The paper contributes to the theory of thermal radiation. In addition to the common energy spectrum of radiating surfaces, the entropic and exergetic spectrum of the perfectly black or perfectly grey surfaces is proposed. Also, besides the traditional surface emissivity, called in the considerations the energetic emissivity, the entropic and exergetic emissivities are introduced. The theoretical considerations are developed for a better understanding and interpretation of the radiative ability of a surface in comparison to the nature of the emission product.
Keywords: engineering thermodynamics, radiation spectra, exergy, thermal radiation, entropy, surface radiation