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

Content 1.Introduction 2.Calibration Methods 3.Measurement Correction Methods 4.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 [O 3 ] –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

1. Introduction Source: Global Solar UV Index: A Practical Guide, WHO, 2002, ISBN ,

2. Calibration Method Location: JRC, Italy ( 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  1C) 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 #3 CIE 1987 Erythema

2. Calibration Method Radiometric Sensitivity (“calibration factor”): U UVS (  ): Broad band radiometer signal [V] at time  E Bre (  ): UV spectrum (Brewer) [W/m 2 /nm] at time  S Rel (): Relative spectral response function (BB radiometer)

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

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

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

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  0 and [O 3 ] –Corrections of radiometer cosine response function error (implicitly) included

3. Measurement Correction Method a) Model derived correction factors,  UVS E Mod (   O 3,): Modelled UV Spectrum [W/m 2 /nm] S CIE (): Erythema weighting function (CIE-1987)

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

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

3. Measurement correction methods 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 (60 o < 0 <65 o )  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 (I CIE -I UVS ): W/m 2 (or UV Index) Standard deviation (2): W/m 2 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.

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Measurement correction methods (cont.) Results:Comparison

Model Corrected

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