Gain-modulated three-dimensional active imaging with depth-independent depth accuracy

Three-dimensional super-resolution range-gated imaging based on Gaussian-range-intensity model

Current range-gated imaging methods based on triangular or trapezoidal models face significant limitations in terms of the accuracy and precision of three-dimensional (3D) reconstruction. Here, we establish a range-gated 3D super-resolution image reconstruction method based on a Gaussian-range-intensity model. A denoised optimization method for image preprocessing is established to improve the distance accuracy of the reconstruction. The experimental results show that the model can reduce the error of target distance by 64% and possess the reconstruction of multiple kinds of targets with a distance resolution of 1.5 cm and lower noise, realizing better reconstruction results compared with the existing range-gated 3D imaging techniques.

Range resolution modeling for 3D gated range-intensity correlation imaging based on a statistical theory

This paper systematically establishes a range resolution model for 3D gated range-intensity correlation imaging (GRICI) based on the law of error propagation and statistical theory, and especially takes the high-repetition frequency characteristic of 3D GRICI into consideration. The model can theoretically guide the setting of the GRICI system parameters to obtain a higher range resolution compared with existing modeling methods. This paper also verifies the correctness of the proposed model through simulation and experiments, and quantitatively analyzes the influence of the accumulated pulse number in a single frame. In addition, the range resolution for our 3D GRICI system is measured under the guidance of the proposed model, and it reaches the millimeter order.

Polarisation-modulated photon-counting 3D imaging based on a negative parabolic pulse model

Indirect methods based on intensity for time-of-flight measurement have attracted considerable research interest in recent years because they can provide high spatial resolution in 3D imaging. However, the majority of indirect methods are inapplicable when echo signals are small (e.g., less than one photon). We propose a novel polarisation-modulated photon-counting 3D imaging method based on a negative parabolic pulse model (NPPM) to solve this problem. We measure weak signals using the number of received photons after repetitive pulsed laser emission. We establish a computational method by exploring the relationship between photon flight time that corresponds to the polarisation-modulated state of photons controlled by phase shift and calculated photon rates from received photon-counting values based on Poisson negative log-likelihood function to calculate the distance. We specifically utilise the NPPM to estimate distribution of echo signals and reduce ranging error given that echo signals are constantly time-varying. We build the first experimental system for polarisation-modulated photon-counting 3D imaging for verification by integrating it with a dual-axis galvo scanning device. Experimental results demonstrate that the proposed method can achieve ranging accuracy at the millimeter-level and exhibit superior 3D imaging performance even when the average received number of echo signals per pulsed laser emission is smaller than 0.05.

Compressive Sensing Based Three-Dimensional Imaging Method with Electro-Optic Modulation for Nonscanning Laser Radar

Low-cost Laser Detection and Ranging (LiDAR) is crucial to three-dimensional (3D) imaging in applications such as remote sensing, target detection, and machine vision. In conventional nonscanning time-of-flight (TOF) LiDAR, the intensity map is obtained by a detector array and the depth map is measured in the time domain which requires costly sensors and short laser pulses. To overcome such limitations, this paper presents a nonscanning 3D laser imaging method that combines compressive sensing (CS) techniques and electro-optic modulation. In this novel scheme, electro-optic modulation is applied to map the range information into the intensity of echo pulses symmetrically and the measurements of pattern projection with symmetrical structure are received by the low bandwidth detector. The 3D imaging can be extracted from two gain modulated images that are recovered by solving underdetermined inverse problems. An integrated regularization model is proposed for the recovery problems and the minimization functional model is solved by a proposed algorithm applying the alternating direction method of multiplier (ADMM) technique. The simulation results on various subrates for 3D imaging indicate that our proposed method is feasible and achieves performance improvement over conventional methods in systems with hardware limitations. This novel method will be highly valuable for practical applications with advantages of low cost and flexible structure at wavelengths beyond visible spectrum.

