Evaluation of Virtual Sky Domes for the Prediction of Daylight Performance with Radiosity-based Light Simulation Software

Abstract

A Virtual Sky Dome (VSD) is a computational model of a hemisphere resembling the daylight conditions of a particular location, date and time1. It comprises of 145 point light sources, whose individual properties can be adjusted automatically to match the sky luminance distribution as defined by the current set of 15 standard skies2. VSD's can be formatted into any CAD-based lighting simulation software format, thus extending the range of simulation tools for daylight analysis beyond the commonly used software RADIANCE. This paper evaluates the accuracy of the VSD method for the prediction of daylight performance with the Radiosity-based light simulation software LIGHTSCAPE. Methodologically a simulation that used RADIANCE was compared against a simulation using VSD within the software LIGHTSCAPE. The former was based on a published case, which documents Daylight Factors in a reference room across all 15 sky types3. This case is referred to as reference case. LIGHTSCAPE's built-in sky types however follow the obsolete sky standard4, thus offering only a limited set of clear and overcast skies. The VSD method was now applied to create the 15 sky types for LIGHTSCAPE. Subsequently simulations for the reference room have been performed and Daylight Factors obtained. A comparison of Daylight Factors between the reference and VSD case shows a high accordance, the mean deviation over all sky types and locations is less than 2%. It can be concluded that a simulation, using Virtual Sky Domes for Radiosity-based simulation tools, predicts the daylight performance as accurate as the reference case.