# A Review of Light Shelf Designs for Daylit Environments
## 1 Introduction  
Over the last decades, energy and environmental issues have led to increased energy efficiency and improved indoor lighting quality in building design. Yet, despite technological advances in lighting, it has not been possible to match the quality of the illumination produced by daylight. Daylight is preferred by users because of its psychological and physiological effects. These effects are well documented, covering an area from improved health, well-being and productivity for the users, to significant building energy savings [1; 2; 3; 4; 5]. Lighting control systems can reduce energy use from electricity by 20% or even up to 60% in certain conditions [6; 7; 8]. A side benefit to proper daylighting are the reduced costs for cooling, as the amount of heat generated by the lighting system is also reduced [9]. The use of daylighting systems, in an effort to increase core daylight illuminance, may reduce lighting energy consumption, but careful design is needed in order to avoid possible increases in cooling loads especially in cooling-dominated locations [10]. Thus, the main task of the design teams is to balance antagonistic phenomena, in an effort to maximize the users' thermal and visual comfort. For example, simultaneous reduction of solar gains and the provision of sufficient daylight levels with an improved uniformity is a typical problem that should be tackled during the initial design phase. Daylight can be delivered into a building with conventional techniques; windows, cleverages, etc. are common examples of side-lighting applications which have the disadvantage of only effectively illuminating areas that are located near them. The area benefiting most is called a daylit or perimeter area and is affected mainly by window placement and dimensions [11; 12; 13; 14]. Consequently, daylight distribution is not uniform in deep spaces, even when a fully glazed facade is used. In these cases, daylight alone cannot adequately illuminate the space without additional support from artificial lighting, especially in areas away from the facade. One way of addressing the problem is by integrating systems to the aperture that are capable of collecting and distributing daylight into the space, and if possible, enhancing the indoor daylight levels at the area further most from the openings. Thus, incoming solar radiation can be controlled by either using shading systems, such as overhangs, solar screens, venetian blinds, rollers and louvers, etc., where problems associated with solar gains and potential glare can be moderated [15; 16] or by adopting daylighting systems whose primary function is to redirect a significant part of the incoming light flux towards the ceiling. Thus, a more balanced distribution of daylight accompanied by a reduction of excessive illuminance levels near the windows can be achieved [17; 18; 19; 20]. There have been substantial research efforts placed on designing daylighting systems, in order to protect occupants from direct solar glare, but also to improve daylight distribution. The latter can lead to a reduction in electrical consumption for lighting [21; 22; 23]. Attempts to classify different daylighting systems have been presented in several articles and review papers [24; 25; 26; 27; 28]. Light shelves represent one of the most popular design choices in contemporary buildings and are often suggested in literature as effective devices that can improve the lighting quality of a space and offer energy savings especially when daylight controls are used. Light shelves are possibly the simplest among daylighting systems; a horizontal/inclined surface which is placed on a window above eye level, either internally or externally, is capable of controlling and redistributing incoming daylight through reflection on its upper surface. Due to its position in relation to the opening, it divides the window into two parts; an upper part (clerestory) which can be considered as daylight provider and a lower one (view) window.