1. 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.

2. 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

3. 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)

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

5. 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

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: 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

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. 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

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
Spectral Irradiance output :
comparable to 1000 W FEL Lamp
Nearly Constant output over 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 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
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
from Zong et al, 2006
Calculated Straylight
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 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

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 ( 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)
Switched Integrator
Si photodiode S 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

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
Workshops
International Radiation Symposium, Berlin, August 2012
UVNet Workshop & ENV03 session, Davos, 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:

25. 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
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

26. Solid State Detector Systems (SSDS)
Switched Integrator
Si photodiode S 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

27. 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, (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)

28. 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