A Method to Construct Virtual Sky Domes for Use in Standard CAD-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 time. VSD's allow users of standard CAD-based simulation software to simulate architecture and assess daylight performance criteria making full use of the 15 standard sky luminance distribution models, which have just recently been accepted as a CIE standard [1]. This paper introduces the method to construct VSD. Prior to this any daylight simulation using standard CAD based light simulation software was limited to a smaller set of standard skies: clear, overcast and intermediate with non-changeable parameters as in the case of the software Lightscape. The all-weather-sky tool developed for the software Radiance follows the sky definitions defined by Perez. Neither of these tools considers the recently developed standard of 15 sky types, which is to replace previous standards[2] to further differentiate the skies to better match the real daylight conditions. This is of particular significance for predominately cloudy regions like Singapore, where a wide range of overcast and intermediate skies with significant different luminance distributions occur. A VSD is a CAD model of the hemisphere comprising 145 point lights of controllable parameters, each one sitting in the centre of a particular sky patch. Their luminous flux is derived from the luminance of the individual sky patch calculated using Kittler's equation[3]. A computational method has been developed that creates a VSD for a given sky type based on the input of solar azimuth and elevation, hence representing a particular time and location. This paper explains the method and shows results using VSD's to simulate daylight with the standard CAD-based light simulation software Lightscape.