Effect of ambient temperature onRobertson–Berger-type erythemal dosimeters

Martin Huber, Mario Blumthaler, Josef Schreder, Alkis Bais, andChrysanthi Topaloglou

To quantify the effect of ambient temperature on the voltage signal of Solar Light UV-Biometers, spectralresponse functions of two instruments were determined in the laboratory under various external tem-perature conditions. Despite the biometer’s internal temperature stabilization, a temperature increaseof 20 °C at the outside of an instrument’s housing resulted in a reduction of the instrument’s spectralresponse by as much as 10% in the UVB range and by as much as a factor of 2 in the UVA range,depending on the individual instrument and on its internal relative humidity. The significance of thiseffect for outdoor measurements is demonstrated by data from an intercomparison campaign of erythe-mal radiometers in Thessaloniki, Greece, organized by the Laboratory of Atmospheric Physics �AristotleUniversity of Thessaloniki�, the Cooperation in Science and Technology �European Commission�, and theWorld Meteorological Organization. On 16 September 1999, 12 of 16 Solar Light Biometers showedsignificant diurnal variation in their sensitivity �as much as 10% for some individual instruments�, whichcan be explained through a heating of the instruments’ housings due to direct solar radiation. © 2002Optical Society of America

OCIS codes: 120.0120, 120.3930, 120.5630.

1. Introduction

As a result of public and scientific demand forground-based solar UV measurements, the number oferythemal dosimeters in routine operation is increas-ing on a worldwide scale. As recommended by theWorld Meteorological Organization1 �WMO�, the so-called UV index is generally accepted as a unit forbiologically relevant UV irradiance. Its definition isbased on erythemally weighted global solar UV irra-diance from the upper hemisphere.2

The Solar Light UV Biometer model 501 solar lightUV biometer is one of the most common detectorsused for determination of the UV index. Because ofthe large number of instruments in routine operationworldwide, the sensitivity of this type of detector toenvironmental parameters such as ambient temper-

M. Huber �martin.huber@uibk.ac.at� and M. Blumthaler arewith the Institute for Medical Physics, University of Innsbruck,Muellerstrasse 44, 6020 Innsbruck, Austria. J. Schreder is withCalibration Measurement Software Solutions, Oberndorf 116,6322 Kirchbichl, Austria. A. Bais and C. Topaloglou are with theLaboratory for Atmospheric Physics, Aristotle University of Thes-saloniki, Campus Box 149, 54006 Thessaloniki, Greece.

Received 5 November 2001; revised manuscript received 1 Feb-ruary 2002.

0003-6935�02�214273-05$15.00�0© 2002 Optical Society of America

ature and humidity is an important issue with re-spect to UV data consistency.

2. Materials and Methods

Under the assumption of linearity, a broadband de-tector is generally characterized by its relative spec-tral response function r��� and its calibration factor c,so the relation between spectral irradiance I��� andthe detector’s voltage signal S is given by3

S �1c �I���r���d�. (1)

As with all Robertson–Berger-type detectors, the So-lar Light Biometer is designed to show a spectralresponse function that is close to the erythemal ac-tion spectrum in the UVB range as well as in the UVArange �Fig. 1�. Although differences among individ-ual instruments can be observed,4 this goal isachieved to a large extent by combination of an OG11filter, a layer of phosphorescent material �magnesiumtungstate �MgWO4��, a green filter, and a photo-diode.5 The instrument’s operational range of am-bient temperature is specified from �40 to 50 °C.The instrument is equipped with an internal temper-ature sensor and a Peltier element6 to compensatefor temperature effects observed with previousRobertson–Berger-type instruments.7 During all

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indoor and outdoor investigations described in thispaper the internal temperature regulation was set to25 °C. According to the internal sensor, this tem-perature was maintained within �0.5 °C.

A Pt100 sensor was used to record the temperatureat the outside of the instrument’s housing. As thevoltage signal of a Solar Light Biometer is alreadyknown to depend on internal relative humidity,8 anadditional capacitive humidity sensor was mountedat the inner side of the instrument’s bottom plate tomonitor internal relative humidity. Humidity con-ditions inside the detector were varied by successiveexchange of dessicant and by means of a moist spongeplaced into the mounting of the dessicant’s container.

