Analysis And Corrections For The Diffraction Effects In Solar Irradiance Absolute Radiometer

Solar radiation is the most important energy source for the earth,which has profound impact on the the Earth's ecological environment and human survival.For the sustainable development,our human beings have early begun to monitor solar radiation.Long-term,uninterrupted and overlapping measurements of solar irradiance can provide essential basic scientific data for studying solar activity and are important for studying solar physics as well as for global radiation budgets and climate change.The Solar Irradiance Absolute Radiometer(SIAR)is a kind of radiometer developed by Changchun Institute of Optics,Fine Mechanics and Physics in the 1990 s.After nearly 20 years of continuous improvement and design,it has been equipped with "Shenzhou III" and " Fengyun III " meteorological satellite,to achieve the long-term monitoring of solar irradiance.The solar radiation enters the SIAR cone cavity through a series of aperture structures.The diffraction effect by the structure on the solar radiation is an important part of the system error of the radiometer measurement data.With the increasing demand for measurement accuracy,diffraction effects have become an important factor in radiation measurement which can not be ignored.Diffraction effects exist widely in radiation metrology,the calculation of which requires sufficient accuracy and less difficulty in operation.However,this problem has not been solved properly,making diffractioncorrection not been widely used in radiation measurement.So it is necessary to systematically research this problem and obtain an effective calculation method so that diffraction effect can be corrected practically for the radiation measurement.First,the Kirchhoff diffraction theory is used to derive the scalar field distribution of light diffraction in Gaussian optical approximation from the basic electromagnetic theory.According to the relation between the scalar field distribution and the radiation energy density as well as the energy flow,the diffraction theory is further transferred to the diffraction effect of radiation flux.According to the practical optical structure,SAD model(Source-Aperture-Detector,SAD)is constructed to describe the diffraction effect.From Lommel's point light source energy density distribution to Wolf's point light source integral flux formula and finally to Shirley's complete Diffraction effect expression,step by step processes of the diffraction effect formula derivation are shown.Then,according to the asymptotic property of the diffraction effect formulas,two expressions of the diffraction effect are obtained,as well as two calculating methods,namely the numerical integration method and the asymptotic formula method.Then,according to the diffraction effect theory in radiation measurement,the specific form of the diffraction effect formula is deduced,and the calculation process is introduced in detail to make the diffraction effect theory practical.Then,the diffraction effects which are given by some other team are calculated again by using the formula derived in this paper.By comparing the results,the validity of the formula is verified,and it is also shown in what situations the two calculation methods are more accurate.In order to calculate the diffraction effect of the SIAR,the effect of the aperture structure is analyzed.By constructing the SAD model,the correction for the diffraction effect in the SIAR measurement is realized.There are some differences in the calculation of the SIAR diffraction effects using the two methods,and this difference becomes more apparent when the aperture is closer to the detector.The range of application of the diffraction effect formula is pointed out,and the design of the aperture structure in the radiometer is proposed.In this paper,the theory of diffraction effect in radiation measurement has been systematically and comprehensively studied,and two practical formulas for calculating the diffraction effects have been deduced,which can be easily and accurately applied to SIAR diffraction analysis and corrections,which will reduce the Systematic error effectively.According to the analysis,the guiding design of the apertures structure is put forward,which is of great significance to the further improvement in the accuracy for radiometry measurements.