Researching the design of a glass-bead retro-reflective material to reduce downward reflection for urban heat island mitigation

Abstract

Recently, retro-reflective (RR) materials were proposed for building facades rather than using diffuse highly reflective materials for urban heat island mitigation. However, existing research indicated that when using RR materials, retro-reflection starts to decrease and the downward reflection increases when the incident angle of light exceeds approximately 50° with respect to the surface normal, leading to a negative impact on pedestrians and worsening the urban heat island. To improve the negative impact of downward reflection, this study focuses on designing the surface of glass-bead RR materials to reduce the downward reflection component without changing its retro-reflectivity. The angular retro-reflectivity and angular reflection intensity distribution of a newly designed glass-bead (NDGB) RR sample, a normal glass-bead (NGB) RR sample, and a capsule RR sample (that is already sold on the market) are evaluated and compared using a laboratory-based optical experiment. Moreover, the upward and downward reflection ratios of three RR building walls are measured and compared using an outdoor building-wall model experiment. The results show that the angular retro-reflectivity of the NDGB RR sample is almost the same as the NGB RR sample and only 1.4% lower than that of the capsule RR sample; however, when the incident angle exceeds 60°, the downward (or specular) reflection of the NDGB RR sample is significantly reduced by approximately 97% and 98%, respectively, compared with the NGB and the capsule RR samples.