# A Review of Light Shelf Designs for Daylit Environments
## 2 Overview
### Performance Analysis of Static Horizontal Light Shelves  
This is the simplest form of a light shelf. The types of light shelves that have been examined are horizontal and tilted with mirrored/diffuse upper reflecting surfaces. [PERSON], et al. [33], suggested that under sunny skies, light shelves are effective at increasing light levels at the back of the space compared to standard clear glass windows and concluded that in general, simple light shelf designs provide improvements in daylight penetration. They suggested that maximum exterior light shelf depth (\(d_{ext\ lightshelf,max}\)) should be less than 1.5 times the height of the clerestory window (\(h_{clerestory}\)) above it. \[d_{ext\ lightshelf,max}\leq 1.5\times h_{clerestory} \tag{1}\]  
According to [PERSON] et al. [34] the depth of the exterior horizontal light shelf should be 1.25-1.5 times the height of the clerestory window for South oriented facades (for 20\({}^{\circ}\) either side of South) and 1.5-2 times the clerestory height for facade azimuth angles greater than 20\({}^{\circ}\) and lower than 160\({}^{\circ}\) (azimuth from North 0\({}^{\circ}\), clockwise). [PERSON] [35] continued his previous work simulating the behavior of light shelves. His findings show an increase in daylight uniformity but without significantly increasing illuminances at the back of the room. Light shelves performed better when there was an external obstruction increasing core illuminance values by around 15%. This analysis provided some information on optimum light shelf dimensions. The interior depth (\(d_{int\ lightshelf}\)) should be equal to the height of the clerestory window above it (\(h_{clerestory}\)), while external depth (\(d_{ext\ lightshelf}\)) should be smaller than the difference between the distances of the light shelf's height from the floor (\(h_{lightshelf}\)) and the working plane (\(h_{work-plane}\)). Upper light shelf reflectance should be as high as possible while its performance increases when it is used in high ceiling spaces, as illustrated in Figure 4.  
Figure 4: Cross-section of a side-lighted space with an extended light shelf (internal and external) and main dimension parameters. \[d_{int\ lightshelf}=h_{clersetory} \tag{2}\] \[d_{ext\ lightshelf}\leq h_{lightshelf}-h_{work-plane} \tag{3}\]  
[PERSON] et al. (2016) investigated the impact of the light shelf depth on daylight working surface illuminance using internal, external and combined light shelves. Upper light shelf surface reflectance was 80% and light shelves depth varied from 0 to 3.5 m for the external and from 0 to 3 m for the internal ones, mounted on the window 2 m above the floor. The evaluations were based on scale models for daylighting measurements and simulation models for energy estimations in Texas, USA. The scale model represented a South oriented open plan office space with dimensions of 9 \(\times\) 9 m and a height of 3.5 m with Window-to-wall Ratio of 100%. The light shelf was designed to provide adequate shading to the lower view window and of course, to reflect light deep into the space through the clerestory window above the shelf. Different light shelf depths were evaluated and compared against overhangs of variable sizes corresponding to lengths of 1.2 m, 2.5 m and 3.65 m. In terms of illuminance levels, uniformity and glare discomfort, the results showed that combined (internal and external) light shelf systems perform more effectively, considerably improving uniformity. Among the cases examined, an exterior light shelf with a depth of two to three times the height of the view window and an interior light shelf with a depth of two to three times the height of the daylight window performed in the best way. However, as the authors stated, the results corresponded to the data collected for a low solar altitude angle (30\({}^{\circ}\)) and this explains the large size of the shelfs. [PERSON] et al.