Traceability of the spectral solar UV irradiance scale to SI units

Project Description

The World Meteorological Organization has designated the European UV Calibration Center (EUVC) operated at PMOD/WRC as reference center for solar UV Radiation measurements in Europe. An important task of the EUVC is to maintain a stable reference for spectral irradiance over long time scales to allow the detection of long-term changes of solar UV radiation.

The traceability to SI units is obtained through the calibration of double monochromator spectroradiometers with secondary transfer standards realised by 1000 W tungsten halogen lamps. Since one of the tasks of the EUVC is to perform site audits at UV monitoring stations in Europe, a portable reference spectroradiometer system was developed which uses a portable calibration system to transfer the home calibration to field sites. Currently the EUVC irradiance reference is realised through the average of seven secondary standard lamps calibrated by PTB between 2002 and 2009 against the primary reference for spectral irradiance, blackbody BB3200pg. The long-term stability of the EUVC reference is checked by yearly measurements of these lamps. In addition, lamps belonging to the EUVC reference are re-calibrated by PTB on a regular basis.

The proposed project aims at:

1) Investigating the long-term stability of the EUVC irradiance reference by direct comparison of this reference to the primary spectral irradiance reference of the PTB.

2) Providing direct traceability of the EUVC irradiance reference to the SI unit of Wm-2nm-1 by measurements of the primary irradiance reference BB3200pg of the PTB with the portable EUVC reference spectroradiometer system.

3) Decreasing the present uncertainty of the EUVC irradiance reference by reducing the calibration chain between the EUVC spectroradiometer system and the primary spectral irradiance reference.


Progress Report 2019-01-19

The results fo the 2013/2014 calibration of the portable reference spectroradiometer QASUME at PTB showed slight differences between the tunable laser and high temperature blackbody based realisation of spectral irradiance. The differences are within the uncertainties of the measurements. Nevertheless, it is planned to repeat the measurements with an improved QASUME system to reduce the uncertainties and hopefully be able to resolve the observed differences. The work is aimed at taking place at an improved facility at PTB in 2019 or 2020.

During a guest scientist stay at PTB, the spectral irradiance of a transfer standard calibrated at the facilities of PMOD/WRC were directly compared to the spectral irradiance of the primary spectral irradiance standard of the PTB. The agreement between the two calibrations was well within the measurement uncertainties, with an average difference of 0.1% over the wavelength range 250 nm to 1200 nm.


Progress Report 2018-02-23

The results for the 2013/2014 calibration of the portable reference spectroradiometer QASUME at PTB showed slight differences between the tunable laser and high temperature blackbody based realisation of spectral irradiance. The differences are within the uncertainties of the measurements. Nevertheless, it is planned to repeat the measurements with an improved QASUME system to reduce the uncertainties and hopefully be able to resolve the observed differences. The work is aimed at taking place at an improved facility at PTB in 2019.


Progress Report 2017-01-04

A publication describing the improved uncertainty in solar UV irradiance measurements achieved with the QASUME spectroradiometer has been published in the peer-reviewed literature:

1. Hülsen, G., J. Gröbner, S. Nevas, P. Sperfeld, L. Egli, G. Porrovecchio, and M. Smid, Traceability of solar UV measurements using the QASUME reference spectroradiometer, Applied Optics, 55, 26, 7265-7275, 2016.

The new uncertainties represent a significant improvement of nearly a factor 2. In view of these results, PMOD/WRC would like to submit a new CMC for global solar irradiance measurements, as well as update existing CMCs to reflect the new reduced uncertainties.

Plans for 2017: None so far.


Progress Report 2016-01-04

In the frame of the EMRP Project SolarUV, 2 quality assurance / Calibration campaigns were performed at the premises of PTB to validate the spectral irradiance scale realised at PMOD/WRC using transfer standard tungsten halogen lamps. The campaigns were held in 2013 and 2014, and produced consistent results to within the uncertainties between the calibrations performed at PMOD/WRC and those obtained at PTB using the primary spectral irradiance standard. Furthermore, the tunable laser system TULIP was used to reduce the traceability chain from the primary radiation standard to the QASUME spectroradiometer. A publication describing the results is in progress.

Subjects
Photometry and Radiometry (PR)
Coordinating Institute
PMOD/WRC (Switzerland)
Participating Partners
PTB (Germany)