Shafts for daylighting underground spaces: sizing guidelines

This paper presents an experimental study on Daylight Factors (DFs) distribution in underground multi-level open spaces which use continuous light shafts to reduce energy consumption by the artificial lighting system and to improve the human acceptability of the spaces. Experimental measurements were carried out in a scale model, which could be assembled to provide different shapes and could be placed in an unobstructed site. The use of a real sky, which very seldom shows a fully overcast CIE luminance distribution, required the development of a special procedure to guarantee the accuracy of the experimental results. Level height, slab floor thickness, aspect ratio and glass covering of the light shaft were some of the parameters considered. Measured DFs were correlated by dimensionless parameters and reported in the form of charts which can be used to size the width of the shafts on the basis of the required illuminance and on the frequency distribution of the diffuse natural illuminance. An example of DFs calculation for an underground space is also reported.