# A Review of Light Shelf Designs for Daylit Environments
## 4 Conclusions  
It is not easy to draw practical rules from the review since design problems involving the adoption of light shelves may differ considerably in each case. In addition, since different test room dimensions have been used in different climates, either for measurements or simulation, the comparison of the performance of various light shelf designs using existing research results is also difficult. Nevertheless, there are some common findings:  
1. Light shelves perform differently under various sky conditions. The least favorable condition is that associated with overcast sky conditions. This situation results in an overall reduction in interior daylight illuminances although a slight increase of uniformity can be achieved. However, it seems that the use of an anidolic shelf can increase the daylight factors in comparison to a reference facade with conventional double glazing. In the deeper parts of the room, this increase was 1.7 times the value of the reference case when an urban environment was simulated [61]. 2. Light shelves performed best when installed in south-oriented facades. 3. Light shelves are usually placed above eye level in an effort to prevent glare form their upper surface. However, in the College la Vanoise in Modane, France, south window sills are used as specular light shelves having a tilt towards the south to avoid glare. Because of this tilt, the light shelf presents a seasonal selectivity to reflected sunlight. 4. Exterior light shelves can shade the lower window and if designed properly their performance can be better than conventional shading systems (i.e., overhangs) [39; 46]. 5. Uniformity of daylight illuminance do increase although luminance contrast can be increased giving rise to glare problems. The maximum uniformity increase that was reported is 178.6% in comparison with the reference case (i.e., window without any shading system) under clear sky conditions [63]. Under overcast conditions, uniformity is also improved but this is due to the reduction of illuminance levels in the area near the window. Another way to increase uniformity is to use an optically treated upper surface [47]. By using small mirrored surfaces oriented properly, uniformity can increase by 29.9-34.3% for the external light shelf and 10.4-13.7% for the internal, compared to the existing flat mirror-type light shelf. 6. In very broad terms, the maximum value of an external light shelf depth (\(d_{ext\ lightshelf\_max}\)) can be defined by [33; 34] using the equation: \[d_{ext\ lightshelf\_max}\leq 1.5\times h_{cleesteroy}\] (6) An extreme value of two to three times the height of the clerestory window was proposed by [PERSON] et al. [36] when the light shelf needs to be optimized for low sun altitudes. For internal light shelves, their maximum depth value reported in the literature is again by [PERSON] et al. [36]. They proposed that the internal light shelf depth should be two to three times the height of the clerestory window in an effort to reduce sun patches on the working surface. However, in the majority of the cases, internal light shelf depths ranging from 0.4-1.85 m were examined. 7. In terms of work-plane illuminances, when a light shelf is used there is an overall decrease of illuminance values between 20% and 60% in comparison with a reference case (unshaded, no light shelf) while its effectiveness decreased after 6-7 m from the window. However, light shelves can increase work-plane illuminance levels in the secondary non-daylit zone ranging from 10% to over 70% when highly reflective materials or mirror surfaces are used. This relative increase is inversely proportional to the window size. [40; 45; 50; 53; 56; 59; 63; 67; 69]. A slight tilt of a light shelf can increase the light flux entering the room especially during summer months in the northern hemisphere, increasing at the same time the cooling load. The increase of the latitude decreases this angle [47]. In [60] [PERSON] et al.