# Performance of Retro-Reflective Building Envelope Materials with Fixed Glass Beads
## 1 Introduction
It was shown that the RR capacity of these RR materials becomes weaker as the incidence angle of light is increased from low to high values, and the specular (mirror) reflection becomes stronger. It was concluded that the glass bead-type RR materials are more effective when the incidence angle of sunlight is low. Several RR materials using glass beads with refractive indices of 1.5 and 1.9 have been developed by a research [16]. The reflective directional characteristics of these developed glass bead RR materials were investigated using optical apparatus in the laboratory. It showed that the RR material with a refractive index of 1.9 has the best RR capacity in terms of mitigating UHI effect and saving building energy. In addition, the difference between building envelope samples using glass microspheres and the same sample without the glass microspheres was compared in terms of retro-reflectivity at different incidence of light. It showed that the envelope samples without glass microspheres have almost the same overall solar reflectivity, however, the retro-reflectivity at different incidence is very small compared to the envelope samples with glass microspheres. Thus, it was concluded that the glass microspheres have a positive effect in the development of RR materials. New ceramic colored tiles using RR microspheres have been developed, and the continuous industrial process of producing the new RR tiles in an economic and sustainable way was introduced in a research [17]. The optical performance of these RR ceramic colored tiles was evaluated by spectrophotometric, angular reflectance and colorimetric analysis. Research investigated and compared the performance of three materials used for external wall surfaces, including a common ceramic commercial tile, a glass tile obtained by covering the ceramic tile with a transparent paint on which glass spheres are spread, and a barium tile obtained by spreading treated barium microspheres on the wet transparent paint on the ceramic tile [18]. The optic analysis by spectrophotometer indicated that the barium tile has the highest overall solar reflectivity (39%), compared to the common ceramic tile (30%) and the glass tile (32%). The angular reflectance analysis showed that both the glass tile and the barium tile have stronger RR behavior for most of the incident directions, compared to the common ceramic tile. The analysis on a potential in UHI reduction showed that both the glass tile and the barium tile are more effective to mitigate the UHI effect, compared to the common ceramic tile. The RR performance of three-mirror and four-mirror types of corner RR samples that are another-type RR materials (different from glass microsphere-type) had been analyzed using geometrical optical principles in the research [10]. A type of prism RR material coated with a type of glass sheet was developed [11]. The durability of the developed prism RR materials was investigated by thermal measurement for a long-term exposure (about 1.5 years) in the outdoor environment. It showed that both the solar reflectivity and retro-reflectivity of the prism RR material showed no significant decrease during the exposure period of about 1.5 years. Additionally, a simulation analysis of different reflective characteristics on the urban albedo was implemented. It showed that the prism RR envelope applied to simulated buildings in an urban canyon has the largest potential in terms of mitigating the UHI effect, compared to diffuse reflective (DR) and mirror reflective (MR) envelopes. Based on the above studies of RR materials for possible application to building facades to fight the UHI phenomenon, this study aims to use the glass beads to develop some glass bead-type RR samples and evaluate their retro-reflectivity for different incident angles.