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.

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

Project Overview

http://projects.pmodwrc.ch/env03/

Motivation

Challenges

Large natural variability

Dynamic range >105

Radiation levels µW m-2nm-1

Wavelength uncertainty <0.02% (0.05 nm)

500

10

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

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 Julian Gröbner PMOD/WRC, All

Project Structure

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

Project Outputs

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

WP 1: Spectral Irradiance Traceability (PTB)

Cryogenic radiometerCryogenic radiometer

Spectral irradiance standard

Spectral irradiance standard

cw-Laser Sources cw-Laser Sources

Si-trap detector + apertureSi-trap detector + aperture

Filter Radiometer Filter Radiometer

Blackbody + aperture Blackbody + aperture

Detector

Source

SpectroradiometerSpectroradiometer

Spectrally tuneable source Spectrally tuneable source

QASUME QASUME

EMRP ENV03: Traceability chain for spectral irradiance

(QASUME was calibrated directly against the blackbody in 2004)

Stability controlled by relative measurements using portable Sources

(Aim of the project)

Tunable LasersTunable Lasers

QASUME QASUME

Validated in this Project

Validation of the QASUME irradiance reference in 2004

blackbody BB3200pg at PTB

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.

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%

0 20 40 60 80 100 120Burning time / h

0,94

0,96

0,98

1,00

Nor

mal

ized

sig

nal

2,0010-2

2,0110-2

2,0210-2

2,0310-2

2,0410-2

2,0510-2

I / A

6,6

6,8

7,0

7,2U

/ V: E

: I

: U

Portable sources using UV-LEDs

210-4 h-1

Design goal: aging rate of 0.05 %h-1

Compact LDLS source as transfer standard

Source stability < 0.2%

Spectral Irradiance output :

•comparable to 1000 W FEL Lamp

•Nearly Constant output over UV range

Preliminary results

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 Spectroradiometer characterisation (METAS)

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)

WP 2: Array Spectroradiometers …

Mica based Fabry Perrot

Effect of stray light on solar irradiance measurements

array spectroradiometer

Double monochromator

In-range Straylight

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).

Array Spectroradiometer with nominal wavelength range 280-440 nm.

Stray light correction procedure for array spectroradiometer

Slit Functions obtained from tunable laser setup (PLACOS-PTB)

1IB meas meas

Y A Y C Y

from Zong et al, 2006

In-range straylight matrix

Calculated Straylight

Example for in- and out-range Straylight

Raw MeasurementOut-range Corrected

In&Out-range Corrected

This Array Spectroradiometer

•Nominal Sensitivity: 280 – 440 nm•Out-range Radiation from 440 nm to ~1100 (Silicon) nm

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

Ratio to Double Monochromator

Double MC

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

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

WP 3: Improvement of Reference Spectroradiometers (CMI)

Solid State Detector Systems (SSDS)

Si photodiode S1227 33 BQ

Noise Equivalent Power measured with V/I gain of 1011 (0.1 s)

Calculated SSDS noise performances for QASUME typical UV solar spectral

measurement

+

-

iD

iL

CINT

Vout

Reset

Rsh

iL - iD Hold Switched Integrator

ININT

INTout I

C

ttV

1 % at 298 nm

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

WP 4: New Technologies (Aalto)

Task 4.2 New Diffuser design

1) Design software

2) Validation through prototype

measurements

3) Realisation and commercialisation

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

WP 4: New Technologies …

• 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:

Knowledge Dissemination

http://projects.pmodwrc.ch/env03/