# A Review of Light Shelf Designs for Daylit Environments
## 2 Overview  
Although the use of mirrors as a means to redirect sunlight has been known since antiquity (for example the specular characteristics of the concave side of shields used by [PERSON] in 212 BC to burn the sails of the Roman fleet, or the use of mirrors to reflect sunlight at the theater stage) it was [PERSON] and [PERSON] who designed an early type of the light shelf for Hagia Sophia in Constantinople (present day Istanbul, Turkey) in the 6 th century AD. In the initial design, [PERSON] used reflective window sills, which were located around the dome so that sunlight was reflected to the interior surface of the dome in an effort to increase its brightness [29; 30]. A schematic representation of the system is presented in the following Figure 1.  
Although in late nineteenth century reflectors were in production to increase indoor illuminances like the Tageslicht reflector form [PERSON]. Hanifch and Co. which was presented in Berlin trade fair during 1889, illustrated in Figure 2.  
It is not quite clear when the term \"light shelf\" started to be used. An early study for the use of light shelves in hospitals was carried out by [PERSON] during the 1950s [32]. Since the light shelf represents one of the simplest systems which reduces solar gains and redistributes daylight into the building interior, a large number of papers have been written in an effort to examine all related parameters affecting their performance. Focused research on the performance of light shelves begun during the 1980s. Although the light shelf's effect on a building's energy balance (especially in cooling loads during summer) is important, in this review, the term \"performance\" is related to the resulting luminance and illuminance distribution on the surfaces of the space associated with it. This performance varies according to the prevailing sky conditions, as light shelves control diffuse and direct illuminance differently according to the shelf's geometry and the material used in its upper surface. External light shelves offer daylight redirection together with shading of the lower window while the interior ones can minimize the sun-patch area on a working surface, especially in low altitude angles, thus, improving luminance distribution. It is therefore evident that their depth can significantly affect illuminance levels. An example is presented in the following Figure 3.  
Figure 1: Schematic representation of the parapet of Hagia Sophia based on [PERSON]’s reflector superimposed over the cross-section of the original dome and parapet presented to [29]. Figure 2: Daylight reflector by W. Hanifch and Co. [31]. Thus, the parameters identified in this review, affecting light shelf's performance are the following:  
1. Geometrical (width, length, mounting height, tilt angle and shape). 2. Reflectance type (specular or diffuse). 3. Material type (metal perforated or not, transparent). 4. Position adjustment (rotation, operation). 5. Building data (room dimensions including ceiling shape). 6. Climatic conditions. It must be mentioned at this point that direct comparison of the performance of various light shelf designs is quite difficult as there is no analysis on component level in the bibliography (except anidolic systems). The existence of light shelf metrics, could give an indication of their daylight implications. Today, most of building standards are oriented to the performance based approach, setting specific targets for the whole building. This in turn, reduced the usefulness of the light shelf metrics making their comparison in the initial design phase cumbersome. A light shelf metric could be the estimation of bidirectional transmittance distribution function which represents the variation of optical properties with the angle of incidence. In all papers examined, light shelf's performance was evaluated according to the illuminance levels on the working surface of a room or the luminance distribution on its surfaces. Since room dimensions used by various research teams varied widely, optimized designs varied as well.