Objective Emissivity is an important physical quantity to characterize the radiation ability of material surface. It is an important thermal physical parameter and has very important applications in many fields. Emissivity is not the intrinsic property of an object. It is a physical quantity that is difficult to measure accurately. It is related to temperature, wavelength, angle and so on. Its measurement is more complicated. At present, most of the emissivity measurement methods are only contact measurement of a single substance. It is impossible to measure the three-dimensional distribution of emissivity of complex targets. Combined with the characteristics of LiDAR, a non-contact three-dimensional distribution measurement method of emissivity is proposed based on LiDAR echo.
Methods Firstly, the echo intensity characteristics are analyzed based on the LiDAR transmission distance equation, the main factors affecting the LiDAR echo intensity are explored, and the target point cloud data with intensity information by line array scanning of 95% reflectivity standard diffuse reflector plate are obtained by line array LiDAR (Fig.4). The stacked multi-frame single line point cloud (Fig.5) and the three-dimensional point cloud image of radar intensity with reflective spectral characteristics is obtained (Fig.8). Secondly, the piecewise polynomial model is used to fit the relationship between distance-intensity and incident angle-intensity (Tab.4). Based on the obtained piecewise polynomial model, the echo intensity under the influence of distance and incident angle is corrected, so that the measured echo intensity under different distance and incident angle can truly reflect the reflection spectrum characteristics of the target, and the validity of the correction model is verified (Fig.15-16). Finally, based on the obtained piecewise polynomial correction model, the intensity correction of the three-dimensional point cloud image of the radar intensity of the scaled satellite model with the reflectivity true value patch is carried out. The reflectivity of the target surface is calculated by using the reflectance spectral characteristics of the corrected echo intensity. The emissivity is further deduced by the reflection method, and the three-dimensional distribution of the emissivity of the scaled satellite model is obtained (Fig.21).
Results and Discussions The correction results show that the standard deviation of echo intensity STD under the influence of distance before and after standard diffuse reflection correction is reduced from 50.58 to 3.49 (Tab.5), and the standard deviation of echo intensity STD under the influence of incident angle is reduced from 19.25 to 3.17 (Tab.5). The coefficient of variation of echo intensity under the influence of standard diffuse reflection plate distance effect and incident angle effect is reduced from 0.267 6 and 0.343 8 to 0.042 0 and 0.041 2 (Tab.5) respectively. And the consistency of echo intensity is increased by 84.31% and 88.02% respectively (Tab.6). The average emissivity deviations of the surface patches of the three satellite models can be controlled at 3.33%, 4.84% and 4.44%, respectively (Tab.7).
Conclusions In view of the fact that most of the current emissivity measurement methods are only contact measurement of a single substance, it is impossible to measure the three-dimensional distribution of emissivity of complex targets. Combined with the characteristics of LiDAR, a non-contact emissivity three-dimensional distribution measurement method is proposed based on M8 LiDAR echo. The band of the M8 linear array LiDAR used in this paper is 905 nm, which belongs to the near-infrared band in the infrared band, and the emissivity measurement in the medium and long infrared band cannot be involved. The reflectivity of the materials used in this paper is the true value at room temperature, and the influence of temperature on emissivity measurement is not involved. In addition, due to the differences in physical factors of different LiDARs and the inconsistent expression of the unit and numerical scale of the echo intensity, the current measurement method is only applicable to specific scanning instruments. The robustness and universality of the emissivity measurement method are worthy of further discussion. Therefore, the next step will further improve the universality of the method and introduce the influence of temperature on the emissivity measurement.
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