Optimized Scalable and Learnable Binary Quantization Network for LiDAR Point Cloud
Zhao Zhi, Ma Yanxin, Xu Ke, and Wan Jianwei
To solve the time-consuming and storage problems of the LiDAR point cloud deep learning network models in the deployment of embedded devices on the mobile terminal, a learnable binary quantization network model for LiDAR point clouds is proposed. The model refers to the idea of feature-based knowledge distillation and transfers the statistical feature knowledge of each layer of the full-precision network to the binary quantization network, which greatly improves quantification accuracy. A genetic-algorithm based learnable optimization algorithm for scale factor recovery of binary quantization is proposed, which searches for the initial optimal layer-wise scale recovery factor, and greatly reduces amount of network parameters through network self-learning. A statistical adaptive pooling loss minimization algorithm is proposed, including quantitative network self-adjustment and full-precision network transferring adjustment, which solves the problem of greater pooling information loss of quantitative networks. Experimental results show that the proposed algorithm achieves larger compression ratio and speedup ratio while obtaining high precision. Theoretically, it can compress PointNet by 23 times and accelerate it by 35 times at least or more, and also achieves good scalability for other mainstream point cloud deep networks.
  • Jun. 15, 2022
  • Acta Optica Sinica
  • Vol. 42 Issue 12 1212005 (2022)
  • DOI:10.3788/AOS202242.1212005
Accurate Local Homography-Based Projector Calibration Method
Sun Lijun, Zhang Dandan, Chen Tianfei, Huang Zhiyuan, and Li Shi
In fringe projection profilometry, projector calibration is an important part of system calibration. Accurate projector calibration is key to ensuring the accuracy of three-dimensional shape measurements. The profilometry projector cannot capture the image directly. Thus, obtaining the correspondence between pixels and object points is difficult, which makes projector calibration complex with low precision. To address this problem, we analyzed the factors that influence projector calibration and presented a new projector calibration method based on local homography matrix. First, we conducted experiments to determine the optimum size of the local area. Then, we calculated the local homography matrix of the area where the center of the circle is located and completed the mapping of the projector's coordinates. Finally, we used the camera calibration method to calibrate the projector. Experimental results show that this method reduces the influence of center deviation and phase error on the calibration accuracy of the projector. Based on the idea of local linearization, the subpixel mapping of camera image coordinates to projector image coordinates is established, and the accuracy of projector calibration is improved.
  • Jun. 09, 2022
  • Laser & Optoelectronics Progress
  • Vol. 59 Issue 13 1312003 (2022)
  • DOI:10.3788/LOP202259.1312003
Hybrid-Coded Phase-Shifting Profilometry for Structured Light Measurement
Zeng Xinyi, Wu Shiqian, and Chen Bin
In this paper, we proposed a hybrid coding method that includes both phase order and relative wrapped phase to quickly and accurately obtain three-dimensional information of spatially discontinuous objects. Based on the traditional phase-shifting profilometry, the phase order was modulated into the phase-shifting image using the De Bruijn sequence. The length of the De Bruijn sequence used is the number of periods, where each code value corresponds to a phase period. After calculating the wrapped phase, the code sequence corresponding to each period was matched with the De Bruijn sequence to verify whether the location of the code in the De Bruijn sequence is the phase order. The proposed method need not project additional images to encode the phase order as both the phase order and the wrapped phase required by the unwrapped phase can be obtained from the projected phase-shifting patterns. The experimental results show that when only four images are projected, the three-dimensional morphology of the objects to be measured can be reconstructed, and the standard deviation of the reconstructed plate is 0.319 mm.
  • Jun. 09, 2022
  • Laser & Optoelectronics Progress
  • Vol. 59 Issue 13 1312002 (2022)
  • DOI:10.3788/LOP202259.1312002
Study on Diopter Measurement Technology of Progressive Multi-Focus Lens Using Phase-Shifting Talbot Interferometer
Xu Haifei, and He Yong
In this paper, we present a method for accurately measuring the diopter of both progressive multi-focus and large-range single focal lens. Based on Talbot interferometer methods, we investigated the principle of lens diopter measurement and deduced the formula of optimal diopter measurement. The main parameters of the Talbot interferometer were designed for the range of -8 D to 8 D, and include the grating period, grating angle, and grating spacing. Furthermore, a phase-shifting Talbot interferometer measuring system was constructed and we adopted the calibration method, which uses the target coordinate and grating coordinate systems. Here we applied a five-step phase-shifting algorithm to solve the moiré fringe. The multiple experimental results of multiple diopters and progressive multi-focus lens show that the measurement error is better than 0.1%.
