Evaluation of retro-reflective properties and upward to downward reflection ratio of glass bead retro-reflective material using a numerical model

Abstract Recently, retro-reflective (RR) materials have been proposed to be applied to building facades instead of diffuse reflective (DR) materials for urban heat island (UHI) mitigation. This paper focuses on analyzing the RR performance of glass bead RR material under different incident angles of light rays by using the proposed bead model. The results show that the retro-reflectivity of bead model increases when the incident angle changes from 10o to 50o with respect to the surface normal, and the retro-reflectivity begins to decrease when the incident angle exceeds 50o with respect to the surface normal. Additionally, this study also analyzes the upward to downward reflection ratio by different incident angles. The results show that the upward to downward reflection ratio decreases and the specular reflection increases when the incident angle changes from small to large. There is strong relationship between upward to downward reflection ratio and incident angle, showing the correlation coefficient (R) is approximately 0.92. Moreover, the retro-reflectivity of bead on representative two days (Jun. 21 and Sep. 23, 2020) in Osaka is calculated using the proposed model. The results show that the retro-reflectivity gradually increases from morning to noon, reaches the maximum at noon, and then gradually decreases from noon.