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An Assessment of the UV Broad Band Filter Radiometer Measurement Accuracy

An Assessment of the UV Broad Band Filter Radiometer Measurement Accuracy. A. Los 1 and J. Gröbner 2. Kipp & Zonen, Delft (The Netherlands) JRC, Ispra (Italy), now at PMOD/WRC Davos (Switzerland). Content. Introduction Calibration Methods Measurement Correction Methods

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An Assessment of the UV Broad Band Filter Radiometer Measurement Accuracy

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  1. An Assessment of the UV Broad Band Filter Radiometer Measurement Accuracy A. Los1 and J. Gröbner2 • Kipp & Zonen, Delft (The Netherlands) • JRC, Ispra (Italy), now at PMOD/WRC Davos (Switzerland)

  2. Content • Introduction • Calibration Methods • Measurement Correction Methods • Results and Conclusions

  3. 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

  4. Source: Global Solar UV Index: A Practical Guide, WHO, 2002, ISBN 92 4 159007 6 1. Introduction UV Spectrum < 320nm most important: • Determination [O3] • UV  biosphere: e.g. sun burn (Erythema)  UV Index Large intensity differences difficult to measure

  5. 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/

  6. 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-West view North-East view

  7. Broad Band Radiometers: SL501 (Solar Light) UV-S-E-T (Kipp) UVB-1 (YES) 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)

  8. 2. Calibration Method Minimum requirements for outdoor calibrations: Broad band radiometer properties to be known: • Spectral response function (max ~ 0.3 nm) • Radiometer (filter) thermostating (max 1C) • Outdoor calibration measurement conditions: • tmax ~ 5 sec. (UVS  Brewer) • Direct solar irradiances (cloud free sky, ideally)

  9. 2. Calibration Method #1 #2 #3 CIE 1987 Erythema

  10. 2. Calibration Method Radiometric Sensitivity (“calibration factor”): UUVS(): Broad band radiometer signal [V] at time  EBre(l,): UV spectrum (Brewer) [W/m2/nm] at time  SRel(l): Relative spectral response function (BB radiometer)

  11. 2. Calibration Method Uncorrected Erythema weighted irradiance (using UVS only):

  12. 2. Calibration Method Uncorrected Erythema weighted irradiance (using UVS only): • JD 160 (June 9th 2004) • Radiometric sensitivity ruvs #1, #2, #3 • Brewer derived Erythema weighted irradiance • o

  13. 2. Calibration Method • Results using ruvs only do not provide acceptable results • Measurement corrections absolutely required Typical UV spectrum Typical SRF Erythema weighting function (CIE-1987) CIE weighted UV Spectrum SRF weighted UV Spectrum

  14. 3. Measurement Correction Methods • Two Correction Methods: a) Model derived correction factors b) Observation derived correction factors • Both methods: • Corrections account for the error due to the spectral mismatch • Correction factors provided as a function of Q0 and [O3] • Corrections of radiometer cosine response function error (implicitly) included

  15. 3. Measurement Correction Method a) Model derived correction factors, UVS EMod(Q0,O3,l): Modelled UV Spectrum [W/m2/nm] SCIE(l): Erythema weighting function (CIE-1987)

  16. 3. Measurement Correction Method Corrected Erythema weighted irradiance using both, UVS and UVS:

  17. 3. Measurement Correction Method Corrected Erythema weighted irradiance using both, UVS and UVS: • JD 160 (June 9th 2004) • [O3]: 352 DU • Broad band: #1, #2, #3 • Brewer derived: o • Modelled:

  18. 3. Measurement correction methods b) Observation derived correction factor, duvs Corrected (obs. deriv.) Ery. weighted irradiance according to:

  19. rUVS ratio= rUVS(60o<Q0<65o) Slope due to cosine and/or spectral response function uncertainties. Fit function built into correction factors UVS 3. Measurement correction methods Corrections of radiometer cosine response function error

  20. 4. Results and Conclusions • Measurement accuracy using correction methods • Long term stability of radiometric sensitivity

  21. 4. Results and Conclusions Measurement accuracy using full correction • Mean (ICIE-IUVS): 0.0006 W/m2 (or 0.024 UV Index) • Standard deviation (2s): 0.003 W/m2 • Note total measurement error: s(UVS) + s(Brewer)

  22. 4. Results and Conclusions For comparison:  using mean correction factor only Mean correction factor Full correction

  23. 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%

  24. 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.

  25. END

  26. Measurement correction methods (cont.)Results:Comparison

  27. Measurement correction methods (cont.)Results:Comparison Model Corrected

  28. Measurement correction methods (cont.)Results:Comparison Observation Corrected Model Corrected

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