However, energy can also be carried by photons . This is radiative heat transport, which has no analog in mass transport. In a closed system, the amount of energy absorbed from the photon gas is equal to that emitted by each object. The principle of detailed balance allows us to assert that even in an open system, objects still radiate the same amount of energy. The peak in the distribution is at an energy of about 3 kT. The radiated energy is in the far infrared at room temperature.
Real objects reflect some of the energy impinging on their surfaces.
Some typical emissivities are shown here. The full treatment of radiation can be extremely complex, since photons can reflect off multiple surfaces and follow complex paths. We'll examine only simple planar geometries to get some sense of what sort of heat fluxes occur in typical situations. Radiation is essentially independent of spacing, whereas conduction is strongly dependent on spacing. For a spacing of about 0.1 mm the two fluxes remain comparable over a wide range of temperatures.