  • Jun. 09, 2022
  • Laser & Optoelectronics Progress
  • Vol. 59 Issue 13 1312001 (2022)
  • DOI:10.3788/LOP202259.1312001
Research and Experiment on Space Debris Daytime Laser Ranging Based on 532 nm Wavelength
Li Zhulian, Zhai Dongsheng, Tang Rufeng, Zhang Haitao, Li Rongwang, Pi Xiaoyu, Fu Honglin, and Li Yuqiang
Laser ranging technology is crucial for the monitoring and early warning of space debris. To improve the monitoring capability of the 1.2 m telescope laser ranging platform of the Yunnan Observatory of the Chinese Academy of Sciences for space debris, the daytime laser ranging technology and method for space debris monitoring were studied. First, the feasibility of using the existing 1.2 m telescope laser ranging platform of the Yunnan Observatory to perform daytime space debris laser ranging was analyzed. Then, the key problems associated with space debris laser ranging during daytime were analyzed and solutions were proposed. Based on a daytime space debris laser ranging experiment, some space debris laser ranging data were obtained. The cross section of the measured space debris radar ranges from 9.0 m2 to 20.0 m2, the perigee ranges from 400 km to 900 km, and the apogee ranges from 500 km to 900 km. Results show that the space debris laser ranging platform of the Yunnan Observatory can perform space debris laser ranging during daytime, which can provide technical support for subsequent research on space debris laser ranging.
  • Jun. 09, 2022
  • Laser & Optoelectronics Progress
  • Vol. 59 Issue 11 1112003 (2022)
  • DOI:10.3788/LOP202259.1112003
Influence of Light Sources of Different Wavelengths on Scattering Rate of Defects
Wang Xudong, Gao Aihua, Yan Lirong, Qin Wengang, and Li Wenjin
Herein, a multi-wavelength defect detection system based on the theory of integral scattering detection is proposed. This system uses three light sources with wavelengths of 635 nm, 525 nm, and 405 nm and employs a photomultiplier tube as the photodetector. Two samples of high-transmittance quartz glass sheets with standard size defects,one with a width and a depth of 20 μm and the other with a width and a depth of 25 μm,are evaluated. The powers of the light source are 50 mW and 80 mW, respectively. Experimental results show that the highest scatter rate is achieved using the 405 nm light source. Therefore, the detection ability of the 405 nm light source is stronger than those of the 635 nm and 525 nm light sources. The findings of this study provide a reference for detecting small defects and achieving defect depth information.
  • Jun. 09, 2022
  • Laser & Optoelectronics Progress
  • Vol. 59 Issue 11 1112002 (2022)
  • DOI:10.3788/LOP202259.1112002
Large Aperture Off-Axis Aspherical Segment Test Using Refraction and Diffraction Mixed Compensation Based on Computer Generated Hologram
Huang Ya, Wang Fengpu, Li Xinnan, Chen Zhe, Li Bo, Xu Chen, and Cao Ting
In this paper, a refraction and diffraction mixed compensation test optical path based on computer-generated hologram (CGH) has been proposed, and the combination of the CGH and aplanatic lens is realized to measure the off-axis segment in null test with focal reducing. The total length has been reduced to 1/4 to 1/8 of that of the conventional method. And the adjustment sensitivity of the small focus ratio segment has been improved. The distortion ratio of the interferogram of the large off-axis segment has been optimized from 12.5 to 1.25. The segments with different off-axis distances can be measured rapidly with high precision by replacing different computer-generated holograms and keeping every elements in place. The method is used to complete the processing and testing of a Φ330 mm off-axis trial mirror, and the root mean square (RMS) value of the test surface figure is 0.0290λ (λ is wavelength). The RMS value of the test surface figure is reduced to 0.0267λ, by calibrating the aplanatic lens using a standard spherical mirror.
