Angular selectivity: Impact on optimised coatings for night sky radiative cooling

To achieve strong net thermal radiation emission from surfaces whose temperature is at or below ambient it is important to have high absorptance between 7.9 μm to 13 μm where the atmosphere is most transparent. Outside of this band the atmosphere behaves like a black body emitter and hence at these wavelengths net radiant heat loss is normally not possible at sub-ambient temperatures. It becomes possible using two types of angular electivity, which also improve emission between 7.9 μm to 13 μm. One is coating based, and one uses external heat mirrors. In the latter low emittance mirrors replace the higher emitting segments of the atmosphere. The coating's net gain is a result of its reflectance rise countering the atmosphere's drop in transparency as ray angles to the zenith approach the horizontal. These ideas are examined in the context of experimental data on coatings which rely on nanostructure to largely limit their spectral absorption to the atmosphere's transparent band. The angular selective coating becomes possible in two multilayer types (a) one nano-layer is strongly reflective (b) one layer has much higher index than the other. Type (a) materials as nanoparticles provide surface phonon resonance in the desired absorption band. © 2009 SPIE