The simultaneous occurrence and relationship of sunlight and skylight under ISO/CIE standard sky types

Abstract

In daylight science the availability of sunlight and skylight at different times has been studied because of the desire to use daylight in both exterior and interior spaces. Exterior skylight illuminances under overcast skies were adopted as a standard for window design in the past. Current ISO/CIE sky types are standardised as relative luminance patterns normalised to the zenith. In this paper, the zenith luminance in candela/square metre and the resulting diffuse illuminance in lux are determined for all sky types. Furthermore, the proportions of sunlight and skylight under different levels of turbidity are found and documented. Efforts to harmonise electric lighting and daylighting standards need to analyse the data on available daylight in physical units in order to ensure energy savings that respect human requirements as well as providing information suitable for computer-aided design.