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Insa Lohse - One of the best experts on this subject based on the ideXlab platform.
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calibration and evaluation of ccd Spectroradiometers for ground based and airborne measurements of spectral actinic flux densities
Atmospheric Measurement Techniques, 2017Co-Authors: Insa LohseAbstract:The properties and performance of charge-coupled device (CCD) array Spectroradiometers for the measurement of atmospheric spectral actinic flux densities (280–650 nm) and photolysis frequencies were investigated. These instruments are widely used in atmospheric research and are suitable for aircraft applications because of high time resolutions and high sensitivities in the UV range. The laboratory characterization included instrument-specific properties like the wavelength accuracy, dark signal, dark noise and signal-to-noise ratio (SNR). Spectral sensitivities were derived from measurements with spectral irradiance standards. The calibration procedure is described in detail, and a straightforward method to minimize the influence of stray light on spectral sensitivities is introduced. From instrument dark noise, minimum detection limits ≈ 1 × 10 10 cm −2 s −1 nm −1 were derived for spectral actinic flux densities at wavelengths around 300 nm (1 s integration time). As a prerequisite for the determination of stray light under field conditions, atmospheric cutoff wavelengths were defined using radiative transfer calculations as a function of the solar zenith angle (SZA) and total ozone column (TOC). The recommended analysis of field data relies on these cutoff wavelengths and is also described in detail taking data from a research flight on HALO (High Altitude and Long Range Research Aircraft) as an example. An evaluation of field data was performed by ground-based comparisons with a double-monochromator-based, highly sensitive reference spectroradiometer. Spectral actinic flux densities were compared as well as photolysis frequencies j (NO 2 ) and j (O 1 D), representing UV-A and UV-B ranges, respectively. The spectra expectedly revealed increased daytime levels of stray-light-induced signals and noise below atmospheric cutoff wavelengths. The influence of instrument noise and stray-light-induced noise was found to be insignificant for j (NO 2 ) and rather limited for j (O 1 D), resulting in estimated detection limits of 5 × 10 −7 and 1 × 10 −7 s −1 , respectively, derived from nighttime measurements on the ground (0.3 s integration time, 10 s averages). For j (O 1 D) the detection limit could be further reduced by setting spectral actinic flux densities to zero below atmospheric cutoff wavelengths. The accuracies of photolysis frequencies were determined from linear regressions with data from the double-monochromator reference instrument. The agreement was typically within ±5 %. Because optical-receiver aspects are not specific for the CCD Spectroradiometers, they were widely excluded in this work and will be treated in a separate paper, in particular with regard to airborne applications.
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calibration and evaluation of ccd Spectroradiometers for airbornemeasurements of spectral actinic flux densities
Atmospheric Measurement Techniques Discussions, 2017Co-Authors: Insa LohseAbstract:The properties and performance of CCD array Spectroradiometers for the measurement of atmospheric spectral actinic flux densities and photolysis frequencies were investigated. These instruments are widely used in atmospheric research and are suitable for aircraft applications because of high time resolutions and high sensitivities in the UV range. The laboratory characterization included instrument-specific properties like wavelength accuracy, dark signals, dark noise and signal-to-noise ratios. Spectral sensitivities were derived from measurements with spectral irradiance standards. The calibration procedure is described in detail and a straightforward method to minimize the influence of stray light on spectral sensitivities is introduced. Detection limits around 1×10 10 cm −2 s −1 nm −1 were derived for spectral actinic flux densities in a 300 nm range (1 s integration time). As a prerequisite for the determination of stray light under field conditions, atmospheric cutoff wavelengths were defined using radiative transfer calculations as a function of solar zenith angles and ozone columns. The recommended analysis of field data relies on these cutoff wavelengths and is also described in detail taking data from a research flight as an example. An evaluation of field data was performed by ground-based comparisons with a double-monochromator reference spectroradiometer. Spectral actinic flux densities were compared as well as photolysis frequencies j (NO 2 ) and j (O 1 D), representing UV-A and UV-B ranges, respectively. The spectra expectedly revealed an increased daytime level of residual noise below atmospheric cutoff wavelengths that is caused by stray light. The influence of instrument noise and stray light induced noise was found to be insignificant for j (NO 2 ) and rather limited for j (O 1 D), resulting in estimated detection limits of 5×10 −7 s −1 and 1×10 −7 s −1 , respectively. For j (O 1 D) the detection limit could be further reduced by setting spectral actinic flux densities below cutoff wavelengths to zero. The accuracies of photolysis frequencies were determined from linear regressions with reference instrument data. The agreement was typically within ±5 %. Optical receiver aspects were widely excluded in this work and will be treated in a separate paper in particular with regard to airborne applications. Overall, the investigated instruments are clearly suitable for high quality photolysis frequency measurements with high time resolution as required for airborne applications. An example of data from a flight on the research aircraft HALO is presented.
