The luminance distribution of an average sky

Abstract

Daylight in buildings can save energy by reducing artificial lighting energy consumption. An accurate estimate of internal daylight availability is required for energy use predictions and to include the effect of window orientation this must be based on an adequate model of sky luminance distribution. For such energy calculations the 'average sky' (based on measurements for a wide range of real skies) is recommended as a replacement for the CIE overcast sky. This paper analyses luminance data obtained by Wegner to show that the luminance distribution of an average sky is of the form L = ae^-by + c where y is the angle in degrees between the sun and the element of sky under consideration, b is approximately 0.025 and a and c are functions of solar altitude alone, and, significantly, do not depend on the altitude of the element of sky. Using this formula together with a direct solar component the paper derives expressions for horizontal and vertical illuminances, and the average illuminance inside a side-lit room; and briefly outlines further work, under way at BRS, to evaluate the model for weather conditions in the UK and to apply it to problems of daylighting design.