The biometer’s spectral response function was de-termined with quasi-monochromatic light providedby a 1000-W xenon lamp and a Bentham DM150double monochromator. Light from the xenonlamp was passed through a 10-cm-thick water filterin front of the DM150’s entrance slit to absorb in-frared radiation and thus to prevent heating of themonochromator’s optical elements. The biometerwas attached to the DM150’s exit slit together witha reference diode used to correct for temporalchanges in the lamp’s spectral light output. TheDM150’s entrance and exits slits were adjusted to1.25 mm, corresponding to a triangular slit functionof 4-nm full width at half-maximum. The biome-ter’s voltage signal was recorded while the DM150DM’s wavelength setting was changed from 265 to400 nm in steps of 1 nm. To investigate the effectsof ambient temperature on the biometer’s spectralresponse function we heated the instrument with a

warm-air flow. Measurements of the instrument’sspectral response function were taken after a periodof at least 2 h, during which the temperature at theoutside of the instrument’s housing remained sta-ble within �1 °C. The constant readout of the bi-ometer’s internal temperature sensor confirmed thestability of the temperature gradient between theinstrument’s housing and its internal Peltier ele-ment. Special care was taken to isolate the biom-eter thermally from the DM150 and the referencediode. Thus differences between the DM150 DM’stemperature and the laboratory room temperatureamounted to less than �1°C.

For each wavelength setting of the DM150 DM wedetermined the intensity that was illuminating thebiometer with an absolutely calibrated BenthamDTM300 double monochromator. The sameDTM300 DM was used for outdoor measurements,now equipped with a Teflon diffuser specially shapedfor optimum cosine response. During several inter-national intercomparison campaigns, the DTM300DM proved to provide reliable spectral data of solarUV irradiance within �3%.9 Its calibration is ulti-mately traceable to the Physikalisch TechnischeBundesanstalt in Braunschweig, Germany.

3. Results

The influence of changes in ambient temperature onthe spectral response functions of Biometer 910 andBiometer 1240 at different levels of internal rela-tive humidity is shown in Fig. 2 �top�. Althoughdifferences among the individual instruments canbe seen, in each case an increase of ambient tem-perature results in a decrease of the spectral re-sponse function, which is more significant in theUVA range. Heating the detector’s exterior fromroom temperature to 40 °C reduces UVB sensitiv-ity by as much as 10% and UVA sensitivity by asmuch as 60%.

The significance of these results for outdoor mea-surements can be estimated by wavelength integra-tion of an actual solar spectrum weighted with thespectral response functions at different tempera-tures. Figure 2 �bottom� shows the ratios of theseintegral values for spectral solar irradiance datarecorded with the DTM300 during an intercompari-son of erythemal radiometers in Thessaloniki,Greece, organized by the Laboratory of AtmosphericPhysics �LAP; Aristotle University of Thessaloniki�,the Cooperation in Science and Technology �COST;European Commission�, and the World Meteorolog-ical Organization �WMO�. Data were taken on 16September 1999 at an altitude of 60 m above sealevel under cloudless conditions.4 The effect of anincrease in ambient temperature by 20 °C on thedetector’s voltage signal during outdoor measure-ments is dominated largely by the effect of temper-ature increase on the detector’s spectral responsefunction in the UVB range, leading to a generaldecrease of the detector’s sensitivity by as much as10%. At low solar elevations, when the relativecontribution of UVB irradiance to the biometer’s

Fig. 1. Spectral response functions of Solar Light �SL� Biometer910 �at 27 °C ambient temperature and 17% internal relative hu-midity� and of Biometer 1240 �at 21 °C ambient temperature and17% internal relative humidity�. For comparison with the ery-themal action spectrum, the spectra have been normalized to avalue of 1 at their maxima.

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voltage signal is reduced by long optical pathsthrough the ozone layer, changes of the responsefunction in the UVA range further reduce the in-strument’s sensitivity.

These results were confirmed by comparison oferythemal irradiance data simultaneously deter-mined by Biometer 910 and the Bentham DTM300at Jungfraujoch, Switzerland �3576 m above sealevel� on 30 July 2001 under clear-sky conditions.During a period of 90 min the biometer was exposedto a flow of warm air that increased the tempera-ture outside the biometer’s housing from 18 to47 °C. As a consequence, the biometer’s voltageoutput relative to the DTM300 data gradually de-creased by 12%.

