Diffraction effect correction of solar radiometer

Solar radiation measurement is one of the important way to study solar activity and evolution of Earth's climate, which is of great significance to the sustainable development of human society. As one of the main sources of systematic errors in the measurement, diffraction effect must be accurately corrected so as to improve the reliability of the measurement data. China has little research on the diffraction effects in solar radiometers, and there are currently no related corrections. With the increase demand of the accuracy of solar radiation measurements, it is necessary to systematically study the diffraction effect. Firstly, the diffraction effect theory is studied. According to the Kirchhoff diffraction theory as well as the Gaussian optical approximation, the energy transfer relations between points and points, points and faces, and between faces is gradually determined, and the general formula of diffraction effect is derived. Then a simplified calculation method is obtained according to the asymptotic property of the diffraction effect. The diffraction effect of the Solar Irradiation Absolute Radiometer (SIAR) and the diffraction correction factor are then calculated through a simplified method. Finally, based on the diffraction correction results, the scaling factor relative to the World Radiation Reference (WRR) is calculated. As a result, the diffraction effects and diffraction correction factors of SIAR are about 1.002 742 and 0.997 225, respectively. In addition, the calibration coefficient of SIAR for WRR is closer to 1, indicating that the diffraction correction reduces the systematic error and improves the accuracy of the radiation measurement.