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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.

Project overview
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






  • Challenges

  • Large natural variability

  • Dynamic range >105

  • Radiation levels µW m-2nm-1

  • Wavelength uncertainty <0.02% (0.05 nm)

Project objectives
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

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:

  • 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
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

(Aim of the project)

Tunable Lasers

Validated in this Project

Stability controlled by

relative measurements

using portable Sources


EMRP ENV03: Traceability chain for spectral irradiance



cw-Laser Sources


Si-trap detector + aperture

Spectrally tuneable source

Filter Radiometer

Blackbody + aperture


(QASUME was calibrated directly against the blackbody in 2004)

Spectral irradiance standard


Validation of the qasume irradiance reference in 2004
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
Compact LDLS source as transfer standard

SpectralIrradianceoutput :

  • comparableto 1000 W FEL Lamp

  • Nearly Constant outputover UV range

Preliminary results

Source stability < 0.2%

Wp 2 array spectroradiometer characterisation metas
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
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
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
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)

Example for in and out range straylight
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

Modified array spectroradiometer to suppress out range radiation
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).


Uncorrected In-range stray light

Wp 3 improvement of reference spectroradiometers cmi
WP 3: radiationImprovement 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
Solid State Detector Systems (SSDS) radiation

  • 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

Wp 4 new technologies aalto
WP 4: New Technologies (Aalto) radiation

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
Task 4.2 New Diffuser design radiation

Design software

Validation through prototype measurements

Realisation and commercialisation

Wp 4 new technologies
WP 4: New Technologies … radiation

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
Knowledge Dissemination radiation

  • 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: