Transition from the incoherent to the coherent regime for propagative wave-based thermal radiation

The transition from the incoherent to the coherent regime for thermal radiation between bodies trough a transparent medium is discussed. The canonical case of two parallel semi-infinite planar media is used as a basis to provide an insight into the physics and quantities ruling the distance at which coherent effects have an impact on the propagative component of the net heat flux exchanged. A practical criterion is proposed to define the distance below which radiation intensity framework should not be used, but instead fluctuational electrodynamics.