Significance Traditional light detection and ranging (LiDAR) can obtain point cloud data of three-dimensional (3D) scenes, but it is often difficult to obtain high-quality intensity images. Therefore, a technical solution that combines LiDAR and cameras is usually used, where LiDAR senses 3D spatial information and cameras obtain high-definition texture images of the scene. However, this composite technical solution faces the problem of heterogeneous data fusion. For example, in self-driving and driver assistance systems there are different working distances of the two sensors under severe weather or low light level conditions, and it is hard to achieve effective data fusion, which leads to performance degradation or failure. With the advent of the artificial intelligence era, light ranging and imaging (LiRAI) that simultaneously obtains high-resolution intensity images and dense 3D images of targets and scenes, has become a development trend of LiDAR. That means a single sensor can realize light ranging and imaging instead of light detection and ranging, and thus the heterogeneous data mismatch problem of LiDAR and camera composite technology can be solved. In essence, laser range-gated 3D imaging (Gated3D) technology is a kind of gated LiRAI, since it can utilize a single gated camera to simultaneously obtain high-quality 2D intensity images and high-resolution 3D images. Gated3D has gained much attention in the applications of long-range surveillance, advanced driving assistance system and underwater imaging, owing to its long working distance, fast imaging speed, high resolution and the ability to suppress medium backscattering noise. Unlike traditional imaging methods that indiscriminately capture targets and backgrounds within the field-of-view, laser range-gated imaging selectively captures targets within a specific distance range-of-interest (ROI), which filters out medium backscattering noise in the imaging chain, as well as background noise outside the ROI, thereby increasing the imaging distance and enhancing the image quality. Moreover, different from traditional scanning LiDAR, the Gated3D technology employs gated cameras beyond megapixels, and thus offers spatial resolutions surpassing mechanical scanning LiDAR and outperforming flash LiDAR based on avalanche photodiode (APD) arrays. Over the past decade, there has been significant progress domestically and internationally in the development of Gated3D technologies. These advancements have led to the achievement of super range resolution 3D imaging, and promoted their applications.
Progress This paper systematically reviews the advances of Gated3D technologies in conjunction with its applications across various fields. It introduces the working principles of different technologies such as time slicing, gain modulation and range-intensity correlation methods. Their imaging characteristics of working distance, range resolution, imaging speed and depth of field are discussed. In recent years, the applications of Gated3D technologies have been explored in remote surveillance, automatic driving, vegetation measurement, marine life observation, underwater obstacle avoidance and so on. The results indicate that the technology readiness level (TRL) of range-intensity correlation 3D imaging technology is relatively high, generally reaching TRL5-7. It can fully utilize the correlated information between target distance and image intensity in gated images, enabling real-time super-resolution 3D imaging with fewer gated images. The application of deep learning techniques has further improved the performance of range-intensity correlation method. Finally, the paper analyzes the challenges and further development directions and application prospects faced in laser range-gated 3D imaging technology.
Conclusions and Prospects We believe that LiRAI will be the trend of LiDAR. LiRAI refers that with the help of active illumination, it does not rely on ambient light level, and uses a single sensor to simultaneously obtain high-resolution intensity images that reflect the radiation characteristics and texture characteristics of targets, as well as dense point cloud data/3D images that reflect the 3D spatial information of targets and their scene, and has long working distance with a certain ability of imaging through scattering medium. The Gated3D technology utilizes a single gated camera to simultaneously obtain high-quality 2D intensity images and high-resolution 3D images. The pixels in 2D images correspond one-to-one with the voxels in 3D images, inheriting the technical advantages of laser range-gated imaging through scattering medium. It has great potential to achieve high-performance LiRAI. The development trends of Gated3D are expected to focus on long-distance imaging in fog, rain, snow, smoke, dust, and underwater conditions, high-resolution fast 3D imaging in large depth of view, and high-performance color LiRAI. In the future, with the support of computational imaging and artificial intelligence, Gated3D will achieve faster, higher precision, longer working distance, more imaging functions, higher sensing dimensions, stronger adaptability to complex environments, and thus meet diverse scenario task requirements.
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