Solar radiation calibration scale change

WRR solar irradiance scale is expected to go down

WRR contains a systematic error relative to SI. Therefore, the scale will be adapted. It will likely shift by 0.34 %. The change is relevant to climatology, solar resource assessment and solar system performance assessment.


The WRR (World Radiometric Reference) has been used since 1980 as the scale representing the physical units of solar irradiance.

PMOD: designated institute under METAS 

The WRR scale is maintained by PMOD. The formal status of PMOD in international metrology is “designated institute”. For details see the website of the Swiss Institute of Metrology METAS. In other words, PMOD is responsible maintenance of the highest standard for solar irradiance measurement with pyranometers and pyrheliometers.

What will happen to WRR

It has become clear that WRR contains a systematic error relative to SI. The WRR scale will be adapted to the new latest insights. It will likely shift by 0.34 %.

The WRR scale was too high, which means that all irradiance measured with traceability to WRR must be corrected to lower values in [W/m2]. With the correction from WRR to the new scale, the sensitivities of instruments in [V/(W/m2)] will go up by 0.34 %.

The name of the new scale is not yet known.

WMO does not recommend to correct past data. It does however recommend to store information with data obtained with pyranometers and pyrheliometers about the scale that is used. The decision on introduction of the new scale will be made at the WMO congress in 2027.

Literature reference for scale change

A reference for the proposed scale change is: Fehlmann A., Kopp G., Schmutz W. et al. Fourth World Radiometric Reference to SI radiometric scale comparison and implications for on-orbit measurements of the total solar irradiance. Metrologica 49 (2012) p34-38

PMOD pyranometer calibration*
Calibrated quantityresponsivity to solar irradiance
Calibrated instrumentpyranometer
Uncertainty0.66 %
Calibration methodpyranometer comparison to pyrheliometer
Calibration conditions
  • instrument tilt 0.0 ˚
  • angle between normal of receiver surface and solar beam: 24 ˚ to 50 ˚
  • global solar irradiance : 800 W/m2 to 1000 W/m2
Calibration referenceWorld Radiometric Reference (WRR)
Type of sourcesun and clear sky
* Reference: BIPM website

What we do at Hukseflux to prepare

Hukseflux in its latest industrial pyranometers allows to add metadata, to indicate if calibration is traceable to WRR or to the new scale, to on-board calibration records and to on-board records of calibration history. All these records are stored in the instrument.

Download the PDF en read about solar radiation calibration scale change.