Radiative Transfer in Particle Clouds

Abstract

The total rate at which energy is radiated by a source is the flux Φ measured in watts. Figure 2.1 shows a small area dA₁ emitting radiation into the element of solid angle dΩ at the angle θ₁ to the normal n₁. The flux contained within this solid angle is simply proportional to the projected area dA₁cosθ₁ and dΩ, i.e.

$${\rm{d}}\Phi {\rm{ = }}I\,{\rm{d}}{A_1}\,\cos {\theta _1}\,{\rm{d}}\Omega $$

(2.1)

where the constant I is the photometric intensity of the source with dimensions of power per unit area per unit solid angle (Wm⁻² ster⁻¹). In the radiative transfer literature it is common to refer to this merely as the intensity and this convention will be adopted in this chapter. However, I must be distinguished from /used in Chapter 1, which is used for scattered intensity and has the dimensions of power per unit area (Wm⁻²).