All-sky zenith luminance models for the UK

Abstract

Sky luminance distribution data are of great importance to the lighting designer considering daylighting as an option for provision of internal illuminance. Previous studies have shown that the sky vault can be considered as a series of separate sky patches, each with its own individual degree of luminance. Such luminance values can be determined as a function of the corresponding zenith luminance. A lack of long-term zenith luminance data has led to the creation of a variety of zenith luminance models, from those that are complex in construction to those that are operable only under certain sky conditions. This paper presents an evaluation of Moon and Spencer's overcast sky model and the model of Perez et al., which has previously been shown to be efficient for all-sky conditions. Two new zenith luminance models are also proposed in an attempt to simplify the process by which zenith luminance estimations are obtained under any prevailing sky type. In doing so, an opportunity has been created to incorporate more routinely recorded data, such as cloud cover and sunshine fraction, into the design procedure.