where we have used the limits τ τ ∈ ∈ [ τ τ , ∞ ∞ ) as we are integrating outward from some depth within the atmosphere; therefore μ μ ∈ ∈ [ 0 , 1 ] . Even though we have neglected the frequency-dependence of parameters such as S , we know that it is a function of optical depth therefore in order to integrate this we need to have a method for deriving the source function. We now define some important parameters such as energy density U , total flux F and radiation pressure P as follows