Measurement of broadband diffuse solar irradiance using current commercialinstrumentation with a correction for thermal offset errors

Diffuse-sky solar irradiance is an important quantity for radiation budget research, particularly as it relates to climate. Diffuse irradiance is one component of the total downwelling solar irradiance and contains informatio n on the amount of downward-scattered, as opposed to directly transmitted, solar radiation. Additionally, the diffuse component is often required when calibrating total irradiance radiometers. A variety of pyranometers are co mmonly used to measure solar diffuse irradiance. An examination of some ins truments for measuring diffuse irradiance using solar tracking shade disks is presented, along with an evaluation of the achieved accuracy. A data cor rection procedure that is intended to account for the offset caused by ther mal IR exchange between the detector and filter domes in certain common dif fuse pyranometers is developed and validated. The correction factor is deri ved from outputs of a collocated pyrgeometer that measures atmospheric infr ared irradiance.