Emissivity or Emittance is defined as $\epsilon = \frac{Energy\ radiated\ from\ a\ surface}{Energy\ radiated\ by\ a\ black\ body\ at\ same\ temeprature}$ For Thermal Equilibrium of a body $\epsilon = \alpha$ [NASA Reference publication 1121](https://ntrs.nasa.gov/citations/19840015630) is a good source for emissivity of common materials. It is the same list published in the Spacecraft Thermal Control Handbook Appendix. [NASA TPP–2005–212792](https://ntrs.nasa.gov/citations/20070014757) is a good source for coatings Emittance can be - Spectral directional emittance - Spectral, hemispherical emittance - total, hemispherical emittance ## 1. Factors affecting Emissivity Emissivity can be a function of wavelength, direction and temperature ### 1.1. Directional dependence - For metals emittance is low at most angles except at "grazing" angles i.e high [[zenith and azimuth angle|polar angle]] - non-metals typically have high emittance and very low emittance at grazing angles Diffuse - reflects radiation in all directions Specular - changes with source of radiation A diffuse emitter or a "Lambert" surface is one where emittance is same in all directions ### 1.2. Impact of Wavelength For Grey surfaces emittance does not depend on wavelength ### 1.3. Impact of temperature Emissivity can be a function of temperature ## 2. Note on Emissivity vs Emittance Radiative properties of opaque substances are highly influenced by the surface condition of the samples. Factors such as roughness, surface contaminants, paints, oxide layers etc. highly affect their radiative behavior. Hence, NIST recommends using "-ance" for real world rough and contaminated surfaces and "-ity" for pure, perfectly smooth ideal materials.[^1] *This convention is not strictly followed in various publications and caution should be applied when interpreting the terms.*