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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