Songtao Fan

Echo broadening effect in the range-gated active imaging technique

We have presented the Echo Broadening Effect of range-gated active imaging. It can respectively generate a head signal part and a tail signal part both sides of the echo signal profile. Our research demonstrates that the head signal and the tail signal impact the depth of view, detection range and imaging quality, especially the head signal. In order to solve the problems, we establish a model of the echo broadening effect, and analyze the signal energy characteristics and atmospheric backscatter. We have given the depth of view under the effect, verified it in experiment, and found that the reasonable choosing of the illuminator laser pulse time and the camera gate time can mange the effect to optimize range-gated imaging systems.

Three-dimensional range-gated flash LIDAR for land surface remote sensing

Three-dimensional range-gated imaging is a new 3D sensing technique with higher resolution than 3D flash LIDAR, and has great potential in realizing high-resolution real-time 3D imaging to satisfy land surface remote sensing applications. In this paper, three existing approaches of realizing 3D range-gated LIDAR are introduced including their advantages and disadvantages. Among them, the two methods of gain modulation and range-intensity correlation can reconstruct a 3D scene from two gate images, which enable 3D flash imaging. We propose a 3D superresolution range-gated flash LIDAR based on triangular algorithm of range-intensity correlation, and further present a coding method based on triangular algorithm for high depth-to-resolution ratio. Some prototyping experiments and simulations are demonstrated.

Real-time remote three-dimensional superresolution range-gated imaging based on inter-frame correlation

High-resolution real-time three-dimensional imaging is important in 3D video surveillance, robot vision, and automatic navigation. In this paper, a three-dimensional superresolution range-gated imaging based on inter-frame correlation is proposed to realize high-resolution real-time 3D imaging. In this method, a CCD/CMOS with a gated image intensifier is used as image sensor, and depth information collapsed in 2D images is reconstructed by spatial-temporal inter-frame correlation with a resolution of about 1000×1000 full-frame pixels within a frame. Furthermore, under inter-frame correlation a 3D point cloud frame is generated at video rates corresponding to CCD/CMOS utilized. Finally, some proof simulation experiments are demonstrated.

Target detection against strong light based on gate viewing

We present a method of target detection against strong light based on gate viewing. In this method, a nanosecond-scale gate shutter is used to control the exposure time of CCD and reduce the collection of obtrusive light, and a nanosecond-scale pulsed laser is used to illuminate targets and increase signal energy. By matching them, the ratio between signal and obtrusive light will be significantly improved to detect targets against light disturbance. We have analyzed the method in theory, and performed it in experiment. In addition, a stroboscopic time sequence is used, and the setting of temporal parameters is also discussed.

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.

High delay precision based on dynamic phase-shift for range-gated laser imaging technology

The range-gated laser imaging technology has become a useful technique in many applications in recent years. In order to expand the range of imaging detection and improve the measurement range resolution of the imaging system, we used circular step advance delay sequence for the synchronous control. And we developed a method of using dynamic phase-shift technique in FPGA to improve the precision of the delay in the time sequence, which can make the precision of the delay stepper between the two adjacent frames less than global clock period of the FPGA and approach the limit of FPGA’s operating frequency. That is to say, it can equivalently increase the clock frequency. Then we can effectively improve measurement range resolution of the imaging system. In this paper, we have studied how dynamic phase-shift technique can be equivalent to higher clock frequency and performed some experiments. We presented the structure of dynamic phase-shift technique used to improve the precision of delay in the synchronization control time sequence. And the simulation and experimental results are showed in this paper. The results demonstrate that using dynamic phase-shift technique in FPGA can make the precision of the delay between the ICCD’s trigger pulse and the laser’s trigger pulse reach 1ns, which means the resolution of measurement range can be 0.15m theoretically. The timing control signal with dynamic phase-shift technique designed in this paper can be widely used in range-gated imaging because of its high timing control precision and flexible parameter setting.

Surveillance photonic fence based on active range-gated imaging for night intrusion detection

We propose a surveillance photonic fence for night remote intrusion detection, especially in bad environmental conditions. The photonic fence is established by the synchronization of a pulsed infrared laser and a gated imaging sensor. Since the wavelength of the laser is invisible, the photonic fence is also invisible. Only when targets pass the fence, their image information can be collected. Objects and backgrounds out of the fence are all filtered directly which decreases the complexity of image processing about target extraction. For the fence, its location can be easily adjusted by the delay time between the laser pulse and the gate pulse, and its thickness can be set by changing the gate time and the laser pulse width. Furthermore, target space information can also be estimated in terms of the range information of the photonic fence.

Moving target detection in flash mode against stroboscopic mode by active range-gated laser imaging

Moving target detection is important for the application of target tracking and remote surveillance in active range-gated laser imaging. This technique has two operation modes based on the difference of the number of pulses per frame: stroboscopic mode with the accumulation of multiple laser pulses per frame and flash mode with a single shot of laser pulse per frame. In this paper, we have established a range-gated laser imaging system. In the system, two types of lasers with different frequency were chosen for the two modes. Electric fan and horizontal sliding track were selected as the moving targets to compare the moving blurring between two modes. Consequently, the system working in flash mode shows more excellent performance in motion blurring against stroboscopic mode. Furthermore, based on experiments and theoretical analysis, we presented the higher signal-to-noise ratio of image acquired by stroboscopic mode than flash mode in indoor and underwater environment.

3D super resolution range-gated imaging for canopy reconstruction and measurement

In this paper, we proposed a method of canopy reconstruction and measurement based on 3D super resolution range-gated imaging. In this method, high resolution 2D intensity images are grasped by active gate imaging, and 3D images of canopy are reconstructed by triangular-range-intensity correlation algorithm at the same time. A range-gated laser imaging system(RGLIS) is established based on 808 nm diode laser and gated intensified charge-coupled device (ICCD) camera with 1392´1040 pixels. The proof experiments have been performed for potted plants located 75m away and trees located 165m away. The experiments show it that can acquire more than 1 million points per frame, and 3D imaging has the spatial resolution about 0.3mm at the distance of 75m and the distance accuracy about 10 cm. This research is beneficial for high speed acquisition of canopy structure and non-destructive canopy measurement.

Underwater three-dimensional range-gated laser imaging based on triangular-range-intensity profile spatial-correlation method

Underwater 3D range-gated imaging can extend the detection range over underwater stereo cameras, and also has great potentials in real-time high-resolution imaging than 3D laser scanning. In this paper, a triangular-range-intensity profile spatial correlation method is used for underwater 3D range-gated imaging. Different from the traditional trapezoidal method, in our method gate images have triangular range-intensity profiles. Furthermore, inter-frame correlation is used for video-rate 3D imaging. In addition, multi-pulse time delay integration is introduced to shape range-intensify profiles and realize flexible 3D SRGI. Finally, in experiments, 3D images of fish net, seaweed and balls are obtained with mm-scaled spatial and range resolution.