# A Review of Light Shelf Designs for Daylit Environments
## 2 Overview
### Performance Analysis of Active Light Shelves  
In 1983, [PERSON] and [PERSON] [65] proposed a set of tracking mirrors on an equatorial mount. This arrangement was selected because the reflected beam is parallel to the right-ascension axis and collection area can be maximized. The results showed that lighting energy savings of up to 200 Wh/m\({}^{2}\)day can be achieved in areas where normal beam illuminance exceeds 50,000 lx for at least four hours per day. [PERSON] [66] examined the difference between static and dynamic light shelfs (tilt angle adjustment) using simulations and scale model measurements in Sao Paulo, Brazil (Latitude: \(-23^{\circ}\) S). The specific paper deals with heat gains as well, a rather rare fact in the bibliography. The north oriented scale models used had dimensions of 0.6 \(\times\) 1 m with a ceiling height equal to 0.6 m and a window-to-wall Ratio of 100%. Illuminance and radiant energy measurements were collected. Two test cells were used in parallel with one of them having a static light shelf and the other a dynamic one using polished aluminum on the upper surface. In comparison with standard horizontal shading devices the heat gain using light shelves increased, but this does not seem to affect overheating conditions. Static light shelves, improve uniformity, but they decrease illuminance levels under overcast conditions. On the other hand, dynamic ones should be set in its optimum tilt in accordance with the sun's position. Otherwise, they will have a performance less than the static light shelf. [PERSON] and [PERSON] [67; 68] assessed the daylight performance of a deep office space together with its energy balance when an active sunlight redirection system was used mounted on a light shelf (Figure 12). The results from the assessment of the system indicate the possibility of significantly improving the daylight levels of non-daylit areas ranging from 30% to 50%, depending on the mirror area and on improvements in uniformity (<40%) when compared to an unshaded/unobstructed reference case. Although lighting energy savings can be achieved, cooling consumption increases significantly, diminishing the aforementioned lighting energy savings. Figure 12: Schematic representation of the sunlight redirection system examined in [68; 69]. [PERSON] and [PERSON] [69] used a simulation approach to optimize the design of a horizontal light shelf, composed of an array of mirror tiles that can tilt on two axes tracking he sun. Five alternative designs were examined in an effort to select the most efficient variant. This consisted of a frame which holds an array of fifteen mirror tiles. Two stepper motors are used to rotate the system on two axes. The system was simulated in three locations, New York (Latitude: 40.70\({}^{\circ}\)), Anchorage (Latitude: 61.21\({}^{\circ}\)) and Phoenix (Latitude: 33.44\({}^{\circ}\)), using a room of dimensions 9 \(\times\) 12 m with a ceiling height of 3 m. For the first two locations, a south orientation was used while for the third east-west orientations tested as well. The findings show that the dynamic light shelf did increase daylight levels resulting in an increase of estimated lighting energy savings in comparison with a mirror static shelf of 8% (Phoenix EW) to 22% (Phoenix S). [PERSON] [54] examined a light shelf with adaptive geometry. This consisted of an external part which could be rotated, and an internal part whose depth can be adjusted. Using a global optimization algorithm and radiance simulation, managed to estimate the hourly variation of the light shelf angle in an effort to minimize lighting energy consumption in an office building in Singapore (Latitude: 1.35\({}^{\circ}\) N). Results indicate that lighting energy savings of about 12% are possible compared to a traditional static light shelf. [PERSON] and [PERSON] [70] extended the abovementioned study [43] by examining the daylighting performance of various internal light shelf configurations using radiance-based simulations and physical scale model experiments.