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airborne measurements of uv spectral actinic radiation with ccd Spectroradiometers applications aboard zeppelin nt and the research aircraft halo
2014Co-Authors: Insa LohseAbstract:Airborne measurements of spectral actinic flux densities (280-650 nm) were performed using two Spectroradiometers each covering a 2 π field-of-view. A method was developed to compensate for the non-ideal combined angular response of the optical receivers, based on spectral radiance distributions from radiative t ransfer calculations. Correction factors on the order of 1- 3 % were derived dependent on atmospheric conditions. First results of aircraft campaigns with a Zeppelin and the resea rch aircraft HALO are presented.
Saulius Nevas - One of the best experts on this subject based on the ideXlab platform.
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nonlinearity characterization of array Spectroradiometers for the solar uv measurements
Applied Optics, 2017Co-Authors: Tomi Pulli, Saulius Nevas, Omar El Gawhary, Steve Van Den Berg, Janne Askola, Petri Karha, Farshid Manoocheri, Erkki IkoneAbstract:Array Spectroradiometers are attractive alternatives to scanning Spectroradiometers in solar ultraviolet measurements. However, the measurement of solar spectral irradiance imposes stringent requirements for the linearity of the instruments. In this article, two array Spectroradiometers were characterized for nonlinearity. Significant nonlinearities, in excess of 10%, as a function of analog-to-digital converter counts were discovered. Additional nonlinearities as a function of integration time were observed at very long integration times. No clear residual nonlinearity as a function of spectral irradiance was witnessed despite the characterization spanning four orders of magnitude of spectral irradiance. The characterizations were carried out with three measurement setups that are briefly compared.
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stray light correction of array Spectroradiometers for solar uv measurements
Applied Optics, 2014Co-Authors: Saulius Nevas, Luca Egli, Julian Grobner, M BlumthalerAbstract:An approach is presented to characterize and correct stray light in spectra measured with array Spectroradiometers and caused by out-of-spectral range radiation. A prerequisite for out-of-range stray light correction is knowledge of the spectral irradiance not measured by the instrument itself. A way of solving this problem for solar UV measurements is shown. The effect of out-of-range stray light is especially important for solar UV Spectroradiometers typically having a spectral range narrower than that of the silicon detectors in use. Two different types of instruments used for solar UV measurements were characterized and corrected for out-of-range and in-range stray light. As a hardware solution to the out-of-range stray light problem, a bandpass filter was fitted in one array spectroradiometer. Results of test measurements using this modified instrument are also shown.
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characterisation of nonlinearities of array Spectroradiometers in use for measurements of the terrestrial solar uv irradiance
UVNews, 2014Co-Authors: Saulius Nevas, Luca Egli, Tomi Pulli, Omar El Gawhary, Petri Karha, Pete Lattne, Julia GrobneAbstract:Characterisation of nonlinearities of array Spectroradiometers used for measurements of the terrestrial solar UV irradiance over 5 to 6 orders of magnitude is required by the application. It is also one of the tasks within the EMRP ENV03 project. Within this framework, different setups at Aalto, METAS, PTB and VSL have been applied to the characterisations. This paper will present the technical realisations of the setups and result s of comparison measurements, which are meant for the validation of the methods.
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Transferability of stray light corrections among array Spectroradiometers
2013Co-Authors: Saulius Nevas, A. Sperling, Benno OderkerkAbstract:One of the most important properties of array Spectroradiometers limiting their applications, especially in the UV spectral range, is a high level of spectral stray light. The suppression of the stray light can be improved numerically by using a correction matrix derived by comprehensive characterizations using wavelength-tunable lasers. For economic reasons, it is highly desirable that the same stray light correction matrix were effective on a number of array Spectroradiometers of the same type and model. Here we address the properties of the instruments defining the transferability of the stray light corrections from one instrument to another and show examples based on the stray light characterization, hardware optimization and test measurements with a number of array Spectroradiometers.
