title: Absolute spectral radiance responsivity calibration of sun photometers.
abstract: Sun photometers are designed to measure direct solar irradiance and diffused sky radiance for the purpose of atmospheric parameters characterization. A sun photometer is usually calibrated by using a lamp-illuminated integrating sphere source for its band-averaged radiance responsivity, which normally has an uncertainty of 3%-5% at present. Considering the calibration coefficients may also change with time, a regular high precision calibration is important to maintain data quality. In this paper, a tunable-laser-based facility for spectral radiance responsivity calibration has been developed at the Key Laboratory of Optical Calibration and Characterization, Chinese Academy of Sciences. A reference standard radiance radiometer, calibrated against cryogenic radiometer, is used to determine the radiance from a laser-illuminated integrating sphere source. Spectral radiance responsivity of CIMEL CE318-2 sun photometer is calibrated using this new calibration system with a combined standard uncertainty of about 0.8%. As a validation, the derived band-averaged radiance responsivity are compared to that from a Goddard Space Flight Center lamp-based sphere calibration and good agreements (difference <1.4%) are found from 675 to 1020 nm bands.
authors.affiliation: Key Laboratory of Optical Calibration and Characterization, Chinese Academy of Sciences, Hefei 230031, China.
authors.forename: Qiuyun
authors.initials: Q
authors.lastname: Xu
authors.forename: Xiaobing
authors.initials: X
authors.lastname: Zheng
authors.forename: Zhengqiang
authors.initials: Z
authors.lastname: Li
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
authors.forename: Xianhua
authors.initials: X
authors.lastname: Wang
authors.forename: Jianjun
authors.initials: J
authors.lastname: Li
authors.forename: Xin
authors.initials: X
authors.lastname: Li