alpha =0.8
sigmat = 0.5
d = 1
mui = 0.5
mur = 0.5

(* This solution is due to Chandrasekhar and used by Hanrahan & Krueger in SIGGRAPH 1993 *)
fr = (alpha/(4Pi)) (1 - Exp[-sigmat d (1 / mui + 1 / mur)]) / (mui + mur)
Lr = Pi/2 * fr