Long-range three-dimensional active imaging with superresolution depth mapping

Range-Intensity-Profile-Guided Gated Light Ranging and Imaging Based on a Convolutional Neural Network

Three-dimensional (3D) range-gated imaging can obtain high spatial resolution intensity images as well as pixel-wise depth information. Several algorithms have been developed to recover depth from gated images such as the range-intensity correlation algorithm and deep-learning-based algorithm. The traditional range-intensity correlation algorithm requires specific range-intensity profiles, which are hard to generate, while the existing deep-learning-based algorithm requires large number of real-scene training data. In this work, we propose a method of range-intensity-profile-guided gated light ranging and imaging to recover depth from gated images based on a convolutional neural network. In this method, the range-intensity profile (RIP) of a given gated light ranging and imaging system is obtained to generate synthetic training data from Grand Theft Auto V for our range-intensity ratio and semantic network (RIRS-net). The RIRS-net is mainly trained on synthetic data and fine-tuned with RIP data. The network learns both semantic depth cues and range-intensity depth cues in the synthetic data, and learns accurate range-intensity depth cues in the RIP data. In the evaluation experiments on both a real-scene and synthetic test dataset, our method shows a better result compared to other algorithms.

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.

Active imaging through dense fog by utilizing the joint polarization defogging and denoising optimization based on range-gated detection

Imaging through the scattering medium, such as fog, is important for military and civilian applications. However, the fog concentration restricts the current defogging methods; the image will be seriously degraded in dense fog scenes. Here, an imaging technique by developing joint active polarization defogging and denoising optimization methods based on range-gated detection is proposed for the target in fog conditions. The range-gated imaging method shields the scattering light from outside the selected region to improve the signal intensity. The properties of signal light, backscattering light, and forward scattering light in the range-gated imaging way are analyzed experimentally and theoretically. Thus the elimination method of backscattering light is developed in terms of polarization differences in the degree of polarization and angle of polarization, and the block-matching with 3D transform-domain collaborative filtering (BM3D) algorithm is developed to remove the effect of the forward scattering light on the image. By adopting the proposed defogging method, the clear imaging of the target under fog with an optical thickness of up to 5 is realized, and the target contour and detail information are successfully recovered. Compared with the complete failure of the current defogging method, this method can recover targets with high contrast and signal-to-noise ratio in dense fog scenes, which exhibits widespread application potential for target detection and recognition in severe weather and turbid underwater environment.

Long range 3D imaging through atmospheric obscurants using array-based single-photon LiDAR

Single-photon light detection and ranging (LiDAR) has emerged as a strong candidate technology for active imaging applications. In particular, the single-photon sensitivity and picosecond timing resolution permits high-precision three-dimensional (3D) imaging capability through atmospheric obscurants including fog, haze and smoke. Here we demonstrate an array-based single-photon LiDAR system, which is capable of performing 3D imaging in atmospheric obscurant over long ranges. By adopting the optical optimization of system and the photon-efficient imaging algorithm, we acquire depth and intensity images through dense fog equivalent to 2.74 attenuation lengths at distances of 13.4 km and 20.0 km. Furthermore, we demonstrate real-time 3D imaging for moving targets at 20 frames per second in mist weather conditions over 10.5 km. The results indicate great potential for practical applications of vehicle navigation and target recognition in challenging weather.

A boundary migration model for imaging within volumetric scattering media

Effectively imaging within volumetric scattering media is of great importance and challenging especially in macroscopic applications. Recent works have demonstrated the ability to image through scattering media or within the weak volumetric scattering media using spatial distribution or temporal characteristics of the scattered field. Here, we focus on imaging Lambertian objects embedded in highly scattering media, where signal photons are dramatically attenuated during propagation and highly coupled with background photons. We address these challenges by providing a time-to-space boundary migration model (BMM) of the scattered field to convert the scattered measurements in spectral form to the scene information in the temporal domain using all of the optical signals. The experiments are conducted under two typical scattering scenarios: 2D and 3D Lambertian objects embedded in the polyethylene foam and the fog, which demonstrate the effectiveness of the proposed algorithm. It outperforms related works including time gating in terms of reconstruction precision and scattering strength. Even though the proportion of signal photons is only 0.75%, Lambertian objects located at more than 25 transport mean free paths (TMFPs), corresponding to the round-trip scattering length of more than 50 TMFPs, can be reconstructed. Also, the proposed method provides low reconstruction complexity and millisecond-scale runtime, which significantly benefits its application.