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a guide to measuring solar uv spectra using array Spectroradiometers
RADIATION PROCESSES IN THE ATMOSPHERE AND OCEAN (IRS2012): Proceedings of the International Radiation Symposium (IRC IAMAS), 2013Co-Authors: M Lumthale, Luca Egli, Julia Grobne, Saulius NevasAbstract:Array Spectroradiometers are cost-effective instruments allowing fast measurement sequences to monitor the high variability of solar radiation. From the spectra any desired biologically weighted doses within their operational spectral range can be derived by post-processing. Therefore, array Spectroradiometers have the potential to replace filter radiometers currently used in UV monitoring networks. However, they suffer significantly from stray light. Thus for routine operation it is important to follow strict procedures for characterisation, operation and data post-processing, which are suggested in these guidelines. The most important characteristics of array Spectroradiometers are wavelength calibration, slit function, stray light properties, spectral structure of the dark signal, linearity, noise equivalent irradiance and spectral responsivity. For routine operation, temperature stabilization is absolutely necessary as well as automated dark signal measurements. Integration time and number of repetitions control the noise level and they have to be defined according to the measurement requirements. Data post-processing includes consideration of nonlinearity, dark signal with its spectral structure, integration time, stray light correction, spectral response function and weighting function.
Tapani Koskela - One of the best experts on this subject based on the ideXlab platform.
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estimation of the optical receiving plane positions of solar Spectroradiometers with spherical diffusers on the basis of spatial responsivity data
Optics Letters, 2009Co-Authors: Pasi Mannine, Tapani Koskela, Petri Karha, Lasse Ylianttila, Erkki IkoneAbstract:A straightforward method for estimating the position of the optical receiving plane of a spherical, dome-shaped diffuser from its spatial responsivity data is presented. The method is tested with two diffusers, types J1002 and J1015 from CMS Schreder, commonly used in solar UV Spectroradiometers. The shift of the receiving plane from its nominal position determines a potential measurement error that occurs when measurements and calibrations are carried out with sources at different distances from the diffuser. Such information is particularly valuable for voluminous solar UV monitoring Spectroradiometers that cannot easily be transported to laboratory calibrations. The results suggest that systematic measurement errors are at least of the order of 1%, if the position of the receiving plane is not properly taken into account, thus indicating a need to study the effect more carefully. This method can also be used to minimize measurement errors when designing diffusers.
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portable device for characterizing the angular response of uv Spectroradiometers
Applied Optics, 2005Co-Authors: A F Ais, M Lumthale, A R Webb, Gunthe Seckmeye, Tapani Koskela, S Kazadzis, Katerina Garane, Natalia Kouremeti, Julia Grobne, Pete GortsAbstract:This paper introduces a device that was developed to measure the angular response of UV Spectroradiometers in the field. This device is designed to be used at the operating position of Spectroradiometers; thus the derived angular response also includes any effects from imperfect leveling of the diffuser and corresponds to the actual operational angular response. The design and characterization of the device and the results from its application on 11 different Spectroradiometers that operate at different European UV stations are presented. Various sources of uncertainties that were identified result in a combined uncertainty in determining the angular response, which ranges between approximately 1.5% and 10%, depending on the incidence angle and the characteristics of the diffuser. For the 11 instruments, the error in reporting the diffuse irradiance ranges between 2% and −13%, assuming isotropic distribution of the downwelling radiances.
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suspen intercomparison of ultraviolet Spectroradiometers
Journal of Geophysical Research, 2001Co-Authors: A F Ais, Germa Ernhard, Richard Mckenzie, G Gardine, H Slape, M Lumthale, A R Webb, Gunthe Seckmeye, Eri Kjeldstad, Tapani KoskelaAbstract:Results from an intercomparison campaign of ultraviolet Spectroradiometers that was organized at Nea Michaniona, Greece, July 1–13, 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within ±20%. About half of the instruments agreed to within ±5%, while only three fell outside the ±10% agreement limit. As for the accuracy of the wavelength registration of the recorded spectra, for most of the Spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy, and in 1993 in Garmisch-Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating Spectroradiometers operating may be regarded as a core group of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.
