Objective Lidar is widely applied in 3D imaging because of its high precision, high resolution, and long working distance. The size and distribution of the illumination spot on the imaging target are important factors affecting the imaging resolution. The smaller spot size results in finer information obtained on the target surface and clearer 3D information of the reconstructed object. The increase in spot size affects the quality of laser scanning 3D imaging mainly in two aspects. On one hand, it will reduce the energy of light signals shining on the surface of the object, thereby leading to weakening return light signals and affecting the detection distance. On the other hand, the area of large light spot shining on the object surface is also large, which causes multiple echo signals generated by the same light spot due to the change of object surface, thus affecting the ranging accuracy. However, most of the existing lidars employ collimated Gaussian beams to emit. When imaging at a long distance, the spatial resolution is significantly reduced with the increasing target distance due to the laser divergence angle and the diffraction limit of the optical system. To this end, this paper improves the system resolution from the aspect of output beam control and scanning mode. Methods In laser scanning 3D imaging, the beam scans and illuminates the target point by point, and detects the return light signals at different positions of the target to obtain the depth information of the detection target surface and realize 3D imaging. In this process, the traditional laser scanning 3D imaging generally irradiates the laser collimated and expanded to a distant target object in a parallel beam mode for reducing the divergence of beams. The size of the light spot at the target object mainly depends on the divergence angle of the beam after expansion and the imaging working distance and is generally far greater than that of the light spot under the diffraction limit. By changing the focal length of the system, this paper focuses the light spot at the imaging target to obtain a small light spot. Sub-pixel scanning is originally proposed as a method to obtain high-resolution (HR) images from multiple low- resolution (LR) images for ordinary digital cameras. In sub-pixel scanning technology, the basic premise to improve spatial resolution is to capture multiple LR images from the same scene. LR images are subsampled ( aliased) and shifted with sub-pixel accuracy. If the LR image is shifted in integer units, each image contains the same information, so there is no new information available to reconstruct the HR image. However, if LR images have different sub-pixel offsets from each other and there is aliasing, HR images can be obtained using the new information contained in each LR image. As an analogy to lidar, sub-pixel scanning can also be employed to enhance the imaging resolution of the system. Results and Discussions A confocal sub- pixel scanning photon counting lidar is designed to improve the spatial resolution of the system. The size of the focused spot at 10 m is about 1/4 of that of the collimated spot. Compared with pixel-bypixel scanning, 1/8 sub-pixel scanning at 7 m and 10 m can recover the letter I on the resolution target ( Fig. 4). Focused lighting can effectively enhance echo energy (Fig. 5) and imaging resolution (Fig. 6) compared with collimated lighting. Combined with sub-pixel scanning, the imaging resolution of the system is increased from 5. 0 mm to 0. 9 mm, which exceeds the diffraction limit of the system. In addition, with the decreasing sub- pixel scanning step, the imaging resolution will also be improved. The experimental results show that through sub-pixel scanning, the aliasing of the reconstructed image is rapidly reduced, and the contour is smoother and more accurate ( Fig. 8). Conclusions This paper demonstrates the sub-pixel scanning high-resolution technology using collimated and focused illumination through experiments. The imaging results of the transmissivity resolution target show that under the same FoV, small spot illumination can effectively improve the imaging resolution. The reconstructed image obtained by subpixel scanning is smoother and clearer than the pixel-by-pixel image. The horizontal resolution is increased from 5. 0 mm per pixel for collimated illumination to 0. 9 mm for focused illumination with sub-pixel scanning and exceeds the limit of system angular resolution. In addition, in the lidar system, the resolution improvement of sub-pixel scanning is also related to the spot size. Small spot illumination and sub-pixel scanning technology have great potential in improving the spatial resolution of long- range lidar.
