# A Review of Light Shelf Designs for Daylit Environments
## 2 Overview
### Performance Analysis of Static Horizontal Light Shelves
(2016) tested an internal light shelf that was 1 m deep at a height of 2.08 m from the floor, mounted in a mockup office facing slightly West of due South (Latitude: 53.35\({}^{\circ}\) N). The office space was 3 \(\times\) 9 m with ceiling height of 2.7 m. Because the system managed to reduce light levels more at the front of the room than at the back, uniformity increased under all conditions but with smaller improvements occurring during overcast sky conditions. Compared with the reference case (conventional glazing) findings from measurements showed a reduction in daylight levels ranging from 10% to 20%. As a general conclusion they suggested that fixed systems can only increase core illuminances under certain well-defined conditions; when the sun shines directly onto the window and their performance could be increased if a movable system that can be adjusted to work more effectively is adopted. It seems that a large increase in core daylight illuminance was not feasible and therefore, in UK, these daylighting systems should be considered as shading devices. [PERSON] and [PERSON] (2016, 2017) used two South oriented scale models with rectangular openings to examine the efficiency of a highly reflective (91%) light shelf, in Madrid, Spain (Latitude: 40.41\({}^{\circ}\)). The models represented spaces with dimensions of 6 \(\times\) 6 m with a ceiling height of 2.8 m and a Window-to-wall Ratio of 50%. One was equipped with the light shelf and the other was left free of obstruction to act as the reference case. Measurements of mean hourly illuminances were obtained during a period of one year from both models. The objective was to show the dependence of solar elevation and solar azimuth on daylighting efficacy. This was defined as the ratio of the mean hourly illuminances obtained by the model equipped with the light shelf and the reference model respectively. As a shading device, the light shelf was designed with a vertical shading angle of 50\({}^{\circ}\). The light shelf provided higher uniformity than the reference model. Measurements show that the efficacy of the system increases together with the solar elevation up to a point and then decreases. Further increase in the solar elevation resulted in a decrease of the light shelf's performance. The maximum values of daylight efficacy (>1) are only obtained on clear days, and approximately between 110 th and 270 th day of the year. This implies that exterior light shelves have a difficulty to treat lower sun positions. In a similar study, [PERSON], and [PERSON] (2016) compared the daylighting performance of two light shelf configurations that had both internal and external parts against overhangs. Four scale models with South facing apertures were constructed using the same space dimensions as ([PERSON], 2016; [PERSON], 2016). The first was equipped with a white opaque metacralite light shelf, the second with a mirror light shelf, the third had an overhang, while the last one was left without a shelf as a reference case. Illuminancemeasurements took place in Madrid, Spain for a period of one year. The light shelves and the overhang were designed to provide the same shading effect (vertical shading angle of 50\({}^{\circ}\)) as shown in Figure 5.  
Findings of that study showed that light shelves, by redirecting light into the interior, performed better than a classic overhang by 10-25% in terms of illuminance levels inside the space. The metacralite light shelf is more efficient than the mirror one, during the central months of the year and for the hours from 08:30 to 11:30 showing improvements ranging from 10% to 35%. The opposite happens at the beginning and at the end of the year. These results emphasize the importance of the light shelf's upper surface reflectance. [PERSON] and [PERSON] [40] assessed the benefits of light shelves, calculating useful daylight illuminances (UDI) in office buildings by conducting annual simulations taking different Window-to-wall Ratios, window shapes, facade orientations and external obstructing elements into consideration.