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lamps as means to homogenize solar ultraviolet irradiance measurements performed with different Spectroradiometers
Journal of Geophysical Research, 2000Co-Authors: Eri Kjeldstad, Jo Johnse, Tapani KoskelaAbstract:Portable lamp irradiance calibration units of different size were used to transfer a relative calibration in a group of Spectroradiometers operating at the same site. Every unit was used as normally in the field or at a home laboratory, i.e., including any errors related to the method itself. The calibration obtained for each instrument was used to reprocess their synchronous solar measurements. Solar measurements deviating by typically ± 10% got a standard deviation as low as 3-4% when corrections derived from a common lamp measurement were applied. Because instrumental features remained the same in the original and in the reprocessed solar data, the improvement of the agreement in the solar measurements was considered as an indication of a more homogeneous irradiance scale. The remaining disagreement must be a sum of the uncertainty related to the use of each calibration unit and any instrumental peculiarities. Each of the lamp units could bring the scales of the specific instrument group closer. This was especially true for distinct outlicrs in the group. Even the smallest units performed almost as well as more sophisticated equipment, and an agreement of about 5% in the sky measurements could be reached, which is better than that seen in most recent intercomparisons.
Gunthe Seckmeye - One of the best experts on this subject based on the ideXlab platform.
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monte carlo based uncertainties of surface uv estimates from models and from Spectroradiometers
Metrologia, 2013Co-Authors: Raul R Cordero, Gunthe Seckmeye, Alessandro Damiani, Fernando Labbe, D LarozeAbstract:Although some of the adverse effects of the ultraviolet (UV) radiation may be strictly proportional to cumulative UV doses, others may relate to the frequency of extreme UV events. Therefore, an improved understanding of the UV global climate, including variability and trends, has become of great interest. Variability and trend analyses require quality-ensured surface UV series. The quality of surface UV data depends on their uncertainty. Building upon our prior efforts, we have used a Monte Carlo-based method to compute, under different conditions, the uncertainties affecting UV data rendered by models (1D radiative transfer models) and by Spectroradiometers (double monochromator-based and CCD array-based). We found that the uncertainty of spectral UV measurements is driven by the signal-to-noise ratio of the detector, while the uncertainty of spectral UV calculations strongly depends on the uncertainty of the ozone input. The presented uncertainty figures allow comparison of the performance of modern UV gathering techniques (models and instruments), and provide a frame to assess the significance of differences when intercomparing.
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monte carlo based uncertainty analysis of uv array Spectroradiometers
Metrologia, 2012Co-Authors: Raul R Cordero, Gunthe Seckmeye, Stefa Riechelma, Alessandro Damiani, Fernando LabbeAbstract:Recent efforts (including effective ex-post stray light corrections) have allowed significant improvement in the performance (tested by intercomparison) of low-cost array Spectroradiometers. However, the problem of uncertainty evaluation of UV measurements by these instruments has not been comprehensibly addressed mainly because of the difficulties involved in the uncertainty propagation. We have applied a Monte Carlo-based approach that allowed us to overcome those difficulties and to evaluate the uncertainty associated with spectral measurements carried out using a 12-bit CCD array of 2048 pixels, model SD2000, manufactured by Ocean Optics®. In the UV-A part of the spectrum the standard uncertainty was found to be roughly constant (about 5%). However, mostly due to the noise affecting the readings, a sharp increment was observed at wavelengths shorter than 315 nm; indeed, the driving factor determining the UV-B uncertainty (and in turn the UV index uncertainty) was the signal-to-noise ratio (SNR). Low SNRs lead to standard uncertainties greater than 20% for irradiances at 300 nm and greater than 10% for UV index values. Although each instrument has an individual character, comparable figures are expected in the case of instruments whose measurements undergo similar quality assurance activities (i.e. similar stray light correction methods). Our uncertainty figures provide a frame to assess the significance of differences when intercomparing, and in turn to check the quality of the measurements. The applied uncertainty evaluation methodology is general and agrees with recommendations of the ISO Guide to the Expression of Uncertainty in Measurement.
