Luminance calibration of a full sky HDR imaging system using sky scanner measurements

A full sky High Dynamic Range imaging system, based on a Single-Lens Reflex camera with a fisheye lens, has been constructed and calibrated with a sky scanner luminance meter. The method considers the geometrical, spectral, timing and orientation issues between instruments. The calibration data sets, having nearly simultaneous measurements under stable sky conditions, were obtained from approximately one month of data using selection variables based in the experimental design. For luminance estimation we use the standard CIEY RGB combination and a Spectrally Matched Luminance (SML) predictor, matching the spectral response of the instruments. With 738 calibration points having luminances up to 23.6 kcd/m(2), covering 98.5% of the sky luminance range, CIEY is linearly correlated with sky scanner measurements with a coefficient of determination R-2 = 0.9927 and a Root Mean Squared Error (RMSE) of 7.7%. SML gives better results, with R-2 = 0.9973 and RMSE = 5.3%. With 253 calibration points with luminances up to 12.9 kcd/m(2), comprising 94.1% of the sky luminance range, both predictors clearly improve, with R-2 = 0.9964 and RMSE = 4.1% in case of CIEY and R-2 = 0.9982 and RMSE = 2.9% in case of SML.