Experimental Verification of Validity for Kirchhoff's Law (E=1-R) in Vacuum and Purged Air

Abstract

The simplest postulation (without wavelength or directional subscripts) for Kirchhoff’s law reads Ɛ=1-R, where Ɛ is emissivity and R is reflectance. However, Kirchhoff’s law was originally postulated for opaque materials in thermodynamic equilibrium with the surrounding medium (meaning the sample is isothermal and at the same temperature of the background to wich it radiates) [1]. Such a condition is rarely found in laboratory on Earth and never met in space; nevertheless, for simplification or lacking of suitable emissivity datasets (and being reflectance measurements much easier than emissivity measurements), Kirchhoff’s law is widely used to substitute emissivity with 1 minus reflectance measurements (and often bi-conical or bi-directional reflectance is used, and not hemispherical as it should be). The purpose of this paper is to show the behavior of emissivity vs. bi-directional reflectance measurements for samples of small to large grain sizes, under vacuum or purged air, at various sample temperatures.