National diffuse irradiance standard and its calibration method

The standard total irradiance is measured by the standard pyrheliometer and standard diffuse pyranometer. It's very important to establish the diffuse irradiance standard and corresponding calibration method and to transfer the World Radiometric Reference(WRR) from a standard pyheliometer directly to the diffuse irradiance standard, in order to make sure the accuracy of standard pyranometer. By comparing shade/unshade method with sum composition method, the sensitivity using shade/unshade method was not affected by thermal offset, the diffuse irradiance result is more accurate. From the perspective of reducing uncertainty, the shade/unshade method is much better as it just uses one standard instrument with least uncertainty which is directly traceable to WRR, while the error caused by other methods involving more standard instruments was larger.