Reduction of Reflected Heat of the Sun by Retroreflective Materials

It is demonstrated that the walls made of retroreflective materials can reduce the reflected heat of the sun in the directions of neighboring roads and buildings. For mitigating the urban heat island effects, the applicable area of retroreflective materials is larger than that of high-reflective paints, because retroreflective materials can be used not only as "cool roofs" but also as "cool walls." Then, the solar retroreflectances of several retroreflective materials, which cannot be measured directly by a spectrophotometer, were measured for the first time. The procedures were as follows: First, the reflectance without retroreflection was measured by using a spectrophotometer with the integrating sphere. Then, the total reflectance was deduced from the amount of temperature rise by solar irradiation. Finally, the retroreflective component was calculated by subtracting the former from the latter. The measured retroreflectances are 20 to 30 percent for the prism-array type, about 20 percent for the capsule-lens type, and about 10 percent for the bead-embedded type.