Imaging in scattering media is challenging due to signal attenuation and strong coupling of scattered and signal photons. The authors present a boundary migration model of the scattered field, converting scattered measurements in spectral form to scene information in temporal domain, and image Lambertian objects in highly scattering media.

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.

Extended Risley scanning system with 30 × 360 coverage

We propose a beam scanning system based on an extended Risley prism structure. This system uses a rotating mirror structure and expands the original construction of the balanced and stable Risley prism. There is much research on extended Risley prism structures, but the scanned angle of their extended systems is less than 180°. The proposed system in this paper can expand scanned angle to 360°, showing practical significance and application value in the field of beam scanning. This is achieved by using reflectors and multiple transmitter and receiver structures with optimized positions and transmission directions. Simulation results show that the scanning range is 360° and ±14^∘ in the horizontal and vertical directions, respectively, at 110 m while the angular resolution is 0.2^∘×2^∘.

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.

Binning-based local-threshold filtering for enhancement of underwater 3D gated range-intensity correlation imaging

3D gated range-intensity correlation imaging (GRICI) can reconstruct a 3D scene with high range resolution in real time. However, in the applications of underwater range-gated imaging, targets with low reflectivity or at a far distance typically have a low signal-to-noise ratio (SNR), especially in turbid water. Usually, a global threshold is set to suppress noise in gated images, which easily results in data holes in the degraded depth map reconstructed by 3D GRICI. To solve the problem, we have proposed a binning-based local-threshold filtering (BLF) algorithm to fill depth data holes. Firstly, raw gated images are added to obtain a sum image, and global threshold filtering and pixel binning for the sum image are used to obtain a reference image. Secondly, hole pixels can be registered according to the reference image. Finally, a smaller local threshold is reset for the hole pixels, and then a repaired depth map is given by 3D GRICI. The experimental results have shown that the proposed method can effectively fill depth data holes and the peak signal-to-noise ratio of the repaired depth map is increased from 10.23 dB to 22.45 dB by suppressing water noise with the improved range resolution from 6.25 mm to 4.12 mm. In addition, the 3D depth of viewing is enlarged, and the limit detection distance is increased by 21% in experiment. The research can promote the practical applications of 3D GRICI.

Compact long-range single-photon imager with dynamic imaging capability

Single-photon light detection and ranging (LiDAR) has emerged as a strong candidate technology for active imaging applications. Benefiting from the single-photon sensitivity in detection, long-range active imaging can be realized with a low-power laser and a small-aperture transceiver. However, existing kilometer-range active imagers are bulky and have a long data acquisition time. Here we present a compact co-axial single-photon LiDAR system for kilometer-range 3D imaging. A fiber-based transceiver with a 2.5 cm effective aperture was employed to realize a robust and compact architecture, while a tailored temporal filtering approach guaranteed the high signal-to-noise level. Moreover, a micro-electro-mechanical system scanning mirror was adopted to achieve fast beam scanning. In experiment, high-resolution 3D images of different targets up to 12.8 km were acquired to demonstrate the long-range imaging capability. Furthermore, it exhibits the ability to achieve dynamic imaging at five frames per second over a distance of ∼1km. The results indicate potential in a variety of applications such as remote sensing and long-range target detection.

Range-intensity-profile prior dehazing method for underwater range-gated imaging

This paper is concerned with the mitigation of backscatter effects in a single gated image. A range-intensity-profile prior dehazing method is proposed to estimate scene depth and finely remove water backscatter at different depths for underwater range-gated imaging. It is based on the prior that the target intensity is distributed with range intensity profiles in gated images. The depth transmission and depth-noise map are then calculated from the scene depth. A high-quality image is restored by subtracting the depth-noise map and dividing the depth transmission. The simulation and experimental results show that the proposed method works well even if a portion of the estimated depth may be smaller than its real value, and the peak signal-to-noise ratio of dehazing images gets up to a doubled increase.

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.

