K1 Radiation of Surfaces

1 Introduction

All substances, regardless of shape and consistency, emit and absorb radiation energy when their temperature is above absolute zero (T > 0 K). For nontransparent opaque bodies, the absorption and emission processes are confined to their surfaces. Thermal radiation energy can be regarded as electromagnetic waves carrying energy and entropy in the wavelength range between 0.1 μm and about 1000 μm (0.1 μm < λ<1000 μm). Because of these emission and absorption processes in all substances (solids, liquids, and gases) around there will be a net transfer of energy between those bodies which face each other and whose surfaces have different temperatures. This net thermal radiation energy is, from a thermodynamic view, a heat flux, as its only driving force is a temperature difference and as it carries entropy. In thermal equilibrium situations, the emission and absorption processes still exist, but the net radiation energy will be zero. The calculation of thermal radiation...