AN OVERVIEW OF IMPORTANT FACTORS TO CONSIDER WHEN CALIBRATING LEDS IN PHOTOMETRY WITH DIFFERENT DETECTORS

LED-based lighting products are the fastest developing light sources on the general lighting market. These include not only white light sources, but also coloured light sources. SI-traceable calibrations with a low measurement uncertainty are challenging because their unique spectral and geometric properties must be taken into account. The requirements on the measuring equipment and resulting uncertainty contributions differ significantly from those for the calibration of conventional incandescent or fluorescent light sources. Furthermore, different detector types can be used when calibrating LEDs. These include classic photometers, radiometers, spectroradiometers but also imaging systems such as ILMDs. Each of these measuring devices has its specific advantages and disadvantages. In this paper, traceable calibrations on 9 coloured and 9 white LEDs are discussed. Various array spectroradiometers, photometers, radiometers and a double monochromator were used for this purpose. Each of these measuring instruments is discussed in terms of its suitability for measuring different physical quantities (e.g. array spectrometer for measuring illuminance).

Total ozone column intercomparison of Brewers, Dobsons, and BTS-Solar at Hohenpeißenberg and Davos in 2019/2020

During the 2019/2020 measurement campaign at Hohenpeißenberg (Germany) and Davos (Switzerland) we compared the well-established Dobson and Brewer spectrometers (single- and double-monochromator Brewer) with newer BTS array-spectroradiometer-based systems in terms of total ozone column (TOC) determination. The aim of this study is to validate the BTS performance in a longer-term TOC analysis over more than 1 year with seasonal and weather influences. Two different BTS setups have been used – a fibre-coupled entrance optic version by PMOD/WRC called Koherent and a diffusor optic version from Gigahertz Optik GmbH called BTS-Solar, which proved to be simpler in terms of calibration. The array-spectrometer-based BTS systems have been calibrated with traceability to NMI, and both versions of TOC retrieval algorithms are based on spectral measurements in the range of 305 to 350 nm instead of single-wavelength or wavelength pair measurements as per Brewer or Dobson. The two BTS-based systems, however, used fundamentally different retrieval algorithms for the TOC assessment, whereby the retrieval of the BTS-Solar turned out to achieve significantly smaller seasonal drifts. The intercomparison showed a difference of the BTS-Solar to Brewers of < 0.1 % with an expanded standard deviation (k=2) of < 1.5 % over the whole measurement campaign. Koherent showed a difference of 1.7 % with an expanded standard deviation (k=2) of 2.7 % mostly caused by a significant seasonal variation. To summarize, the BTS-Solar performed at the level of Brewers in the comparison in Hohenpeißenberg. The BTS-Solar showed very small dependence on the slant path column compared to the double-monochromator Brewer and performed better than the single-monochromator Brewer. Koherent showed a strong seasonal variation in Davos due to the sensitivity of its ozone retrieval algorithm to stratospheric temperature.

TOC intercomparison of Brewer, Dobson and BTS Solar at Hohenpeißenberg and Davos 2019/2020

In the 2019/2020 measurement campaign at Hohenpeißenberg (Germany) and Davos (Switzerland) we compared the well-established Dobson and Brewer spectrometers (single and double monochromator Brewer) with newer BTS array spectroradiometer based systems in terms of total ozone column (TOC) determination. The aim of this study is to validate the BTS performance in a longer-term TOC analysis over more than one year with seasonal and weather influences. Two different BTS setups have been used. A fibre coupled entrance optic version by PMOD/WRC called Koherent and a diffusor optic which proved to be simpler in terms of calibration from Gigahertz-Optik GmbH called BTS Solar. The array-spectrometer based BTS systems have been traceable calibrated to National Metrology Institutes (NMI) and the used TOC retrieval algorithms are based on spectral measurements in the range of 305 nm and 350 nm instead of single wavelength measurements as for Brewer or Dobson. The two BTS based systems, however, used fundamentally different retrieval algorithms for the TOC assessment, whereby the retrieval of the BTS solar turned out to achieve significantly smaller seasonal drifts. The intercomparison showed a deviation of the BTS Solar to Brewers of < 0.1 % with an expanded standard deviation of < 1.5 % within the whole measurement campaign. Koherent showed a deviation of 1.7 % with an expanded standard deviation of 2.7 % mostly given by a significant seasonal drift. Resulting, the BTS Solar performance is comparable to Brewers at the comparison in Hohenpeißenberg. The slant path slope is in-between double monochromator and single monochromator Brewer. Koherent shows a strong seasonal variation in Davos due to the sensitivity of its ozone retrieval algorithm to stratospheric temperature similar to the Dobson results.

