Optical analysis of glass bead retro-reflective materials for urban heat island mitigation

• Developed several glass bead retro-reflective (RR) samples for application to building walls.
• RR capacity of samples was investigated by an optical emitting–receiving fiber system.
• Retro-reflectance and angular distribution of RR samples are assessed by optical apparatus.
• Among glass bead samples, refractive index of 1.9 in a white reflective layer are most effective.
• Glass beads have better retro-reflectance than prism at high incident angle, about 70–80°.

Retro-reflective (RR) envelopes can reduce the urban heat island (UHI) effect. To develop a lower cost method for imparting RR properties to external wall surfaces, this research measures RR performance of RR surfaces made with glass beads. 8 glass bead RR samples were created, as well as 1 prism and 1 capsule type for comparison. Their angular retro-reflectance and angular distribution were measured by an emitting–receiving optical fiber system developed in our laboratory. It shows that the retro-reflectance of all RR samples at low incident angle with respect to the perpendicular is better than at high angles. The RR capacity of glass bead samples with a refractive index of 1.9 is better than those of 1.5. The retro-reflectance of prism sample is much larger than the all glass bead samples at incident angles up to about 60–70°. The performance decreased sharply at incident angles of 70° and above. At high incident angles, the glass beads are slightly better than prisms. This could make them better suited to apply to outer walls with the goal of reducing reflection of midday sunlight onto streets and buildings. Also, considering their simple construction, glass beads may be more cost-effective the prism in reducing UHI.