The daylight distribution in a typical sidelit room is very uneven, with very high level of daylight in the window zone and very low level in the rear zone of the room. A lightshelf can be used to even out the daylighting. It is usually placed in a nearly horizontal position between lower and upper part of the window. It functions as a sunshading for the lower part of the window, reducing daylighting in the window zone. Light which is reflected to the ceiling increases the daylight in the rear part of the room.
On a site with predominantly clear sky with sun, both inside and outside lightshelves can be used. The outside lightshelf functions better as sunshading. A specular lightshelf reflects daylight more effectively. If there is a danger of solar glare, a diffuse material is recommended. The slope of the lightshelf should be designed in dependence of the mean solar altitude and geometry of the room.
For a site with predominantly overcast sky conditions and for north-facing windows a specular, outside, sloped lightshelf is recommended.