For the case of optically thick medium of length L, i.e., σ ph L >> 1, and using the gas kinetic theory, the photon conductivity k ph is 16 σ SB T /3 σ ph (σ SB : Stefan–Boltzmann constant, σ ph : average photon absorption), and photon heat capacity n ph c v,ph is 16 σ SB T / u ph.