Project Overview

1. EMRP ENV03 “Traceability for surface spectral solar ultraviolet radiation”Julian GröbnerPhysikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos SwitzerlandThe European Metrology Research Programme (EMRP) is jointly funded by the EMRP participating countries within EURAMET and the European Union.

2. Project Overview • Duration: August 2011 to July 2014 • Total Budget: 3.9 M€ • Project coordinator : Julian Gröbner Davos, Switzerland • 8Partners from European Metrology Institutes and 2 from Industry • 4 Researcher Excellence Grants (REG) from Universities & Health Institute • 5 Collaborators, open-call http://projects.pmodwrc.ch/env03/

3. 500 Motivation 10 • Challenges • Large natural variability • Dynamic range >105 • Radiation levels µW m-2nm-1 • Wavelength uncertainty <0.02% (0.05 nm)

4. Project Objectives • Enhance the reliability of spectral solar UV radiation measured at the Earth surface • Improved SI traceability, improved methodologies, improved devices • Uncertainties better than 2% in the wavelength region 300 nm – 400 nm (current state of the art is 5%) • Develop techniques and devices for using cost-effective array-spectroradiometers for solar UV measurements Calibrated UV Network End-User Devices Transfer Standard Reference Spectroradiometer Primary Irradiance Standard

5. Project Structure

6. Project Outputs • Devices: • UV LED-based transfer standards (WP 1) • Transfer standard based on Laser Driven Light Source LDLS (WP 1) • Wavelength scale characterisation devices (WP2) • Solid state detectors to replace PMTs (WP 3) • Modified Fourier-Transform Spectrometer (WP 3) • UV hyperspectral imaging camera (sky radiance) (WP 4) • Global input optics (improved diffuser design) (WP 4) • Two array spectroradiometers optimized for UV (WP 4)

7. Project Outputs • Software: • Tool to determine the uncertainty budget for array spectroradiometers (WP 2) • Tool for bandwidth and wavelength homogenisation and stray light correction (WP 2) • Knowledge Transfer: • Guidelines (WP 2) • Conference Presentations (WP 5) • Technical Workshops (WP 5) • Refereed publications (WP 5) • Intercomparison campaign at Davos

8. WP 1: Spectral Irradiance Traceability (PTB) Goal: Shorten the traceability chain of solar UV measurements to SI units and reduce transfer uncertainties (U = 1 - 2%) 1 Detector-based traceability chain using an absolute radiometer and tunable UV laser facility (PTB) • Tuneable laser source 280 – 400 nm • Traceability to the primary standard cryogenic radiometer via a trap detector 2 Development of stable, portable and robust reference sources based on UV-LEDs (PTB) • For monitoring purposes, near-field conditions • Replacement of halogen lamps susceptible to transportation and aging 3 Compact laser-induced UV source as transfer standard (VSL) • Laser Driven Light Source (LDLS™) from Energetic

9. (Aim of the project) Tunable Lasers Validated in this Project Stability controlled by relative measurements using portable Sources QASUME EMRP ENV03: Traceability chain for spectral irradiance Cryogenicradiometer Detector cw-Laser Sources Source Si-trap detector + aperture Spectrally tuneable source Filter Radiometer Blackbody + aperture Spectroradiometer (QASUME was calibrated directly against the blackbody in 2004) Spectral irradiance standard QASUME

10. Validation of the QASUME irradiance reference in 2004 blackbody BB3200pg at PTB Measurement of BB3200pg at PTB on 15 June 2004 Expanded uncertainty of PTB transfer standards±3% New expanded uncertainty of the QASUME irradiance reference (based on these blackbody measurements) ±2% Gröbner J., and P. Sperfeld, Direct traceability of the portable QASUME irradiance scale to the primary irradiance standard of the PTB, Metrologia, 42, 134—139, 2005.

11. Portable sources using UV-LEDs Design goal: aging rate of 0.05 %h-1 210-4 h-1

12. Compact LDLS source as transfer standard SpectralIrradianceoutput : • comparableto 1000 W FEL Lamp • Nearly Constant outputover UV range Preliminary results Source stability < 0.2%

13. WP 2: Array Spectroradiometer characterisation (METAS) Goal: New characterisation techniques for the most relevant uncertainty components – stray light, bandwidth, linearity, wavelength 1 “A guide to measuring solar UV spectra using array spectroradiometers” (IMU) • Specification of array spectrometers to meet the requirements for solar UV measurements • Recommended measurement sequences for typical measurement setup • A standardized protocol for saving measurement data, and ancillary information 2 “Uncertainty estimation in array spectroradiometer measurements of Solar UV spectra” (LNE) • Guideline, software and methodology 3 Stray light characterisation and correction methods (PTB)

