General classification of PDRC materials. From top to bottom, different options are shown based on whether solar reflectance is provided by a metallic mirror, a porous IR-transparent membrane, or a strongly emitting disordered layer

Passive Radiative Cooling is a renewable cooling method by which an object on Earth can cool below ambient temperature by radiating its thermal energy through the atmosphere, to outer space.

This is possible due to the existence of a so-called "Atmospheric Transparency Window" in the thermal infrared range between 8 and 13 μm, through which there are no significant absorption bands of atmospheric gases. This wavelength range coincides with the peak of the thermal (black-body) emission of an object at ambient temperature (300 K), which can therefore dump part of its thermal energy to the cold sink of outer space (3 K). This is a well-known phenomenon which explains why, after a clear sky night, we sometimes find a thin layer of ice over our car windshield or over grass blades, even when the ambient temperature remained above zero (silica and cellulose are both strong emitters in the thermal infrared).

During the day, the down-welling intensity from the Sun is too intense to observe this effect due to the fact that most common materials absorb some fraction of the solar wavelengths, which obliterates the radiative cooling effect.

This limitation can be overcome by engineering the spectral properties of materials so that they exhibit a negligible absorption at solar wavelengths, and emit as much infrared radiation as possible inside the atmospheric transparency window. By doing so, even during the day and underneath direct sunlight illumination, a material can lose more thermal energy than it absorbs from the Sun, thus cooling itself and its substrate to a new equilibrium condition below the ambient temperature.

At LENS, the Complex Photonics group works on the fabrication and numerical modeling of new sustainable structures and materials for daytime passive radiative cooling, and on the development of accurate testing methods to evaluate their radiative cooling performance under realistic application conditions.