Spectrally integrated window transmittance measurements for a cryogenic solar absolute radiometer

Abstract

A recently built cryogenic solar absolute radiometer aims to reduce the uncertainty of terrestrially measured direct solar irradiance from 0.3% to 0.01%. Because solar irradiance entering a cryogenic radiometer is partly reflected and absorbed by the entrance window, the spectrally integrated transmittance of the broadband solar irradiance needs to be determined in parallel to correct the power reading of the cryogenic radiometer for these losses. To meet the accuracy requirements of the radiometer, the Monitor to Measure the Integral Transmittance of Windows (MITRA) aims to measure the spectrally integrated transmittance of a window identical to that used for the cryogenic radiometer with an uncertainty of less than 0.01%. The current model of the MITRA instrument measures transmittances with an instrument stability of 0.015% under laboratory conditions, almost achieving the instrument requirement. This proves that the accuracy of terrestrial irradiance measurements can be significantly increased by means of a cryogenic solar absolute radiometer, which can then be utilized as a new calibration standard for terrestrial absolute radiometry.