Logo of the Physikalisch-Technische Bundesanstalt
Into the Future with Metrology - The Challenges of Our Environment and Climate

Measuring the greenhouse effect accurately

News on Environment and Climate

A new reference standard for measuring the infrared radiation emitted by the atmosphere

PTB-News 1.2023
Especially interesting for


environmental and climate research

Within the European series of research projects labeled “Metrology for Earth Observation and Climate”, a new reference blackbody called the Hemispherical Blackbody (HSBB) was developed.

The new HSBB and a transfer standard.

The infrared radiation that is emitted from the atmosphere and incident on the Earth (longwave downward radiation) is constantly being measured at many stations around the globe. It is a benchmark for the intensity of the greenhouse effect whose increase is responsible for climate change. Atmospheric longwave downward radiation is typically measured by means of pyrgeometers. These instruments are spectrally broadband infrared detectors having a hemispherical acceptance angle. To ensure the significance and comparability of the measurement data over the long term, pyrgeometers need to be regularly calibrated (i.e., metrologically traced to standards).

The new reference blackbody known as the Hemispherical Blackbody (HSBB) was developed at PTB in cooperation with the Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center (PMOD/WRC). The HSBB fulfills the specific requirements for such calibrations and is traceable via PTB’s Radiation Temperature Scale to the International Temperature Scale ITS-90 and hence to the SI, the International System of Units. The results of comparison measurements between the blackbody standard of PMOD/WRC ‒ a well-established reference for longwave downward radiation measurements ‒ and the new HSBB show very good agreement within an uncertainty of 0.5 W/m2. The typical uncertainties when measuring atmospheric longwave downward radiation are currently on the order of 5 W/m2. One of the instruments used as a transfer standard for the comparison was an Infrared Integrating Sphere instrument.

While the traceability procedure at PMOD/WRC has, to date, been based on contact thermometry and optical simulations, the HSBB and the comparison measurements performed enable a second independent path of traceability. At the same time, the agreement observed between the PMOD/WRC and PTB radiometric scales for irradiance serves to validate the previously established traceability. Discrepancies that were encountered to date in the worldwide measurements of atmospheric longwave downward radiation and that were factored into the uncertainty can now be precluded and the measurement uncertainty of atmospheric longwave downward radiation reduced.


Moritz Feierabend
Department 7.3
Detector Radiometry and Radiation Thermometry
Phone: +49 30 3481-7610
Opens local program for sending emailmoritz.feierabend(at)ptb.de

Scientific publication

M. Feierabend, J. Gröbner, I. Müller, M. Reiniger, C. Monte: Bilateral comparison of irradiance scales between PMOD/WRC and PTB for longwave downward radiation measurements. Metrologia, accepted (2022)