Distance determination based on the delay time-intensity profile analysis in range-gated imaging

A method for distance determination with the help of range-gated viewing systems is proposed for an arbitrary shape of the illumination pulse. The method is based on finding the maximum of the reflected pulse energy as a function of the delay time. At the equal pulse and gate durations, there is a strict maximum, which turns into a plateau when the pulse is shorter than the gate duration. The delay times corresponding to the strict maximum or the far boundary of the plateau are directly related to the distance to an object. These results are confirmed by the numerical study of the dependence of the detected energy on the delay time for the different pulse shapes. Experimental verification of the proposed method showed accuracy 1-2 m in a measuring range of 15-120 m. The obtained results can be useful in the development of 3D vision systems.

High resolution flash three-dimensional LIDAR systems based on polarization modulation

Two high resolution flash LIDAR systems based on polarization modulation are demonstrated in this paper. One utilizes a polarization beam splitter (PBS) and two charge coupled device (CCD) arrays; the other utilizes a micro-polarizer array and a CCD array. Compared with the traditional flash LIDAR systems, the main advantage of the presented flash LIDAR systems is replacing high bandwidth detectors with a polarization modulator and low bandwidth detectors. The polarization modulator is used to modulate the polarization state of the received laser pulse in time and the range information can be mapped into the intensity received by the low bandwidth detectors. The distance between the target and the system can be derived from intensity images that can be read out using low bandwidth detectors. Comparisons of the two systems indicate that they have the same range precision. Both demonstrated systems can achieve a high range precision of several millimeters. The system using a PBS and two CCD arrays can obtain higher resolution images, but it is crucial to align the two CCD arrays precisely.

Electro-optic modulation methods in range-gated active imaging

A time-resolved imaging method based on electro-optic modulation is proposed in this paper. To implement range resolution, two kinds of polarization-modulated methods are designed, and high spatial and range resolution can be achieved by the active imaging system. In the system, with polarization beam splitting the incident light is split into two parts, one of which is modulated with cos(2) function and the other is modulated with sin(2) function. Afterward, a depth map can be obtained from two simultaneously received images by dual electron multiplying charge-coupled devices. Furthermore, an intensity image can also be obtained from the two images. Comparisons of the two polarization-modulated methods indicate that range accuracy will be promoted when the polarized light is modulated before beam splitting.

Triangular-range-intensity profile spatial-correlation method for 3D super-resolution range-gated imaging

We present a triangular-range-intensity profile (RIP) spatial-correlation method for 3D range-gated imaging with a depth of super resolution. In this method, spatial sampling volumes with triangular-RIPs are established by matching laser-pulse width and sensor gate time, and then depth information collapsed in gate images can be reconstructed by spatial correlation of overlapped gate images corresponding to sampling volumes. Compared with super-resolution depth mapping under trapezoidal-RIPs, range accuracy and precision are improved, and a large range fluctuation due to noise disturbance is smoothed by noise suppression under triangular-RIPs. In this paper, a proof experiment is demonstrated with a range precision 2.5 times better than that obtained under trapezoidal-RIPs.

Depth fingerprint map of active range-gated imaging for real-time three-dimensional measurement of foreground objects

We present a depth fingerprint map to obtain three-dimensional (3D) spatial information from objects in a large deterministic background by active range-gated imaging, for night remote surveillance. This method first gives the depth fingerprint map of the region of interest in the background by gate viewing in the form of contour bands in range. The map is then embedded in the range-gated laser surveillance system. Finally, 3D spatial information such as target scale and location can be estimated by segmenting the target from the background and matching them with the depth fingerprint map. The measurement is performed by computer background processing. Therefore, the method has no influence on the frame rate of surveillance systems and can realize real-time surveillance. In this paper, the approach to acquisition of the depth fingerprint map is also demonstrated without an echo-broadening effect.

Overcoming the shot-noise limitation of three-dimensional active imaging

Depth resolution is limited by the photoelectron shot noise in conventional gain-modulated active three-dimensional (3D) imaging methods. A proposed method, which is based on photon intensity correlation, is presented to overcome the depth resolution limitation. The signal photons are amplified by an imaging intensifier, and are then divided into two beams by a beam splitter. The theory shows that the shot-noise limitation is broken using the strong intensity coherence between the two beams. The experiment results show that the depth resolution of the correlated active 3D imaging method is three times better than that of the shot-noise limitation.