Analysis of Sky Luminance Experimental Data and Comparison with Calculation Methods

Data of sky luminance measured in Osaka (Japan) and in Arcavacata di Rende (Italy) during a year wereanalysed and compared with the values predicted by the Perez, Igawa and CIE calculation methods. Thebest predictions of absolute, relative and zenith luminances were obtained by the CIE method. If themeasured luminances in a locality are not available, only by the Perez and the Igawa method is possibleto predict the sky luminance distribution under different meteorological conditions. Of these twomethods, the Igawa method appears to be more accurate for the prediction of the absolute luminance,and the Perez method for the predictions of the relative luminance.It should be stressed that, before comparison with the calculation models, the sky luminancesmeasured in each scanning have to be corrected in order to make the horizontal illuminance obtained byintegration of the luminances on the sky vault equal to the value of the horizontal illuminance measuredby photometers. If such a correction is not made owing to the absence of this latter measurement, theerrors of the predictions obtained by the models become larger, but they can be reduced by applyinga new methodology proposed by the authors.