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

Project Overview http://projects.pmodwrc.ch/env03/ Duration: August 2011 to July 2014 Total Budget: 3.9 M€ Project coordinator : Julian Gröbner Davos, Switzerland 8 Partners 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/

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

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 Primary Irradiance Standard Transfer Standard Reference Spectroradiometer End-User Devices Calibrated UV Network

Project Structure WP WP Name WP Leader Active JRP Partners WP1 Spectral Irradiance Traceability Saulius Nevas PTB, METAS, SFI Davos, VSL, Kipp WP2 Array Spectroradiometer Characterisation Peter Blattner METAS, Aalto, LNE, PTB, PMOD/WRC, VSL, REG(IMU) WP3 Improvement of Reference Spectroradiometers Marek Šmíd CMI, INRIM, PTB, PMOD/WRC WP4 New Technologies Petri Kärhä Aalto, CMI, METAS, INRIM, LNE, PTB, PMOD/WRC, CMS, Kipp, REG(IMU) WP5 Impact Julian Gröbner PMOD/WRC, All WP6 Management

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)

Project Outputs Software: Knowledge Transfer: 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

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

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

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.

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

Compact LDLS source as transfer standard Spectral Irradiance output : comparable to 1000 W FEL Lamp Nearly Constant output over UV range Preliminary results Source stability < 0.2%

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)

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

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).

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

Example for in- and out-range Straylight This Array Spectroradiometer Nominal Sensitivity: 280 – 440 nm Out-range Radiation from 440 nm to ~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

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

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

Solid State Detector Systems (SSDS) Switched Integrator Si photodiode S1227 33 BQ 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

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

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

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

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/

Spectrum of a Xe lamp with interference filter IF 406 nm Feasibility study of a Fourier transform spectrometer for solar UV irradiance measurements Entrance Key features Spectrum measured within <30 sec high wavelength accuracy High spectral Resolution Moving mirror Fix UV beam splitter UV detector Entrance --- Signal x 1000 --- Mean value Signal Ghost Ghosts HeNe laser 1000 500 333 250 200 167 l / nm -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 log10(signal) Spectrum of a Xe lamp with interference filter IF 406 nm

Solid State Detector Systems (SSDS) Switched Integrator Si photodiode S1227 33 BQ 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

Spectral stray light characterization of array spectroradiometers using tuneable lasers PLACOS setup at PTB (Pulsed Laser for Advanced Characterisation of Spectroradiometers) pulsed operation (20 Hz, 5 ns) easy to handle automatic change of wavelength OPO, optical parametric oscillator SHG, second harmonic generator * S. Nevas, M. Lindemann, A. Sperling, A. Teuber, R. Maass, “Colorimetry of LEDs with Array Spectroradiometers”, MAPAN - Journal of Metrology Society of India 24, 153-162 (2009) ** S. Nevas, G. Wübbeler, A. Sperling, C. Elster, A. Teuber, “Simultaneous correction of bandpass and stray-light effects in array spectroradiometer data”, Metrologia 49, S43-S47 (2012)

Spectral stray light characterization of array spectroradiometers using tuneable lasers Characterization and correction of UV array spectroradiometers for in-range and out-of-range stray light effects