Mixing Calibration Method for Spectral Sun-Photometer
TANG Xiao-xiao1, 2, LI Jian-yu1, 3, 4*, XU Gang1, 3, 4, SUN Feng-ying1, 3, 4, DAI Cong-ming1, 3, 4, WEI He-li1, 3, 4
1. Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China
2. University of Science and Techology of China, Hefei 230031, China
3. Advanced Laser Technology Laboratory of Anhui Province, Hefei 230037, China
4. State Key Laboratory of Pulsed Power Laser Technology, Hefei 230037, China
Abstract：Spectral solar radiometers can directly measure the variation of solar radiation and reflect the radiation information corresponding to each band. The calibration accuracy of the instrument in the whole band of the direct ray channel directly affects the inversion accuracy of atmospheric parameters. The commonly used Langley fitting method has low calibration accuracy in the atmospheric strong absorption band, and there are certain errors in the final calculation of precipitable water and strong absorption band transmittance data. In order to meet the requirements of high-precision measurement of the visible-near-infrared full-band solar spectrum, this paper proposes a Mixing calibration method combining the Langley calibration method of the non-absorption band and the Mixing calibration method based on theoretical calculation of solar irradiance at the top of the atmosphere of the strong absorption band. It obtains the calibration value of the full band of the spectral radiometer. Because the instrumental response function changes slowly with wavelength, the instrumental response function in the strong absorption band is obtained by linear interpolation of the calibration instrument response function in the non-absorption band according to wavelength. Then the instrumental response function in the strong absorption band is obtained by combining the relationship between the solar irradiance at the top of the atmosphere and the instrumental calibration value. By comparing the change curves of instrument calibration values of the Langley calibration method, improved Langley calibration method and Mixing calibration method, it is found that the calibration values of the former two methods have obvious mutations in the strong absorption band, while the calibration values of the Mixing calibration method change more gradually in the strong absorption band, which conforms to the law of instrument response. By comparing the relative deviation between the atmospheric transmittance measured by different calibration methods and the transmittance calculated by CART theory, it is found that the average deviation of the Mixing calibration method is reduced by 1.15%, which is mainly attributed to the improvement of the measurement accuracy of atmospheric transmittance in the strong absorption band by the Mixing calibration method. The precipitation data calculated by the improved Langley method and the Mixing calibration method are compared with those measured by the same type of POM radiometer abroad. The results calculated by the Mixing calibration method are almost consistent with those calculated by the POM radiometer, and the relative error is less than 10%. Compared with the improved Langley scaling method, it reduced by 40% on average. For the atmospheric transmittance measured, compared with the POM radiometer transmittance data, the relative error of the Mixing calibration measurement is reduced by 25% at the 940 nm vapor absorption zone. Therefore, the Mixing calibration method is of great application value to the full-band calibration of a direct channel of spectral solar radiometer, the calculation of precipitable water and the calculation of transmittance of the strong absorption band, and improves the calibration accuracy of the strong absorption band.
Key words：Spectral solar radiometer; Calibration method; Langley; Irradiance; Absorption band
