An Assessment of the UV Broad Band Filter Radiometer
Measurement Accuracy
A. Los1 and J. Gröbner2
1) Kipp & Zonen, Delft (The Netherlands)2) JRC, Ispra (Italy), now at PMOD/WRC Davos (Switzerland)
Content
1. Introduction2. Calibration Methods3. Measurement Correction Methods4. Results and Conclusions
1. Introduction
• What to achieve: The accurate measurement of Erythema weighted (CIE-1987) UV irradiance using broad band filter radiometers.
• Which accuracy level: (broad band) 1.05 * (spectral)
• How: – Perform careful outdoor calibrations– Apply dedicated measurement corrections
1. Introduction
UV Spectrum < 320nm most important:
– Determination [O3]
– UV biosphere:
e.g. sun burn (Erythema)
UV Index
Large intensity differences difficult to measure
Source: Global Solar UV Index: A Practical Guide, WHO, 2002, ISBN 92 4 159007 6
1. Introduction
Source: Global Solar UV Index: A Practical Guide, WHO, 2002, ISBN 92 4 159007 6, http://www.who.int/uv/publications/globalindex/en/
2. Calibration Method
• Location: JRC, Italy (45.803 N, 8.627E)• Elevation: 240 m.a.s.• Duration: since July 2003• Instruments: Brewer MK III (ECUV), broad band UV radiometers
North-East view North-West view
2. Calibration Method
Radiometers used in this study:• 1 x Brewer MKIII No. 163 spectrophotometer• 3 x UV broad band filter radiometers
1 x SL501 (Solar Light) 1 x SET (Kipp & Zonen)1 x UVB-1 (YES Inc.)(referred to as #1, #2, and #3)
UVB-1 (YES)UV-S-E-T (Kipp)SL501 (Solar Light)
Broad Band Radiometers:
2. Calibration Method
Broad band radiometer properties to be known:– Spectral response function (max ~ 0.3 nm)
– Radiometer (filter) thermostating (max 1C)
Outdoor calibration measurement conditions: max ~ 5 sec. (UVS Brewer)
– Direct solar irradiances (cloud free sky, ideally)
Minimum requirements for outdoor calibrations:
2. Calibration Method
#1
#2
#3CIE 1987Erythema
2. Calibration Method
Radiometric Sensitivity (“calibration factor”):
400
280
Re )(),(
)(
dSE
U
lBre
UVSUVS
UUVS(): Broad band radiometer signal [V] at time
EBre(): UV spectrum (Brewer) [W/m2/nm] at time SRel(): Relative spectral response function (BB radiometer)
2. Calibration Method
UVS
UVSUVS
UI
Uncorrected Erythema weighted irradiance (using UVS only):
2. Calibration Method
• JD 160 (June 9th 2004)• Radiometric sensitivity
uvs
#1, #2, #3
• Brewer derived Erythema weighted irradiance– o
UVS
UVSUVS
UI
Uncorrected Erythema weighted irradiance (using UVS only):
2. Calibration Method
• Results using uvs only do not provide acceptable results
• Measurement corrections absolutely required
Spectral Response Funtion (BB) and CIE-1987
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
280 300 320 340 360 380 400
Wavelength [nm]
Rel
ativ
e Va
lues
Typical SRF
Typical UV spectrum
Erythema weightingfunction (CIE-1987)
CIE weightedUV Spectrum
SRF weightedUV Spectrum
3. Measurement Correction Methods
• Two Correction Methods:a) Model derived correction factorsb) Observation derived correction factors
• Both methods: – Corrections account for the error due to
the spectral mismatch– Correction factors provided as a function
of 0 and [O3]
– Corrections of radiometer cosine response function error (implicitly) included
3. Measurement Correction Method
a) Model derived correction factors, UVS
UVS(0,O3) EMod (0,O3,)SRe l () d
280
400
EMod (0,O3,)SCIE () d280
400
EMod(O3,): Modelled UV Spectrum [W/m2/nm]
SCIE(): Erythema weighting function (CIE-1987)
3. Measurement Correction Method
IUVS UUVS
UVS UVS (0,O3)
Corrected Erythema weighted irradiance using both, UVS and UVS:
3. Measurement Correction Method
• JD 160 (June 9th 2004)– [O3]: 352 DU
• Broad band:#1, #2, #3
• Brewer derived: o• Modelled:
IUVS UUVS
UVS UVS (0,O3)
Corrected Erythema weighted irradiance using both, UVS and UVS:
3. Measurement correction methods
IUVS UUVS
UVS (0,O3)
Corrected (obs. deriv.) Ery. weighted irradiance according to:
b) Observation derived correction factor, uvs
3. Measurement correction methods
Corrections of radiometer cosine response function error
UVS(60o<0<65o)
UVS
ratio=
Fit function built into
correction factors UVS
Slope due to cosine and/or spectral response function uncertainties.
4. Results and Conclusions
• Measurement accuracy using correction methods
• Long term stability of radiometric sensitivity
4. Results and Conclusions
• Mean (ICIE-IUVS):
0.0006 W/m2
(or 0.024 UV Index)
• Standard deviation (2):
0.003 W/m2
• Note total measurement error: (UVS) + (Brewer)
Measurement accuracy using full correction
4. Results and Conclusions
For comparison: using mean correction factor only
Full correction
Mean correction factor
4. Results and Conclusions
Long term stability of radiometric sensitivity
Changes of radiometric sensitivity per year:
#1: (-11.0%)
#2: +2.0%
#3: -1.0%
Conclusions
• Results are promising, show good agreement with Erythema weighted spectral measurements.
• Accurate UV Index possible with corrected broad band UV radiometer measurements.
• Careful calibration against well maintained spectrophotometer recommended.
• Measurement correction absolutely required.• Regular re-calibration and spectral response
function measurement highly recommended.• Future plans: perform similar measurements at
other location.
END
Measurement correction methods (cont.)Results:Comparison
Measurement correction methods (cont.)Results:Comparison
Mod
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Measurement correction methods (cont.)Results:Comparison
Ob
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Corr
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Mod
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