Daylighting: Measuring The Performance Of Light Shelves And Occupant-Controlled Blinds On A Dimmed Lighting System

Secondary Author(s)

Floyd, David

Report Number





Windows; Buildings


The design of a daylighted space is both an art and a science. The biggest challenge facing the lighting designer is to admit only as much light as necessary and distribute it evenly throughout the space without introducing glare or heat. In warm climates such as Florida, it has become common practice in windowed spaces to specify blinds and glazing with high shading coefficients to control glare and minimize heat gain. However, this practice reduces the effectiveness of lighting systems that dim automatically. Improved systems are needed to capture natural daylight and distribute it uniformly throughout a space while controlling heat gain and glare. One such system is the light shelf. Light shelves shade the space from direct sunlight and reflect this sunlight onto the ceiling for a deeper and more uniform distribution. While this is not a new idea, little unbiased empirical data has been collected, outside the laboratory, that compares the performance (energy savings, uniformity, and level) of an automatic daylighting system.This study measures the effectiveness of light shelves and manually controlled horizontal blinds in an automatic daylighting system. Power consumption and interior work-plane lighting levels were compared in four essentially identical private offices. Two offices were configured with an interior light shelf, one with a white diffuse top surface and the other with a specular surface. The third office had no window treatment and the fourth office had horizontal blinds, which were manually adjusted by the user. All offices had two lamp fluorescent luminaires with dimming ballasts (min. 20%) controlled by a ceiling mounted photosensor. The study showed that daytime savings ranged from 29% to 46%, with the largest savings from the office with the light shelves. The office with horizontal blinds showed the poor savings (32%) and also the poorest light uniformity and level.

Date Published



Buildings - Windows

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