Assessing and predicting average daylight factors of adjoining spaces in atrium buildings under overcast sky

Daylight use in an atrium is particularly beneficial as the natural light can illuminate potentially dark core areas and decrease energy consumption from electric lighting. This study has investigated, for overcast sky conditions, the horizontal daylight levels in spaces adjoining atria and the vertical daylight levels on atrium well walls in atria. The daylight levels in the rooms and on the walls were derived from scale model measurements, theoretical calculations and predictions from the lighting simulation package Radiance. A comparison of measured data and modelling has validated Radiance simulations of atrium daylighting. The simulations show generally good agreement with theory while some limitations in the calculations used in determining the daylight factors in rooms with large widow area to total wall area ratios were observed. The average daylight levels in rooms displayed a linear relationship with the vertical daylight levels on the well wall. In terms of the well geometry and well façades (decided by the ratio of window area to solid wall area) and well surface reflectance, the variations of daylight level in the adjoining rooms have been analysed and some conclusions and design strategies for supporting preliminary design decisions are presented.