Bilateral comparison of irradiance scales between PMOD/WRC and PTB for longwave downward radiation measurements

In this work, comparison measurements are presented between two independently realised and characterised blackbody cavities which serve as irradiance standards, namely the well-established Tilted Bottom Cavity BB2007 and the new Hemispherical Blackbody (HSBB). Both are used to realise the unit for thermal infrared irradiance. The BB2007 at the Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center has long provided the reference for longwave downward radiation within the Baseline Surface Radiation Network. Longwave downward radiation is constantly measured at multiple stations around the world using specific broadband infrared radiometers with a hemispherical acceptance angle, for example pyrgeometers. The HSBB, developed at the Physikalisch-Technische Bundesanstalt (PTB) in recent years, was specifically designed to calibrate radiometers with a hemispherical acceptance angle which measure longwave downward radiation. The HSBB is directly traceable to the Radiation Temperature Scale of PTB and, in turn, via this scale to the SI. Comparison measurements between the BB2007 and HSBB in this work were carried out with three transfer instruments: the dedicated Temperature Stabilised Radiation Thermometer, an Infrared Integrating Sphere (IRIS) instrument and a Kipp and Zonen CG4 pyrgeometer. The results show good agreement with respect to the target irradiance uncertainty of 0.5 -2 provided by the HSBB. This study thus supports and validates the traceability of atmospheric longwave downward radiation to the SI linking measurements performed with the World Infrared Standard Group of pyrgeometers to the traceability of the reference blackbody BB2007 using IRIS instruments as the transfer standard.