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new Spectroradiometers complying with the ndsc standards
Journal of Atmospheric and Oceanic Technology, 2006Co-Authors: Sigrid Wuttke, Germa Ernhard, Richard Mckenzie, James C Ehramjia, Gunthe Seckmeye, Paul E Johnsto, Michael OneillAbstract:Abstract The investigation of the effect of solar ultraviolet (UV) and visible radiation on biological organisms and photochemical reactions requires spectral measurements of the desired radiation parameters of high accuracy. The Network for the Detection of Stratospheric Change (NDSC) and the World Meteorological Organization have set up stringent requirements for high-quality spectral measurements of ultraviolet radiation. It is shown that two new instruments comply with these standards. One is the newly developed spectroradiometer of the Institute of Meteorology and Climatology, University of Hannover, Hannover, Germany. It is capable of covering the spectral range from the UV to the near-infrared (290–1050 nm) in a comparably fine resolution. One major aim is to deploy this instrument as a traveling NDSC spectroradiometer. The other new instrument is built for the U.S. National Science Foundation's UV Monitoring Network. It is designed to monitor UV and visible irradiance at high latitudes and covers ...
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portable device for characterizing the angular response of uv Spectroradiometers
Applied Optics, 2005Co-Authors: A F Ais, M Lumthale, A R Webb, Gunthe Seckmeye, Tapani Koskela, S Kazadzis, Katerina Garane, Natalia Kouremeti, Julia Grobne, Pete GortsAbstract:This paper introduces a device that was developed to measure the angular response of UV Spectroradiometers in the field. This device is designed to be used at the operating position of Spectroradiometers; thus the derived angular response also includes any effects from imperfect leveling of the diffuser and corresponds to the actual operational angular response. The design and characterization of the device and the results from its application on 11 different Spectroradiometers that operate at different European UV stations are presented. Various sources of uncertainties that were identified result in a combined uncertainty in determining the angular response, which ranges between approximately 1.5% and 10%, depending on the incidence angle and the characteristics of the diffuser. For the 11 instruments, the error in reporting the diffuse irradiance ranges between 2% and −13%, assuming isotropic distribution of the downwelling radiances.
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suspen intercomparison of ultraviolet Spectroradiometers
Journal of Geophysical Research, 2001Co-Authors: A F Ais, Germa Ernhard, Richard Mckenzie, G Gardine, H Slape, M Lumthale, A R Webb, Gunthe Seckmeye, Eri Kjeldstad, Tapani KoskelaAbstract:Results from an intercomparison campaign of ultraviolet Spectroradiometers that was organized at Nea Michaniona, Greece, July 1–13, 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within ±20%. About half of the instruments agreed to within ±5%, while only three fell outside the ±10% agreement limit. As for the accuracy of the wavelength registration of the recorded spectra, for most of the Spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy, and in 1993 in Garmisch-Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating Spectroradiometers operating may be regarded as a core group of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.
M Lumthale - One of the best experts on this subject based on the ideXlab platform.
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quality assessment of solar uv irradiance measured with array Spectroradiometers
Atmospheric Measurement Techniques, 2015Co-Authors: Luca Egli, Marina Khazova, M Lumthale, Julia Grobne, Grego Hulse, Luciano Achma, Jimmy Dubard, Richard Kif, Kees Hoogendijk, A SerranoAbstract:Abstract. The reliable quantification of ultraviolet (UV) radiation at the earth's surface requires accurate measurements of spectral global solar UV irradiance in order to determine the UV exposure to human skin and to understand long-term trends in this parameter. Array Spectroradiometers (ASRMs) are small, light, robust and cost-effective instruments, and are increasingly used for spectral irradiance measurements. Within the European EMRP ENV03 project “Solar UV”, new devices, guidelines and characterization methods have been developed to improve solar UV measurements with ASRMs, and support to the end user community has been provided. In order to assess the quality of 14 end user ASRMs, a solar UV intercomparison was held on the measurement platform of the World Radiation Center (PMOD/WRC) in Davos, Switzerland, from 10 to 17 July 2014. The results of the blind intercomparison revealed that ASRMs, currently used for solar UV measurements, show a large variation in the quality of their solar UV measurements. Most of the instruments overestimate the erythema-weighted UV index – in particular at large solar zenith angles – due to stray light contribution in the UV-B range. The spectral analysis of global solar UV irradiance further supported the finding that the uncertainties in the UV-B range are very large due to stray light contribution in this wavelength range. In summary, the UV index may be detected by some commercially available ASRMs within 5 % compared to the world reference spectroradiometer, if well characterized and calibrated, but only for a limited range of solar zenith angles. Generally, the tested instruments are not yet suitable for solar UV measurements for the entire range between 290 and 400 nm under all atmospheric conditions.
