For an absorbing plane parallel plate, a simple reciprocal transformation exists between the expressions for the reflectance ℛ and transmittance T, enabling easy passage from one to the other. Then it is shown that the expression a = 1-ℛ-T is always positive and, further, may be identified with the energy computed for the dissipation by the Joule effect in the plate, provided that the resultant electric field is calculated at each point inside the plate.
H. DAMANY and V. CHANDRASEKHARAN, "Energy Dissipated in an Absorbing Plane Parallel Plate," J. Opt. Soc. Am. 58, 763-765 (1968)