# A Review of Light Shelf Designs for Daylit Environments
## 2 Overview
### Performance Analysis of Active Light Shelves
Again, side by side measurements in scale models were used, one equipped with the internal light shelve configurations (Figure 13) and the other acting as a reference case. The findings suggest that the height of the clerestory window plays an essential role in determining the performance of light shelves. Compared to the reference case, a decrease in illuminance values was observed, but there was a significant increase of uniformity. Light shelves with 0.9 m clerestory height had higher indoor illuminance than light shelves with 0.6 m clerestory height, but when compared the optimum cases to the base case a decrease in the percentage of DF ranging from \(-\)62.0% to 34.1% was recorded. The optimum cases showed a significant increase in the percentage of uniformity up to 178.6%. Based on the findings, the authors suggested that under clear sky conditions, the daylighting performance of a light shelf depends on the dynamic movement of solar position and proposed a dynamic light shelf control with adjustable height that responded to the optimal configurations. The authors suggested a three-step control of deployment of the shelves that could be achieved by using an illuminance sensor to measure the outdoor illuminance and if values below 20 K lx where measured (which was assumed to represent an overcast sky in the tropic) the first configuration could  
Figure 13: Schematic representation of the dynamic light shelf operation based on the optimum cases (Modes’ (**A–C**)) proposed in [70]. be employed. During an intermediate day, the dynamic internal light shelf could be programmed to configure the other two steps, based on the orientations, months and times. [PERSON] and [PERSON] (2018) examined a method of control based on the tilt angle of an exterior mirror light shelf according to the position of the sun and a predefined target area on the ceiling and analyzed the resulting lighting energy savings. Two case studies were simulated in a deep, South oriented office space with dimensions 4 \(\times\) 7 \(\times\) 2.8 m: (a) with static and, (b) with an automatically controlled exterior light shelf in Athens, Greece (Latitude: 37.8\({}^{\circ}\)). Using the above Figure 14, the tilting angle of the light shelf (\(\vartheta\)) was defined as follows:  
\[\vartheta=\
icefrac{{(\theta_{sun}-\omega)}}{{2}} \tag{5}\]  
where \(\theta_{sun}\) is the projected sun's elevation angle, and \(\omega\) a constant angle defined by the aiming point. The resulting daylight autonomy values strongly depended on window size. Using only the cleerstory window (window-to-floor Ratio 6.2%) the increase in daylight autonomy was 5 times its value of the reference case (without light-shelf) while when view window was used the aforementioned increase was only 1.15. An interesting concept is the one that was proposed by [PERSON] et al. (2017). VALRA (Variable Area Light Reflecting Assemblies) is a tracking light shelf system that reflects light into a building at the south elevation or the roof and utilizes both direct and diffuse sunlight. This configuration can perform more efficiently under all incident sun angles. Using a direct current (DC) motor, the reflective plastic film surface can be adjusted over a spring-loaded tracking roller assembly as presented in Figure 15. The effectiveness of the system is limited to a depth of about 9 m depending on sky conditions and mounting limits. The payback period for the VALRA was said to vary ranging from three to 20 years depending several parameters like climate, the ratio of the collection area per illuminated floor area, building type, utility rate, etc. Figure 14: Tilted light shelf reflecting sunlight on a specified area on the ceiling examined in (2018).