The paper presents the calibration activity on the imaging system of the MINLU instrument, an autonomous sensor suite designed for monitoring light pollution using commercial off-the-shelf components. The system is extremely compact and with an overall mass below 3 kg can be easily installed as a payload for drones or sounding balloons. Drones and air balloons can in fact play an important role in completing upward light emission measurement from satellites allowing an increased spatial and time resolution from convenient altitudes and positions. The proposed system can efficiently measure the luminous intensity and the spectral power density of on-ground emissions providing a useful tool to identify polluting sources and to quantify upward light flux. The metrological performance of the imaging system has been verified through an extensive laboratory test activity using referenced light sources: the overall uncertainty of the multi-luminance meter has been calculated to be 7% of the reading, while the multi-spectrometer has shown a full width at half maximum (FWHM) equal to 10 nm within the measuring range between 400 nm and 700 nm. When operating at an altitude of 200 m, the system can achieve a horizontal resolution at a ground level of 0.12 m with a wavelength resolution able to identify the different lamp technology of outdoor light sources, including light-emitting diode (LED) lights that are undetected by satellites.
Keywords: UAV; artificial lighting; image luminance measuring device; imaging system; light pollution; photometry; remote sensing; spectral analysis; urban analysis.