An Assessment of the UV Broad Band Filter Radiometer Measurement Accuracy
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Transcript of An Assessment of the UV Broad Band Filter Radiometer Measurement Accuracy
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
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Measurement correction methods (cont.)Results:Comparison
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