Light shelves can increase daylight penetration into a building from the typical 1.5x the height of the top of windows up to 2.5x.

Light shelves are surfaces with reflective upper sides, located near the top of windows. They allow light to penetrate further into a building by reflecting some of the light onto the ceiling, which in turn reflects that light further back into the room.

In addition to increasing light penetration, this also has the effect of slightly reducing the amount of light close to the window, decreasing harsh contrasts in lighting as you move further into a room, away from the windows.

To be most effective, a light shelf should extend on both the exterior and interior side of windows, although benefit is still gained by exterior-only light shelves, and some lesser benefit is also still gained from interior-only light shelves.

Light shelves are also most effective when used on the south side of a building if in the northern hemisphere, and the north side if in the southern.