Emissivity as a function of wavelength, direction, and temperature correlates to a material's efficiency in radiating thermal energy. Knowledge of emissivity is essential for designing and developing radiation-cooled thermal protective systems for hypersonic applications. It is desirable to achieve a high emissivity (with a value close to 1) to maximize heat radiation from a hot surface of a hypersonic vehicle's leading edge during atmospheric re-entry. With the goal of providing the hypersonic materials community with this necessary knowledge, this article offers a basic understanding of thermal radiation, methods for measuring emissivity at high temperatures, and a comprehensive overview of the emissivity of ultra-high temperature ceramics.

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