Measuring the dark with a solar simulator: indoor calibration of sky brightness meters

Pravettoni, Mauro and Strepparava, Davide and Cereghetti, Nerio and Klett, Stefano and Andretta, Marco and Steiger, Marco (2015) Measuring the dark with a solar simulator: indoor calibration of sky brightness meters. In: UNSPECIFIED.

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The indoor calibration of sky brightness meters requires, in the most accurate and reproducible way, extremely low values of irradiance which is rather unusual for the photovoltaic (PV) testing laboratory at the University of Applied Sciences and Arts of Southern Switzerland (SUPSI), the Swiss ISO 17025 accredited PV testing centre. Researchers at SUPSI are used to test PV devices from 100 to 1000 W/m 2 total irradiance on solar simulators, roughly spanning the global irradiance at central European and North American latitudes all over the year. Within the collaboration between SUPSI, Dark­Sky Switzerland and the Republic and Canton of Ticino and in the framework of OASI 5 project, SUPSI's testing equipment was modified to reach controlled total irradiance of several orders of magnitude below the standard values, down to fractions of nW/cm 2 . 16 sky brightness meters were tested (Sky Quality Meter ® , SQM­LE, by Unihedron), coming from meteo stations at different sites and different altitudes in the Alpine region of Southern Switzerland, to assess the impact of their covering protective glass filters to the calibration coefficients and their ageing status. A steady­state 1000 W/m 2 (100 mW/cm 2 ) solar simulator by Oriel­Newport (±2% spatial non­uniformity, ±0.5% temporal instability, class A spectral irradiance in the 400­900 nm wavelength band) was used to test the meters indoors. The output power was first adjusted to give 25 mW/cm 2 total irradiance (measured with a calibrated reference PV cell); then a set of calibrated neutral density filters was used, with optical density from 1.0 to 7.5, giving the minimum irradiance of 0.79 nW/cm 2 (maximum brightness: 19 mag/arcsec 2 ), while keeping the testing meters at controlled temperature (25±1°C). The brightness of each meter was then tested as a function of irradiance. The equipment allowed to calibrate accurately the meters from 6 to 19 mag/arcsec 2 , giving confirmation of the linearity of the instruments, both with and without the protective glass filter, as shown in the chart below. In this work, the authors present full results of the calibration campaign: brightness calibration, with and without protective glass filters; transmittance measurement of the filters; spectral responsivity of the meters and a detailed uncertainty analysis. The work will serve as a useful tool and best­practice indication to any research centre using similar meters for dark sky monitoring. Further ongoing activities include the characterization of the meters at different temperatures to assess the meter performance at low temperatures

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