Accurate traceability of LiDAR(Light Detection and Ranging) is still a difficult issue in the autonomous driving industry, as it serves as crucial 'eyes' for perception. To address this issue, a device was developed for absolute measurement of reflectance at small angles near θ/θ reflection condition of LiDAR. Through the fine design of the small angle optical path, the problem of mutual occlusion between the light source and the detector due to the actual size under this condition was optimized, the accurate value of the incident and reflected radiant flux were obtained, and the absolute measurements of the reflectance with the minimum angle better than 0.1°/-0.1° were realized. To verify the accuracy and reliability of the device, one 1 -inch diameter UV-enhanced aluminum film plane mirror and one 1-inch diameter silver film plane mirror were used. Absolute reflectance measurements were conducted at light source wavelengths of 532nm and 905nm, with reflection angles of 12°/-12°and 45°/-45°. The measured results were then compared with their calibrated values, achieving an optimal relative deviation of 0.4%, thereby providing preliminary validation of the device's measurement accuracy and reliability. Meanwhile, the light source and detection part of this device are planned to be extended to 1550 nm, providing better support for LiDAR reflection traceability in the autonomous driving industry.
As a pair of "wise eyes" for autonomous vehicles to perceive the external environment, Lidar (Light detecting and Ranging) plays a crucial role in detecting target characteristics in driving scenarios. To ensure the accuracy and reliability of Lidar, precise measurement of target reflectance under θ/θ reflection conditions is an indispensable step. Determining the reflectance value of targets under θ/θ reflection conditions is a critical part of completing Lidar calibration and traceability. When designing the θ/θ optical path, a significant challenge lies in achieving almost perfect overlap between the lighting and detecting paths while ensuring system compactness and measurement accuracy. Fiber optic spectrometers, known for their fast and accurate measurement capabilities, can be directly applied to measure target reflectance. Therefore, combining a compact θ/θ reflection optical path with a fiber optic spectrometer is key to achieving small-angle reflectance measurements for Lidar, marking an important step towards improving the calibration and traceability chain. For Lidar under θ/θ reflection conditions, various "N+1" lighting/detecting combined optical paths based on fiber bundles have been designed. Simulation analysis of these designs has been conducted using ray-tracing methods, comparing the uniformity and optical flux efficiency of the tested samples. The results indicate that when N=6, the uniformity and incident flux efficiency are optimal. Based on the simulation results, a "6+1" lighting/detecting fiber optic spectrometer has been developed, and actual measurements have been performed on a standard diffuser. The measurement data shows that the angular accuracy under 0/0 and 45/45 conditions is better than 0.1°, and the optimal relative error of the reflection measurement results in the 905nm laser wavelength was less than 0.5%. This meets the requirements for on-site measurements and is significant for further improving the Lidar measurement traceability chain.
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