Automated Aerosol Classification from Spectral UV Measurements Using Machine Learning Clustering

In this study, we present an aerosol classification technique based on measurements of a double monochromator Brewer spectrophotometer during the period 1998–2017 in Thessaloniki, Greece. A machine learning clustering procedure was applied based on the Mahalanobis distance metric. The classification process utilizes the UV Single Scattering Albedo (SSA) at 340 nm and the Extinction Angstrom Exponent (EAE) at 320–360 nm that are obtained from the spectrophotometer. The analysis is supported by measurements from a CIMEL sunphotometer that were deployed in order to establish the training dataset of Brewer measurements. By applying the Mahalanobis distance algorithm to the Brewer timeseries, we automatically assigned measurements in one of the following clusters: Fine Non Absorbing Mixtures (FNA): 64.7%, Black Carbon Mixtures (BC): 17.4%, Dust Mixtures (DUST): 8.1%, and Mixed: 9.8%. We examined the clustering potential of the algorithm by reclassifying the training dataset and comparing it with the original one and also by using manually classified cases. The typing score of the Mahalanobis algorithm is high for all predominant clusters FNA: 77.0%, BC: 63.9%, and DUST: 80.3% when compared with the training dataset. We obtained high scores as well FNA: 100.0%, BC: 66.7%, and DUST: 83.3% when comparing it with the manually classified dataset. The flags obtained here were applied in the timeseries of the Aerosol Optical Depth (AOD) at 340 nm of the Brewer and the CIMEL in order to compare between the two and also stress the future impact of the proposed clustering technique in climatological studies of the station.

Supercontinuum laser-based monochromatic radiant source for detector spectral responsivity characterization in the range 0.9–1.6 μm using absolute cryogenic radiometer

An improved monochromatic radiant source with spectral bandwidth of 4 nm based on supercontinuum laser and a double monochromator was included in absolute cryogenic radiometer-based facility to improve the accuracy of spectral responsivity measurement in the range 0.9–1.6 μm. The developed feedback system ensures stabilization of monochromatic radiant power with standard deviation up to 0.025 %. Radiant power that proceeds detector under test or absolute cryogenic radiometer varies from 0.1 to 1.5 mW in dependence of wavelength. The spectral power distribution of its monochromatic source for various operating mode is presented.

Deriving Aerosol Absorption Properties from Solar Ultraviolet Radiation Spectral Measurements at Thessaloniki, Greece

The gap in knowledge regarding the radiative effects of aerosols in the UV region of the solar spectrum is large, mainly due to the lack of systematic measurements of the aerosol single scattering albedo (SSA) and absorption optical depth (AAOD). In the present study, spectral UV measurements performed in Thessaloniki, Greece by a double monochromator Brewer spectrophotometer in the period 1998–2017 are used for the calculation of the aforementioned optical properties. The main uncertainty factors have been described and there is an effort to quantify the overall uncertainties in SSA and AAOD. Analysis of the results suggests that the absorption by aerosols is much stronger in the UV relative to the visible. SSA follows a clear annual pattern ranging from ~0.7 in winter to ~0.85 in summer at wavelengths 320–360 nm, while AAOD peaks in summer and winter. The average AAOD for 2009–2011 is ~50% above the 2003–2006 average, possibly due to increased emissions of absorbing aerosols related to the economic crisis and the metro-railway construction works in the city center.

Deriving Aerosol Absorption Properties from Solar Ultraviolet Radiation Spectral Measurements at Thessaloniki, Greece

The gap in knowledge regarding the radiative effects of aerosols in the UV region of the solar spectrum is large, mainly due to the lack of systematic measurements of the aerosol single scattering albedo (SSA) and absorption optical depth (AAOD). In the present study, spectral UV measurements performed in Thessaloniki, Greece by a double monochromator Brewer spectrophotometer in the period 1998 - 2017 are used for the calculation of the aforementioned optical properties. The main uncertainty factors have been described and there is an effort to quantify the overall uncertainties in SSA and AAOD. Analysis of the results suggests that the absorption by aerosols is much stronger in the UV relative to the visible. SSA follows a clear annual pattern ranging from ~0.7 in winter to ~0.85 in summer at wavelengths 320 &ndash; 360 nm, while AAOD peaks in summer and winter. The average AAOD for 2009 &ndash; 2011 is ~50% above the 2003 &ndash; 2006 average, possibly due to increased emissions of absorbing aerosols related to the economical crisis and the metro-railway construction works in the city center. A detailed analysis of the uncertainties in the retrieval of the SSA and the AAOD from the Brewer spectrophotometer has been also performed.

The effect of instrumental stray light on Brewer and Dobson total ozone measurements

Dobson and Brewer spectrophotometers are the primary, standard instruments for ground-based ozone measurements under the World Meteorological Organization's (WMO) Global Atmosphere Watch program. The accuracy of the data retrieval for both instruments depends on a knowledge of the ozone absorption coefficients and some assumptions underlying the data analysis. Instrumental stray light causes nonlinearity in the response of both the Brewer and Dobson to ozone at large ozone slant paths. In addition, it affects the effective ozone absorption coefficients and extraterrestrial constants that are both instrument-dependent. This effect has not been taken into account in the calculation of ozone absorption coefficients that are currently recommended by WMO for the Dobson network. The ozone absorption coefficients are calculated for each Brewer instrument individually, but in the current procedure the effect of stray light is not considered. This study documents the error caused by the effect of stray light in the Brewer and Dobson total ozone measurements using a physical model for each instrument. For the first time, new ozone absorption coefficients are calculated for the Brewer and Dobson instruments, taking into account the stray light effect. The analyses show that the differences detected between the total ozone amounts deduced from Dobson AD and CD pair wavelengths are related to the level of stray light within the instrument. The discrepancy introduced by the assumption of a fixed height for the ozone layer for ozone measurements at high latitude sites is also evaluated. The ozone data collected by two Dobson instruments during the period of December 2008 to December 2014 are compared with ozone data from a collocated double monochromator Brewer spectrophotometer (Mark III). The results illustrate the dependence of Dobson AD and CD pair measurements on stray light.