A light shelf is a horizontal reflective surface, at or above eye level, used both to passively channel natural daylighting into an occupied space and provide shading. Sunlight is reflected from the upper surface of the light shelf into the room interior and particularly onto the ceiling where it provides additional diffuse light thus helping give uniform illumination and also allows sunlight to penetrate deeper into the interior of a building.

 

Under conditions of an overcast sky, light shelves cannot increase the lighting level. They operate most effectively in sunlight. In this context ceilings are designed to be higher than normal for best operation.

 

Some degree of control is possible by modifying the angle of the light shelf; either internally or externally in combination. Problems with low-angle winter sunlight penetration can give rise to glare. Difficulties can be experienced in cleaning light shelves, especially the external type.

 

Basic principle of a light shelf Credit: Alcoa