Underwater 3D deblurring-gated range-intensity correlation imaging

Three-dimensional (3D) range-gated imaging has great potential in underwater target detection, navigation, and marine scientific research due to good backscatter suppression. However, in turbid water, apparent backscatter leads to bad range resolution and accuracy in 3D reconstruction. To solve this problem, a 3D deblurring-gated range-intensity correlation imaging method is proposed based on light propagation property in water. In the method, only the water attenuation coefficient and a reference image are needed to calculate the depth-noise maps (DNM) of target gate images at different ranges. By subtracting the DNMs from target gate images, new gate images with less noise can be obtained, and then 3D images with high range resolution and accuracy are reconstructed. To prove the feasibility of the proposed method, experiments have been performed in pools under different water conditions. The results show that a higher peak signal-to-noise ratio improvement is about 9 dB in new gated images.

Intracavity image upconversion system with fast and flexible electro-optic image gating based on polarization-frustrated phase-matching for range-gated applications

We report on a new image gating mechanism for intracavity nonlinear image upconversion systems that uses sum-frequency mixing of an external infrared image and a pump laser beam. Fast and flexible time duration gating of the upconverted image is achieved through transient electro-optic frustration of the phase-matching condition in a nonlinear crystal placed inside the cavity of the pump beam. The phase-matching condition is controlled by altering the polarization state of the laser cavity beam without interrupting laser oscillation, using a Pockels cell in one arm of an L-folded standing-wave resonator. In this way, an external image shutter mechanism is added to an image upconverter system that allows for using low shutter-speed EMCCDs (Electron Multiplying CCD) in range-gated imaging systems across the whole IR and potentially in the THz range.

Bayesian reconstruction method for underwater 3D range-gated imaging enhancement

We investigate a systematic improvement for 3D range-gated imaging in scattering environments. Drawbacks including absorption, ambient light, and scattering effect are studied. The former two are compensated through parameter estimation and preprocessing. With regard to the scattering effect, we propose a new 3D reconfiguration algorithm using a Bayesian approach that incorporates spatial constraints through a general Gaussian Markov random field. The model takes both scene depth and albedo into account, which provides a more informative and accurate restoration result. Hyper-parameters for the statistical mechanism are evaluated adaptively in the procedure and an iterated conditional mode optimization algorithm is employed to find an optimum solution. The performance of our method was assessed via conducting various experiments and the results also indicate that the proposed method is helpful for restoring the 2D image of a scene with improved visibility.

Polarization-modulated three-dimensional imaging using a large-aperture electro-optic modulator

To implement high-resolution and low-light sensitive three-dimensional (3D) imaging for long-range applications while simplifying data collection and reducing collection time, a polarization-modulated 3D imaging structure, using a large-aperture electro-optic modulator (EOM) and electron-multiplying CCD (EMCCD), is proposed in this paper. As the EMCCD camera itself has no ability of time resolution and high-speed gating due to the time integration mechanism, large-aperture EOMs are used to provide time resolution and high-speed shutter simultaneously for the EMCCD cameras to obtain the polarization-modulated images from which a 3D image can be reconstructed. A narrow field of view was designed to match the divergence of laser beam for long-range imaging, and therefore through the receiver, the incident angle on the EOM would still be limited to within a small angle, which would not degrade the modulation performance significantly during electro-optic modulation. Ultimately, we found that the polarization-modulated 3D imaging lidar showed very promising performance on time-resolved imaging in a field of view of 0.9 mrad.

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.

Restoration of longitudinal laser tomography target image from inhomogeneous medium degradation under common conditions

In order to overcome the shortages of the target image restoration method for longitudinal laser tomography using self-calibration, a more general restoration method through backscattering medium images associated with prior parameters is developed for common conditions. The system parameters are extracted from pre-calibration, and the LIDAR ratio is estimated according to the medium types. Assisted by these prior parameters, the degradation caused by inhomogeneous turbid media can be established with the backscattering medium images, which can further be used for removal of the interferences of turbid media. The results of simulations and experiments demonstrate that the proposed image restoration method can effectively eliminate the inhomogeneous interferences of turbid media and achieve exactly the reflectivity distribution of targets behind inhomogeneous turbid media. Furthermore, the restoration method can work beyond the limitation of the previous method that only works well under the conditions of localized turbid attenuations and some types of targets with fairly uniform reflectivity distributions.

Single-shot coherent noise suppression by spatial interferometric heterodyning

Coherent noise affects the information content in active imaging systems. Here we show that noise reduction can be accomplished using the intrinsic coherence properties of the electromagnetic fields. We demonstrate numerically and experimentally that a single-shot measurement using interferometric spatial heterodyning detection in conjuncture with computational image processing, permits suppressing coherent noise in conditions of low signal-to-noise ratios, through spectral upshifting of coherent information.

