Measurements of the luminous efficacy of daylight

Abstract

In the calculation of daylight availability and lighting energy use in buildings, luminous efficacy is important because it enables calculation of daylight illuminances from the more widely available radiation data. This paper presents the results of a year's measurement of luminous efficacy at Garston, Hertfordshire, UK. Direct sunlight efficacies tended to increase with solar altitude, typically from 70 lm W -1 at 10° to 95 lm W-1 at 60° solar altitude. Global (sun and sky) efficacies increased slightly with solar altitude, mean values being 107 Im W -1 for clear skies and 109 lm W -1 under average conditions. Diffuse sky efficacies exhibited a roughly linear relationship with cloud cover, decreasing from 144 Im W -1 under clear skies to 115 lm W -1 for overcast conditions. The paper contains empirical functions and data tables to enable daylight data to be generated directly from the most common types of solar radiation data.