This paper presents comparative monitoring of various sized commercial lightpipes at a purposely built test shed with a pitched roof. The purpose of monitoring is to simultaneously compare the performance of lightpipes of different geometries and also to provide data for establishing a formula for performance prediction. The photometric integrating box, which BRE (Building Research Establishment) has adopted as an approximation to the photometric integrating sphere, is employed to determine the lumen output of the lightpipes under real sky condition. This study presents monitoring of four sized lightpipes: straight Phi 530 mm, elbowed Phi 530 mm (with a tilted entrance), elbowed Phi 450 mm and elbowed Phi 230 mm. A mathematical model is proposed to correlate the measured lumen output, horizontal global illuminance, the diameter and length of the lightpipe, the specular reflectance, solar altitude and azimuth, and orientation of elbow if an elbowed lightpipe is tested. The coefficients in the formula are determined by regression of the monitoring data. The formula is then used to predict the lumen output of a lightpipe from the measured global illuminance for the given geometry and time. The agreement between the measured and predicted lumen output is generally satisfactory. The formula may be a useful design tool as it can predict the lumen output of a lightpipe from the global illuminance value such as those given in EnergyPlus weather data. (c) 2012 Elsevier B.V. All rights reserved.