Reconstruction of target image from inhomogeneous degradations through backscattering medium images using self-calibration

Target images recorded with range-gated laser imaging systems and conventional passive imaging systems through rapidly changing turbid mediums inevitably suffer from inhomogeneous degradations. Consequently, this makes the images partly or entirely different from their true targets and eventually has adverse effects on target identification. To date, the inhomogeneous degradations are still not finely eliminable despite utilizing adaptive optical methods and pure mathematical signal improvement techniques. Herein, we demonstrate an image restoration method involving intrinsic physical evolution of light beams based on the backscattering images of a turbid medium. The corresponding mathematical signal processing algorithms are applied for restoring the true target images in the presence of rapidly changing inhomogeneous degradations. This technique would benefit target imaging through moving cloud/mist in air and flowing muddy masses under water.

Lower-upper-threshold correlation for underwater range-gated imaging self-adaptive enhancement

In underwater range-gated imaging (URGI), enhancement of low-brightness and low-contrast images is critical for human observation. Traditional histogram equalizations over-enhance images, with the result of details being lost. To compress over-enhancement, a lower-upper-threshold correlation method is proposed for underwater range-gated imaging self-adaptive enhancement based on double-plateau histogram equalization. The lower threshold determines image details and compresses over-enhancement. It is correlated with the upper threshold. First, the upper threshold is updated by searching for the local maximum in real time, and then the lower threshold is calculated by the upper threshold and the number of nonzero units selected from a filtered histogram. With this method, the backgrounds of underwater images are constrained with enhanced details. Finally, the proof experiments are performed. Peak signal-to-noise-ratio, variance, contrast, and human visual properties are used to evaluate the objective quality of the global and regions of interest images. The evaluation results demonstrate that the proposed method adaptively selects the proper upper and lower thresholds under different conditions. The proposed method contributes to URGI with effective image enhancement for human eyes.

Image restoration method for longitudinal laser tomography based on degradation matrix estimation

Target images captured by longitudinal laser tomography are usually degraded by nonuniform laser beams transmitting through inhomogeneous scattering mediums. An image restoration method with a total variation model is proposed for eliminating the main influence of inhomogeneous scattering mediums from degraded target images. Based on the physical signal relevance between the target layer and the scattering medium layer, the degradation matrix of the target image is approximately estimated by the specified backscattering images of the scattering mediums. Simulations and experiments are performed to verify the validity and feasibility of the proposed method, and all the results demonstrate that the proposed model works well and helps us to achieve the real target images, which represent the reflectivity distributions of the targets standing behind the inhomogeneous scattering mediums and which will benefit target recognition and identification.

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.

Multi-pulse time delay integration method for flexible 3D super-resolution range-gated imaging

Constructing flexible regular-shaped range-intensity profiles by the convolution of illuminator laser pulse and sensor gate pulse is crucial for 3D super-resolution range-gated imaging. However, ns-scale rectangular-shaped laser pulse with tunable pulse width is difficult to be obtained, especially for pulsed solid-stated lasers. In this paper we propose a multi-pulse time delay integration (MPTDI) method to reshape range-intensity profiles (RIP) free from the above limitation of pulsed lasers. An equivalent laser pulse temporal shaping model is established to evaluate and optimize the MPTDI method. By using MPTDI, the RIP shape and depth of viewing can both be flexibly changed as desired. Here typical triangular and trapezoidal RIPs are established for 3D imaging under triangular and trapezoidal range-intensity correlation algorithms. In addition, a prototype experiment is demonstrated to prove the feasibility of MPTDI.

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.

Gated viewing laser imaging with compressive sensing

We present a prototype of gated viewing laser imaging with compressive sensing (GVLICS). By a new framework named compressive sensing, it is possible for us to perform laser imaging using a single-pixel detector where the transverse spatial resolution is obtained. Moreover, combining compressive sensing with gated viewing, the three-dimensional (3D) scene can be reconstructed by the time-slicing technique. The simulations are accomplished to evaluate the characteristics of the proposed GVLICS prototype. Qualitative analysis of Lissajous-type eye-pattern figures indicates that the range accuracy of the reconstructed 3D images is affected by the sampling rate, the image's noise, and the complexity of the scenes.

