Light shelves are reflective horizontal surfaces that extend from the exterior into the interior of a building. They can extend the useful range of perimeter daylighting on a building's south side to about 25 feet on sunny days. Light shelves can prevent unwanted direct sunlight, which is a source of glare, from entering a space. The shelves reflect sunlight onto the ceiling, minimizing glare and boosting light levels in the space. Light shelves work well at high solar angles, but at lower angles the shelves need to extend deeper into the room to catch the sunlight.
Light shelves can also reduce the amount of heat that enters a space.
In the early 1980s, researchers tried to increase the performance of light shelves by developing a reflective light shelf that responds to the angle of the sun. At high solar angles, the roller retracts and forms a short tilted mirror. At low solar angles, the film unrolls to produce a wide, almost horizontal mirror. Refraction is the principle behind prismatic glazing, a material with ridges on one side that bend light to specified angles. Prismatic glazing can be used to exclude sunlight from a space or redirect it, usually to the ceiling.