EMRP-ENV03: Traceability for Surface Spectral Solar Ultraviolet Radiation

• Forschungsstelle: METAS
• Projektnummer: F-5116.30004
• Projekttitel Englisch: EMRP-ENV03: Traceability for Surface Spectral Solar Ultraviolet Radiation

Schlüsselwörter (Englisch)

Radiometry, Solar Radiation Measurements, UV-Radiation

Kurzbeschreibung (Englisch)

This project is a Joint Research Project within the European Metrology Research Program framework, led by the PMOD-WRC, Davos.
The project will shorten the traceability chain of the solar UV measurements to the SI unit and reduce the associated transfer uncertainties. The goal is to approach uncertainties in the field comparable to those currently achieved only for primary spectral irradiance scale realisations at the level of Nationale Metrology Institutes, i.e., at the level of 1 %. To provide traceable solar UV irradiance measurements with an uncertainty of less than 2 %, the portable reference spectroradiometer known as ‘QASUME’ will be fitted with an improved global entrance optic and newly developed solidstate detectors. A Fourier transform spectroradiometer will be adapted for spectral solar UV irradiance measurements to demonstrate the feasibility of using this type of device as alternative reference spectroradiometer.
To support the use of cost-effective array spectroradiometers in UV monitoring networks (as replacements for current UV filter Radiometers), significant progress needs to be achieved in the characterisation of these devices. New characterisation techniques and post-correction methods will be developed to determine and correct the stray light, linearity, and wavelength scale of array spectroradiometers. This will be supported by the design and construction of novel array spectroradiometers with improved stray light characteristics based on band pass filters and micro-electro-mechanical systems (MEMS) such as digital micro mirror devices.
The dissemination of the improved irradiance traceability and the developed tools and methods for using array spectroradiometers in the solar UV range will occur by a large field inter-comparison of spectroradiometers organised at the end of the JRP at the World Radiation Centre, in Davos, Switzerland. Participants from the end-user community involved in solar UV measurements will be invited to the field intercomparison. Participating spectroradiometers from the end-users will be characterised and calibrated by the facilities developed in this project to provide traceability of spectral solar UV irradiance at this new level of uncertainty to the wider European UV monitoring community.

This project is part of the European Metrology Research Programme (EMRP, http://www.euramet.org/index.php?id=emrp); it is partly funded by the European Union on the basis of Decision No 912/2009/EC.

Projektziele (Englisch)

METAS will contribute to the following parts of the project:

• The development of a device for monitoring the responsivity of UV spectroradiometers (e.g. QASUME) over extended periods of time and also under field conditions. The device is based on UV LEDs. The monitoring techniques will cover the solar UV spectral range 280 nm to 400 nm and ensure stability and reproducibility of the spectroradiometer of ± 0.3 % for a lamp-burn time of 7 hours (i.e. representing about one year of calibrations).
• The development of stray light characterisation and correction methodologies. Stray light can originate from inside the spectral range of the solar UV spectroradiometers, or outside of the working spectral range of the instrument (but still within the sensitivity range of the sensor, typically up to 1100 nm for a silicon CCD detector). For this purpose METAS will use its pulsed and picosecond tuneable laser facility to characterise stray light in commercial diode array spectrometers.
• The development of a wavelength scale characterisation device. High spectroradiometer wavelength calibration accuracies are key parameter for solar spectral measurements. The device is based on a Fabry-Perot multilayer structure (etalon), which yields multiple sharp transmission maxima of similar intensity over the full wavelength range (280 nm to 400 nm) under broadband illumination e.g. white light source.

Abstract (Englisch)

The Joint Research Project "Traceability for surface spectral solar ultraviolet radiation" has significantly enhanced the reliability of spectral solar UV radiation in the wavelength range 300 nm to 400 nm measured at the earth surface. Within this project new methods of observation (techniques and devices) were realized to provide traceable solar UV irradiance measurements with an uncertainty of less than 2 %. METAS has been involved mainly in three parts: the wavelength and stray light characterization and the realization of an irradiance transfer standard based on UV LEDs.

• As a major achievement METAS has developed a new wavelength characterization device based on different Fabry-Perot etalons. These devices show an oscillating transmittance behaviour that can theoretically be modelled knowing the optical thicknesses of the device. If the temperature and geometrical conditions are well controlled, the devices can be used as absolute devices and deviations of the wavelength scale can be directly identified. These devices can reduce the uncertainty of the wavelength scale to below 50 pm.