Image coding for three-dimensional range-gated imaging

In the present paper we discuss the method of image coding by multiple exposure of range-gated images. This method enlarges the depth mapping range of range-gated imaging systems exponentially with the number of utilized images. We developed a theoretical model to give a precise prediction of the number of permutations that can be used for image coding. For what we believe is the first time, we realized an image coding sequence for three range-gated images to enlarge the depth mapping range by a factor of 12. We demonstrate three-dimensional imaging in a range of 460 to 1000 m using a laser pulse width of 300 ns. Because of the impact of noise, a critical linking error occurs during the encoding of the intensity images. It is possible to reduce this error by the application of effective noise reduction strategies and the use of a threshold value to the tolerance drift of intensity levels.

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.

Three-dimensional active imaging with maximum depth range

In traditional three-dimensional (3D) active imaging methods, the detection depth range is observed to increase linearly with the detection time, and the intensity information was not fully utilized. However, by encoding the relative values into pseudovalues, the intensity information was fully utilized, and we found the maximum detection depth range increases exponentially with the detection time. Furthermore, we present a 3D imaging system capable of exponentially expanding the detection depth range. A 3D scene reconstruction was undertaken with the targets placed at a distance of 600-1100 m. Experimental results indicate that the method expands the detection depth range exponentially without distance resolution loss as compared with the conventional method.

Multipulse gate-delayed range gating imaging lidar

We present a technique to reconstruct a higher resolution of depth map of range gating imaging lidar by applying the delays of the gates to a typical range gating lidar system during the detection of each returned laser pulse with the encoding of the returned signal. With the consequent delays of the gate, the depth of the scene is extended accordingly. A multipulse gate-delayed range gating lidar system is designed to prove the resolution improvement from 6 to 1.5 m. The unchanged peak power of the laser, the widths of the laser pulse and the sampling period result in a simple structure of the lidar system.

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.

Evaluation metrics for range-gated active imaging systems using a Lissajous-type eye pattern

In this paper a new method to evaluate gated viewing systems and range-gated imaging sequences, using a Lissajous-type eye pattern, is presented. This approach enables the comparison of gated viewing systems and defines clear criteria for depth resolution and depth mapping capabilities of an active imaging system. A distinct parameter can be depicted and sensed within a single chart. Therefore, this approach is a first proposal for an evaluation procedure of gated viewing systems and an enhanced analysis tool in the means of 3D capabilities.

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

We present a three-dimensional imaging method using a pulsed laser as a flood illuminating source and an intensified camera as the receiver with exponentially modulated gain. The depth map of a scene is obtained from two intensity images and the depth accuracy is independent of the depth of the target in the scene. We demonstrate a depth-independent depth accuracy of 0.32 m in an indoor experiment and obtain a depth map of an outdoor scene ranging from 150 to 180 m under a lower signal to noise ratio condition.

Advanced short-wavelength infrared range-gated imaging for ground applications in monostatic and bistatic configurations

Some advanced concepts for gated viewing are presented, including spectral diversity illumination techniques, non-line-of-sight imaging, indirect scene illumination, and in particular setups in bistatic configurations. By using a multiple-wavelength illumination source target speckles could be substantially reduced, leading to an improved image quality and enhanced range accuracy. In non-line-of-sight imaging experiments we observed the scenery through the reflections in a window plane. The scene was illuminated indirectly as well by a diffuse reflection of the laser beam at different nearby objects. In this setup several targets could be spotted, which, e.g., offers the capability to look around the corner in urban situations. In the presented measuring campaigns the advantages of bistatic setups in comparison with common monostatic configurations are discussed. The appearance of shadows or local contrast enhancements as well as the mitigation of retroreflections supports the human observer in interpreting the scene. Furthermore a bistatic configuration contributes to a reduced dazzling risk and to observer convertness.

Pulse-shape-free method for long-range three-dimensional active imaging with high linear accuracy

We present a three-dimensional (3D) imaging method employing linear increasing gain to encode flying time of photons into intensity information. This method obtains both the reflectivity and the depth of scene from only two two-dimensional (2D) images. High linear accuracy between the depth and the intensity information is independent of the laser pulse shape. We demonstrated <1 m linear depth accuracies with two different kinds of laser pulse shape and a 3D scene reconstruction with supperresolution depth mapping when the targets are 800-1100 m away.