  • Jun. 08, 2022
  • Acta Optica Sinica
  • Vol. 42 Issue 12 1212004 (2022)
  • DOI:10.3788/AOS202242.1212004
Conics and Line Fusion Features Based Calibration of Telephoto Camera
Sun Cong, Yang Zhenglei, Jia Mengna, and Yu Qifeng
Aiming at the bottleneck of telephoto camera calibration, a novel calibration method based on conics and line fusion features is proposed. The article reveals the formation mechanism of ill-conditioned calibration equation of the telephoto camera. Given that the cross ratio and tangent point are basic projective invariants, a concentric semi-circular template is designed. Combined with the normalized line homography estimation method and the partitioned regularization estimation algorithm, the high-precision solution of initial values of intrinsic and extrinsic parameters of the telephoto camera is obtained. Further, considering the lens distortion, the integral distance between line segments is introduced, taking the minimum distance from the model line to the projection plane as the cost function, and a nonlinear optimization algorithm is adopted to refine the calibration parameters. Simulation and actual experimental results show that the proposed calibration method of the telephoto camera is effective, and has higher accuracy and robustness compared with the traditional method.
  • Jun. 07, 2022
  • Acta Optica Sinica
  • Vol. 42 Issue 12 1212003 (2022)
  • DOI:10.3788/AOS202242.1212003
High-Precision BRDF Measurement System in Near Infrared Band
Li Zhuoran, Li Yuxiao, and Liu Zilong
Bidirectional reflection distribution function (BRDF) in near infrared band can effectively characterize the reflection properties of materials and is widely used in computer modeling, remote sensing, aerospace, and other fields. In order to measure the BRDF value of near infrared band with high precision, a near infrared band BRDF measurement system is developed, and an absolute measurement scheme based on equal area luminance is proposed. The measurement system takes the center of the tested sample as the base point to re-model the spatial coordinate system of the measurement system, and uses the six-axis manipulator as the angle-changing device to realize the modeling, which can change the three-dimensional orientation of the sample without blind angles. The BRDF values of materials with any angle combination can be measured in 2π hemispheric space. The BRDF values of materials covering the near infrared band of 1000 nm to 3000 nm are measured with high precision. The system is used to measure the BRDF value of the standard diffuse reflection gold plate, and the results show that the relative uncertainty of the measurement result is 1.5%, indicating that the developed system has achieved high-precision measurement.
  • Jun. 07, 2022
  • Acta Optica Sinica
  • Vol. 42 Issue 12 1212002 (2022)
  • DOI:10.3788/AOS202242.1212002
Frequency Scanning Interferometry Absolute Distance Measurement Method Based on Optical Spectrum Calibration
Shang Yue, Wu Tengfei, Lin Jiarui, Yang Linghui, Zhou Qiang, and Zhu Jigui
With high precision and sensitivity, frequency scanning interferometry (FSI) absolute distance measurement technique is getting more and more attention in the field of large-scale equipment manufacturing. Building an optical fiber auxiliary interferometer in an FSI system to realize the precise monitoring of optical frequency changes is necessary for achieving higher ranging accuracy. However, the stability of optical path of the auxiliary interferometer is limited by the environmental factors and dispersion effect, which would reduce the accuracy seriously. An optical spectrum calibration based FSI absolute distance measurement method is proposed in this paper for this problem. The optical path of the auxiliary interferometer in the FSI system is calibrated in situ with the help of the absorption spectrum of hydrogen cyanide (HCN) to provide stable and accurate optical frequency reference. What’s more, a fast compensation method for dispersion mismatch error is proposed to eliminate the influence of dispersion mismatch effect caused by the fiber optic assisted interferometer on measurement results. The proposed method is verified by comparison experiments with commercial interferometer at a range of 20 m. The result shows that the largest ranging deviation of the system is less than 50 μm, and the repeatability is better than ±4 μm.
  • Jun. 07, 2022
  • Acta Optica Sinica
  • Vol. 42 Issue 12 1212001 (2022)
  • DOI:10.3788/AOS202242.1212001