Classification Standard Skies: The use of horizontal sky illuminance

Abstract

This paper is about sky models, mathematical models that simulate the luminance distribution of sky. These models are used in building design and computer rendering. The International Commission on illumination (Commission International de l'Eclairage – CIE) has adopted a general sky model in 2003, which known now as the “ISO/CIE Standard General Skies”. Such model define the sky types into 15 types that range from cloudy to partly cloudy to clear sky. To classify a sky into one of these types, luminance data of the sky are needed. However, these data are not available in many countries. Therefore, and due to the fact that it is difficult to get accurate luminance measurements near the solar disk and due to the fact that the luminance of a single small patch of sky can fluctuate greatly, other methods that use sky illuminance have been proposed to help to fit a sky in one of the CIE 15 sky types. The goals of this paper is to test one of these methods against real luminance measurements.