Verification of program accuracy for illuminance modelling: assumptions, methodology and an examination of conflicting findings

Abstract

,!This paper examines the role of assumptions commonly made in validation studies for lighting simulation programs and quantifies the sensitivity of results to uncertainties in key model parameters such as sky conditions and surface reflectivity. Actually occurring overcast skies are often taken to approximate the CIE standard overcast sky for the purpose of comparing predictions against measurements in real buildings. The validity of this assumption is tested against measurements of the sky luminance distribution for real skies. Illuminance predictions are particularly sensitive to the assigned reflectance of surfaces when the direct component of illumination is small. A number of confounding factors that can lead to imprecise estimates of surface reflectivity for building facades are identified and a methodology to minimize their effect is proposed. This study reveals that commonly made assumptions with respect to sky conditions and moderate imprecision in model parameters can lead to erroneous assessments of program accuracy. The degree to which existing validation findings can be extrapolated to very different application scenarios is discussed in the context of reported conflicting assessments of program accuracy.