Equation of Transfer of Radiation

Abstract

All of the terms of radiative transfer discussed in chapter II are expressed in terms of the specific intensity I _ν. Hence we have to find a relation to determine the specific intensity. The specific intensity is the result of the interaction between radiation and matter which should be studied by the quantum mechanics from the microscopic point of view. However for practical flow problem, our main interest is from the macroscopic point of view. The exact results of microscopic analysis may be represented in the average by means of some coefficients which represent the average physical properties of the medium considered. For the radiative transfer problems, the physical properties of the medium can be expressed in terms of an absorption coefficient k _ν and an emission coefficient j _ν. In the macroscopic analysis, we assume that these coefficients are known functions of the state of the medium (5, 7, 9). The exact form of these functions can be determined either from the microscopic analysis or from experiment. We shall discuss the determination of these coefficients in chapter XI. In this chapter, we shall assume that the medium is homogeneous so that the properties of absorption and emission change continuously in the medium. Special attention should be made for the case of radiation field with discontinuous media. For instance, when a solid body is in a gaseous medium, the radiative properties change suddenly at the surface of the body from those of the gas. We shall discuss the interface problems in chapter VI.