A light shelf is a horizontal or near horizontal baffle fitted part way up a window to control and redistribute incoming sunlight and daylight. This paper describes a computer program to analyse light shelves and other daylighting systems. The program agreed well with measurements made in experimental rooms at the Building Research Establishment. Experimental measurements had revealed that an internal light shelf could improve the uniformity of daylight in a room and provide some solar shading, but without significantly increasing illuminances at the back of the room. The computer analysis confirmed these findings and provided information on optimum shelf depth and the benefits of an exterior light shelf. Light shelves should be as reflective as possible, and to work best need a high ceiling. In relative terms light shelves perform best in rooms with external obstructions, when they can increase core illuminances by around 15%.
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