Mean daylight illuminance in rooms facing sunlit streets

Abstract

The illuminance from windows that are shaded from direct sunlight can depend strongly on reflected light from the ground and from other buildings, while the sky component can be insignificant. The paper describes a method for estimating the mean illuminance on the working plane and on other room surfaces using, as data, solar normal illuminance and diffuse horizontal illuminance. These can be either computed or measured values. The method calculates direct illuminance on a window from sunlight and skylight, from ground reflected light andalso the illuminance from inter-reflection between facades. A split-flux technique is used to determine window direct internal illuminance and the internal inter-reflection, the light entering the window being divided into 3 components. The paper gives a worked example and typical values of window transmittance with louvres, canopies and light shelves.