(color online) The schematic diagram of the optical structure of the quasi-Cantor radiative cooler. It has seven altering layers of SiO₂ and TiO₂ that are arranged in a symmetric pattern. The thickness of the silver (Ag) layer should exceed 200 nm to obtain a high efficiency in cooling. Aluminum (Al) in this figure is to illustrate the base material in practical use, which in some cases, can be the shell of continuous thermal source. The presence of the Al layer does not affect the performance of the cooler.