Radiance Measurements

Radiance measurements carried out using a robotic sky scanner developed from a customized sun tracker INTRA with gears 14:1 (maximum speed 8.5 degrees per second). The unit was programmed to sample the radiance at intervals of 15 degrees along both spherical coordinates (θ and φ) such that a total of 150 measurements resulted from each scan. The input optics consists of a 130 mm long collimator tube. A baffle is used to limit the field of view (FOV) and to avoid stray light. The FOV is approximately 4.5 degrees, which corresponds to a solid angle of 4.8 x 10-3 steradian (sr)12. The radiance was sampled using a double monochromator-based spectroradiometer (Bentham DMc150F-U) with a 150mm focal length and 1800 lines/mm gratings. The system is operated within a weatherproof box fitted with an active temperature-controlling system. The Full Width at Half Maximum (FWHM) of the spectroradiometer is 0.8 nm (in the range 300–800 nm). The instrument complies with the specifications of the Network for the Detection of Atmospheric Composition Change (NDACC) and the World Meteorological Organization (WMO) recommendations for measurements in the UV part of the spectrum. Synchronization of the sky scanner and the spectroradiometer was achieved using a software program in LabView. Sampling the radiance at each point in the sky took about 1 s and, operating at maximum speed, the sky-scanner took about 2 s to move the input optics to the next consecutive point in the sky. Accordingly, a single-wavelength scan of the entire sky (150 measurements) took roughly eight minutes. In our case, the absolute calibration of the system was achieved by using a 200 mm diameter integrating sphere fitted with a baffled 50 W quartz halogen lamp. Based on the calibration certificate of the lamp, the standard uncertainty of our measurements is estimated to be about 5% for wavelengths longer than 320 nm.