(color online) The emissivity of the radiative cooler at near-normal incidence. Panel (a) shows the emissivity within solar spectrum range, which presents a low absorption of the total solar radiation; panel (b) indicates high emissivity within the atmospheric transparency window of hot summer in subtropical region (Guangzhou, Guangdong, China, with moderate air pollution and ambient air temperature of 30° celsius). Panel (c) illustrates the emissivity of the radiative cooler from normal incidence to grazing incidence.