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There is insufficient current knowledge on the sky luminance patterns under ISO/CIE standard sky types in absolute luminance physical units in cd.m−2 and the resulting horizontal skylight illuminance in lux at the ground level. This paper explains research results and formulae to enable determination of computer applications which help in any location and time to calculate the distribution of luminance in any sky element on outdoor surfaces or within window solid angles that cause interior illuminance perception problems and, or discomfort glare. [ABSTRACT FROM AUTHOR]

When exterior global/direct and diffuse illuminance, as well as zenith luminance, is regularly measured and recorded, a selection and specification of the most light efficient skylight conditions is needed. A routine representation of all 1- or 5-minute data on the P-G-D diagram can show extreme diffuse, i.e. skylight ratios normalized to the simultaneous extraterrestrial horizontal illuminance level, which indicate the bright cloudy sky conditions. A new method of cluster analysis was developed to select sky conditions associated with sunlight under specific turbidity or cloudiness conditions at actual solar altitudes, while a rough classification of the sky luminance pattern is possible using the ratio of zenith luminance to diffuse illuminance. Some characteristic data recorded in Bratislava, Slovakia during 1995 and during the three year period 1994-1996, demonstrate the parametrization and selection method used, and document detailed dynamic changes occurring during the brightest days.

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. [ABSTRACT FROM AUTHOR]