a pulsewidth of 100 ns.

Pseudonoise Optical Modulation for Real-Time 3-D Imaging With Minimum Interference

In optical time-of-flight (TOF) range imaging, harmonic intensity modulation of the illumination source is very common. By detecting the phase delay between emitted and reflected sinusoids, the distance can be measured accurately. However, this harmonic approach does not allow for the concurrent operation of several TOF range cameras because the arbitrary superposition of several differently parametrized sinusoids leads to a sinusoid with incorrect phase. To minimize inaccuracies by multi-camera interference (MCI), pseudonoise (PN) modulated intensity signals are employed for robust TOF range imaging. The time of flight is locally measured by correlating the incident light intensity with two time-shifted versions of the PN sequence, making use of smart demodulation pixels. We derive two fundamental expressions for the basic limitations of TOF measurements using PN sequences. Firstly, the precision of the distance measurement is limited by photon shot noise, and it essentially shows an inverse square root dependence of the number of detected photoelectrons. Secondly, MCI causes an inaccurate distance measurement given as the ratio of two sums. The denominator is the sum of two autocorrelation and two cross-correlation values; the nominator is the sum of one autocorrelation and one cross-correlation value. Due to the lack of a strict mathematical theory of correlation properties of m-sequences, an exhaustive numerical simulation was carried out to obtain expectation values of the distance measurement inaccuracy as a function of the sequence length and the number of interfering cameras. For experimental verification, an image sensor with 176 times 144 demodulation pixels was manufactured with a standard CMOS process offering a CCD option. Measurements taken with up to five concurrently operating sensors were in excellent agreement with our theoretical predictions concerning achievable distance accuracy. This confirms the aptness of PN techniques for multi-camera optical TOF range imaging.

Solid-state time-of-flight range camera

The concept of a real-time range camera without moving parts is described, based on the time-of-flight (TOF) principle. It operates with modulated visible and near-infrared radiation, which is detected and demodulated simultaneously by a 2-D array of lock-in pixels employing the charge-coupled device principle. Each pixel individually measures the amplitude, offset and phase of the received radiation. The theoretical resolution limit of this TOF range camera is derived, which depends on the square root of the detected background radiation and the inverse of the modulation amplitude. Actual measurements of 3-D sequences acquired at 10 range images per second show excellent agreement between our theory and the observed results. A range resolution of a few centimeters over a range of 10 m, with an illumination power of a few hundreds of milliwatts is obtained in laboratory scenes for noncooperative, diffusely reflecting objects.