# Performance of Retro-Reflective Building Envelope Materials with Fixed Glass Beads
## 5 Conclusions  
In this study, a total of 10 glass bead RR samples with different refractive indices (1.5 or 1.9) and different color reflective layers (white, yellow, gray, silver, transparent) were developed. In order to compare these developed glass bead RR samples to RR materials commercially available in the market, two types of RR materials were chosen in this study. The angular retro-reflectivity of these developed glass bead RR samples was evaluated by using an emitting-receiving optical fiber system in the laboratory.  
The major points of this research are:  
* The prism and capsule sheet have a relatively higher angular retro-reflectivity than the other developed glass bead RR samples, however, their angular retro-reflectivity decreased sharply when the incident angle of light is below about 60\({}^{\circ}\) (capsule) or 75\({}^{\circ}\) (prism). When the incident angle is increased to 80\({}^{\circ}\), their retro-reflectivity is nearly zero (about 0.02). Thus, it is considered that the prism and capsule sheet commercially available in the market are not effective for retro-reflecting incident sunlight at high incident angles.
* Compared to the prism and capsule sheet, the change in angular retro-reflectivity of the developed glass bead RR samples with white, yellow, gray layers and a refractive index of 1.9 is relatively stable when the incident angle is varied from low to high values. However, the angular retro-reflectivity of these developed glass bead samples is a bit smaller (average value of about 0.13) than that of commercially-available prism and capsule RR sheets. Thus, it is considered that it is possible to use the glass bead with a refractive index of 1.9 and white, yellow, or gray layer to create the RR samples instead of the expensive prism and capsule sheet commercially available in the market.
* Among developed glass bead RR samples with five different colors of reflective layers and two refractive indices, the results showed that RR samples with white, yellow and gray reflective layers and a refractive index of 1.9 have better RR performance and relatively constant change in angular retro-reflectivity over the range of angles than those with silver and transparent reflective layers and a refractive index of 1.5.
* No matter what color reflective layers, the developed glass bead RR samples with a refractive index of 1.5 have smaller angular retro-reflectivity than that with a refractive index of 1.9. Thus, it is considered that the glass bead with a refractive index of 1.9 is more effective in mitigating the UHI effect as a RR material rather than that with a refractive index of 1.5.  
For further work, these glass bead RR materials will be considered for industrial production and will be applied to simulated or actual building facades. The effect of the glass bead RR materials on the indoor thermal comfort and outdoor environment will also be evaluated in the future.  
[PERSON] and [PERSON] created the materials; [PERSON] implemented the optical experiment; [PERSON] and [PERSON] analyzed the data of the experiment; [PERSON] wrote the paper; and [PERSON] checked the English.  
This research received no external funding.  
The authors are sincerely grateful to the UNITIKA Corporation of Japan for providing the glass beads used to create the RR samples.  
The authors declare no conflict of interest.