• One of the highest uncertainty components in the measurements by array spectroradiometers in the solar UV spectral range is caused by a poor suppression of the internally created stray light. METAS has successfully characterized the straylight behaviour of these devices. For this purpose METAS tuneable laser source facility (LASRA) was used to generated pure spectral lines. Due to non-ideal behaviour of the spectrometers the measured spectra of the instruments exhibit straylight signals typically in the order of 10^-4. As these non-ideal responses are highly repetitive the spectrometers can be corrected for this effect. In this project numerical spectral correction algorithms were applied to lower the impact to an acceptable level (<10^-5).

• Instruments measuring solar UV irradiance are generally not stable enough in time intervals between calibrations carried out in the laboratories and the solar measurements in the field. Therefore, they typically need to be often recalibrated, also in the field. State-of-the-art is that their stability is monitored using compact low-power tungsten-halogen lamps. These are fragile and their stability is affected by aging and transportation. To overcome these limitations, a more robust monitoring technique based on UV light emiting diodes (LEDs) has been developed within the project in joint efforts between PTB, METAS and PMOD/WRC. In the first stage of the project, commercially available UV-LEDs covering the solar UV spectral range, 280 nm to 400 nm, have been selected and characterized at PTB and METAS with respect to their temporal stability, generated irradiance levels and spectral properties. The investigations revealed that the commercially available UV-LEDs after certain operating times can reach the temporal stability required for building a monitor source. The operating time needed to reach the required stability, i.e. radiant flux drifts at the level of 0.05 %/h, was found to depend on the specific LED device and especially its wavelength.

PMOD/WRC has tested the UV-LED monitoring source under different operating conditions. The performance of the shows agreement with the current portable system to within 1%. In addition to its intended use as spectral irradiance calibration device, it can also be used for wavelength stability checks, using the spectral structure of the UV-LED device. The dual use of the system was not part of the original task and was found to be very valuable in detecting wavelength shifts during calibration of the order of a few 100 ppm.

Umsetzung und Anwendungen (Englisch)

Overall, the project has produced:

• Reduced uncertainty of a primary reference spectroradiometer QASUME , which will provide improved traceability to UV monitoring stations and better understanding of decadal UV changes
• Validated fast solar UV irradiance spectra measurements, which will enable determination of cloud effects on spectral UV radiation and under rapidly changing conditions.
• Stray- light and bandwidth correction methods for characterised array spectroradiometers, which will allow the use of these robust and cost- effective instruments for UV exposure studies with known uncertainties.
• Global input optics and solid state detectors, which will allow end-users to significantly decrease the measurement uncertainties of their instrumentation.
• Extensive exchange of knowledge between the participants of project and the stakeholders mainly through a two weeks measurement campaign in summer 2014 in Davos/CH.

Publikationen / Ergebnisse (Englisch)

S.Nowy, S. Nevas, M. López, M. Lindemann, A. Sperling, P. Blattner, S.M. Foaleng, „Stability of Light- Emitting Diodes in the Solar UV Spectral Range”, AIP Conference Proceedings 1531, p 833 (2013).

S. Nowy, N. Van Hung, S. Nevas, P. Blattner, J. Gröbner, “Development of monitoring sources based on UV light- emitting diodes,” UVNews 9 , 20– 21 (2013).

S. Nevas, P. Blattner, O. El Gawhary, T. Pulli, P. Kärhä, L. Egli, and J. Gröbner, "Characterisation of nonlinearities of array spectroradiometers in use for measurements of the terrestrial solar UV irradiance," UVNews 10 (2014).

P. Blattner, S.M. Foaleng, S. van den Berg, O. ElGawhary, M. Blumthaler, J. Gröbner, L. Egli, "Devices for characterizing the wavelength scale of UV spectrometers,"UVNews 10 , 28- 29 (2014).

P.Blattner, S.M. Foaleng, S. van den Berg, O. El Gawhary, M. Blumthaler, J. Gröbner, L. Egli, “Devices for characterizing the wavelength scale of UV spectrometers,” Proceedings of NEWRAD 2014, Espoo, Finland, June 24 – 27, 2014, pp. 201 – 202.