14. WP 2: Array Spectroradiometers … 4 Development of two wavelength scale characterisation devices (METAS) • For scanning and array spectroradiometers • U = 0.01 nm, wavelength 280 nm - 400 nm. • 1. Fabry-Perot etalon (METAS) • 2. Polarisation gradient filter (VSL) 5 Linearity of array spectroradiometers (PTB) • Three different procedures and measurement setups for linearity characterisation of array spectroradiometers (broad-band source, monochromator-based and tunable laser source) Mica based Fabry Perrot

15. Effect of stray light on solar irradiance measurements Array Spectroradiometer with nominal wavelength range 280-440 nm. In-range Straylight array spectroradiometer Double monochromator Out-range Straylight Detector arrays are made from silicon (spectral sensitivity up to 1100 nm) and are therefore sensitive to radiation which is not meant to fall on the detector (out-range straylight).

16. Stray light correction procedure for array spectroradiometer In-range straylight matrix Calculated Straylight from Zong et al, 2006 Slit Functions obtained from tunable laser setup (PLACOS-PTB)

17. Example for in- and out-range Straylight This Array Spectroradiometer • Nominal Sensitivity: 280 – 440 nm • Out-range Radiation from 440 nmto ~1100 (Silicon) nm Raw Measurement Out-range Corrected In&Out-range Corrected Double MC Ratio to Double Monochromator NOTE: Out-range Stray-Light Correction requires knowledge of the spectral radiation distribution which is not measured by the instrument itself!! Correction works, but is very complex

18. Modified array Spectroradiometer to suppress out-range radiation We placed a DUG11X solarblind filter in the beam path to suppress out-range radiation in the sensi-tivity range of the silicon CCD detector (390-1100 nm). UG11X Uncorrected In-range stray light

19. WP 3: Improvement of Reference Spectroradiometers (CMI) Goal: New detection systems and entrance optics for scanning spectroradiometers to achieve field measurement uncertainties of 2% for solar UV measurements 1 New detection system for reference scanning spectroradiometers (CMI) • Solid state detectors (Si, SiC, ZnO) and switched integrator amplifier • High sensitivity, high dynamic range, low noise • Substitute to PMT 2 Validation of optimised transportable QASUME reference spectroradiometer (PMOD/WRC) • New Detector-System • New Entrance Optic with improved Cosine response • Improved traceability to SI and stability check using UV LEDs 3 Adaptation of a Fourier-transform spectroradiometer as reference instrument for solar UV irradiance measurements (PTB) • Evaluate suitability of Fourier -transform spectroradiometer as a reference instrument for solar UV irradiance measurements

20. Solid State Detector Systems (SSDS) • Si photodiode S1227 33 BQ Switched Integrator Calculated SSDS noise performances for QASUME typical UV solar spectral measurement Noise Equivalent Power measured with V/I gain of 1011 (0.1 s) 1 % at 298 nm

21. WP 4: New Technologies (Aalto) 1 Realisation of a UV hyperspectral camera (INRIM) • Imaging device for spectral UV sky radiance measurements • Fish-eye UV collection optics • Scanning Fabry-Perot device • Improve cosine correction methods 2 Improved entrance optics for global solar UV spectroradiometers (Aalto) • Cosine error less than ±1 % downto 80° • Material studies and design software • Study new fused silica-based diffuser materials • Two designs for Brewer and fiber coupled optics

22. Task 4.2 New Diffuser design Design software Validation through prototype measurements Realisation and commercialisation

23. WP 4: New Technologies … 3 Array spectroradiometer with improved stray light rejection using adaptive optics (CMI) • Studies and comparison of methods • MEMS tuneable grating technology • Digitally modulated micro mirror devices (DMD). • Prototype of improved spectrograph 4 Array spectroradiometer with improved stray light rejection using band pass filters (LNE) • Jobin-Yvon spectroradiometer optimized for solar UV measurements • Target value for stray light rejection using a tailored band pass filter 106

24. Knowledge Dissemination • UVNET Mailing list at http://metrology.tkk.fi/uvnet/source/lists.html • Workshops • International Radiation Symposium, Berlin, August 2012 • UVNet Workshop & ENV03 session, Davos, 27-28 August 2013 • Spectral solar UV Intercomparison at Davos & Final ENV03 Workshop, 2 Weeks in June/July 2014 Presentations, Guidelines, Publications can be found at the project web-site: http://projects.pmodwrc.ch/env03/