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a guide to measuring solar uv spectra using array Spectroradiometers
RADIATION PROCESSES IN THE ATMOSPHERE AND OCEAN (IRS2012): Proceedings of the International Radiation Symposium (IRC IAMAS), 2013Co-Authors: M Lumthale, Luca Egli, Julia Grobne, Saulius NevasAbstract:Array Spectroradiometers are cost-effective instruments allowing fast measurement sequences to monitor the high variability of solar radiation. From the spectra any desired biologically weighted doses within their operational spectral range can be derived by post-processing. Therefore, array Spectroradiometers have the potential to replace filter radiometers currently used in UV monitoring networks. However, they suffer significantly from stray light. Thus for routine operation it is important to follow strict procedures for characterisation, operation and data post-processing, which are suggested in these guidelines. The most important characteristics of array Spectroradiometers are wavelength calibration, slit function, stray light properties, spectral structure of the dark signal, linearity, noise equivalent irradiance and spectral responsivity. For routine operation, temperature stabilization is absolutely necessary as well as automated dark signal measurements. Integration time and number of repetitions control the noise level and they have to be defined according to the measurement requirements. Data post-processing includes consideration of nonlinearity, dark signal with its spectral structure, integration time, stray light correction, spectral response function and weighting function.
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portable device for characterizing the angular response of uv Spectroradiometers
Applied Optics, 2005Co-Authors: A F Ais, M Lumthale, A R Webb, Gunthe Seckmeye, Tapani Koskela, S Kazadzis, Katerina Garane, Natalia Kouremeti, Julia Grobne, Pete GortsAbstract:This paper introduces a device that was developed to measure the angular response of UV Spectroradiometers in the field. This device is designed to be used at the operating position of Spectroradiometers; thus the derived angular response also includes any effects from imperfect leveling of the diffuser and corresponds to the actual operational angular response. The design and characterization of the device and the results from its application on 11 different Spectroradiometers that operate at different European UV stations are presented. Various sources of uncertainties that were identified result in a combined uncertainty in determining the angular response, which ranges between approximately 1.5% and 10%, depending on the incidence angle and the characteristics of the diffuser. For the 11 instruments, the error in reporting the diffuse irradiance ranges between 2% and −13%, assuming isotropic distribution of the downwelling radiances.
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suspen intercomparison of ultraviolet Spectroradiometers
Journal of Geophysical Research, 2001Co-Authors: A F Ais, Germa Ernhard, Richard Mckenzie, G Gardine, H Slape, M Lumthale, A R Webb, Gunthe Seckmeye, Eri Kjeldstad, Tapani KoskelaAbstract:Results from an intercomparison campaign of ultraviolet Spectroradiometers that was organized at Nea Michaniona, Greece, July 1–13, 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within ±20%. About half of the instruments agreed to within ±5%, while only three fell outside the ±10% agreement limit. As for the accuracy of the wavelength registration of the recorded spectra, for most of the Spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy, and in 1993 in Garmisch-Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating Spectroradiometers operating may be regarded as a core group of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.
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a laboratory investigation of two ultraviolet Spectroradiometers
Photochemistry and Photobiology, 1994Co-Authors: A R Webb, G Gardine, M Lumthale, Piers M Forste, M Hube, P J KrrschAbstract:Spectral measurements of the solar UV irradiance at the earth's surface depend critically on the characteristics of the Spectroradiometers and the procedures used in their calibration. Two UV Spectroradiometers were subjected to a series of laboratory tests to investigate which factors were most significant in limiting the reliability of the absolute irradiance measurements. Three independent standards of spectral irradiance were scanned by both instruments under a range of bench conditions. The results were consistent to within about 3%, most of the uncertainty being due to scattered light in the laboratory. An UV laser was used to determine the slit function of each spectrometer and the influence of internal stray light. Significant departures from the ideal cosine and azimuth responses were measured by a xenon lamp. Both Spectroradiometers were kept indoors throughout the experiments. The relevance of these laboratory results is discussed with respect to the task of measuring solar UV radiation in the field.
