A New Instrument for Angularly and Spectrally Resolved Radiative Emission Measurements of Radiative Cooling Structures

Research on terrestrial radiative cooling has yielded a large volume of theoretical and experimental results for many designs, including thin film multilayer stacks, nanoparticles, and metaphotonic structures, which are capable of passive radiative cooling. However, the most widespread method of characterizing radiative cooling is in an uncontrolled and poorly calibrated outdoor ambient. Direct measurement of the spectral and angular distributions of radiative emission simultaneous with sample surface temperature measurements are thus urgently needed by the community to facilitate inter-comparisons between radiative emission measurements and radiative cooling performance.<br/>Here, we report measurements obtained using a calibrated and extensively modeled custom-designed radiative emission apparatus capable of simultaneous measurements of surface temperature and spectral- and angular-resolved radiative emission. This instrument enables simulation of outdoor ambient environments under well-controlled laboratory conditions. In terrestrial radiative cooling, surfaces are cooled by radiating thermal heat from the terrestrial ambient environment to the cold ambient of space directly through the atmospheric transmission window (8 – 14 µm), while also reflecting sunlight (0.3 – 2 µm) to prevent parasitic solar heat absorption. To simulate an outdoor ambient environment exposed to the clear sky our design utilizes a cryoshroud to simulate the cold ambient of space (2.7 K), a solar simulator, thermoelectric heating elements to control the ambient sample temperature under high vacuum (&lt;1e-9 Bar) as well as ambient conditions, and insulating stage design to reduce convective/conductive heat loss mechanisms. COMSOL thermal and radiation modeling was used to validate our test apparatus design simulating closely outdoor ambient conditions.<br/>We also present thermal and radiative emission results for a nanoparticle-based radiative cooling structure in this instrument, comparing silicon dioxide (SiO₂) and/or silicon nitride (Si₃N₄) nanoparticle films with planar thin film multilayer structures for application in large-scale terrestrial radiative cooling. Nanoparticle films exhibit radiative cooling powers of over 60 W/m² at 300 K.