The significance of the Solar Light Biometer’ssensitivity to ambient temperature for instrumentsin routine operation is demonstrated in results fromthe LAP�COST�WMO intercomparison of erythe-mal radiometers made at Thessaloniki, Greece, on16 September 1999. At the beginning of the cam-paign, every radiometer’s spectral response func-tion was determined at the Laboratory forAtmospheric Physics �Aristotle University of Thes-

saloniki�, and during the succeeding outdoor inter-comparison the Bentham DTM300 was used as areference. Of 16 Solar Light Biometers, 12 showeda pronounced decrease in the ratio between theirvoltage signals and integrated spectra of solarglobal irradiance, weighted with the respective de-tector’s spectral response function. As an extremeexample, for two biometers the reduction of thisrelation at 20° solar elevation in the afternoon rel-ative to that in the morning was 10%. This asym-metry vanished near solar noon, an effect that canbe explained by a decreasing difference between thebiometers’ temperatures at the same solar eleva-tion. In contrast, two other Solar Light Biometersshowed changes of not more than 5% during the day�Fig. 3�. Diurnal asymmetry effects for the otherbiometers had values between these extremes, andnone of the instruments showed a diurnal increase inits relation to the DTM300.4 The stability of theDTM300 within �2% was confirmed by comparisonwith spectral measurements simultaneously re-corded by a Brewer MkIII double monochromator.

Interpretation of these diurnal variations as be-ing the result of a gradual temperature increase of

Fig. 2. Effects of ambient temperature on the spectral response functions of Solar Light Biometer 1240 at 40% and 17% internal relativehumidity �INT. rH� and of solar light biometer 910 at 17% internal relative humidity �top�, described by ratios relative to the correspondingspectral response functions at room temperature. Bottom, resultant effects on the wavelength integral of global solar irradiance�determined on 16 September 1999 in Thessaloniki, Greece�, spectrally weighted with the respective spectral response function.

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the instruments’ housings during the day is af-firmed by the fact that these variations were lesspronounced on 17 and 18 September 1999 underovercast sky, a condition for which smaller temper-ature changes as a result of less intensity in directsolar radiation can be expected �Fig. 3�. For com-parison, the diurnal increase of air temperaturefrom 10.00 am to 17.00 pm local summer timeamounted to 4.9 °C on 16 September and to only 2.2and 0.6 °C on 17 and 18 September, respectively.On 17 and 18 September under changing cloudi-ness, the biometer’s cosine error caused additionalasymmetry effects between morning and afternoonof as much as �3%. This value is in agreementwith model calculations for instruments with angu-lar response functions comparable to the biometer’sangular response function.10

4. Summary

In several cases the Solar Light Biometer’s internaltemperature control proved insufficient to prevent aneffect of ambient temperature on the Solar Light Bi-ometer’s voltage signal. Depending on the individ-ual instrument and its internal humidity status, atemperature increase of 20 °C at the outside of theinstrument’s housing might result in a decrease ofthe instrument’s sensitivity by as much as 10%. Toprevent this effect we recommend use of additionalthermal insulation, external temperature stabiliza-tion, or both. Furthermore, the dessicant should bereplaced at regular intervals, as laboratory measure-ments indicate that the temperature effect is lesssevere at low internal relative humidity. Possibleconsequences of this temperature effect on existingbiometer networks with respect to seasonal bias of

climatological UV data are the focus of our furtherinvestigations.

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Fig. 3. Left, voltage signals of four Solar Light Biometers in the afternoon relative to voltage signals at the respective solar elevationsin the morning, recorded on 16 September 1999 during the LAP�COST�WMO intercomparison of erythemal radiometers in Thessaloniki,Greece. Middle and right, Comparison of these ratios under clear-sky conditions �16 September 1999� and under cloudy-sky conditions�17 and 18 September 1999� for two of these instruments. The biometers’ signals have been normalized by spectrally weighted integralsof simultaneously determined global solar irradiance. Instrument No. 1 is identical with Biometer 1240, which has been extensivelytested in the laboratory.

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