# Radiation

Radiative heat transfer between two or more surfaces can be approximated using the total, normal emissivity. This article provides empirically determined total normal emissivities for a number of materials including metals, metal oxides, common building materials and paints.

At any given temperature, real materials emit less energy than that of a black body. The effectiveness of a material at emitting energy is represented by a radiative property called the emissivity factor, which is the ratio of the actual energy emission of the material to that of a blackbody at the same temperature. This article will provide an overview of emissivity and its many formulations.

A blackbody is an idealized volume which emits and absorbs the maximum possible amount of radiation at a given temperature in all directions over a wide range of wavelengths. Blackbodies are perfect emitters and absorbers of radiation and therefore useful as a standard when studying radiative heat transfer systems where the amount of radiation emitted and absorbed is a also a function of material properties. This article describes the basics of a black body and presents equations to describe its emissive characteristics.

Heat transfer coefficients characterise the transfer of thermal energy in terms of heat flow and temperature difference between two participating media. This article demonstrates how to calculate the radiative heat transfer coefficient and thermal resistance for gray, diffuse radiative exchange.