where A is surface area, E b is energy flux (the rate of emission per unit surface area) and F 1 → → 2 is the view factor from surface 1 to surface 2. Applying both the reciprocity rule for view factors, A 1 F 1 → → 2 = A 2 F 2 → → 1 , and the Stefan–Boltzmann law, E b = σ σ T 4 , yields: