How to define EM radiation due to local temperature (Fluctuation-Dissipation Theorem)

Hi,

I am working on simulation of thermophotovoltaic cell, which is a PV cell illuminated by radiation emitted by a hot body. To simulate the PV cell I need to know the incident radiation from the heated body. So my problem now is how to define the link between the local temperature of an emitting body and the stochastic current density due to this local temperature, which will be input to Maxwell Eq. and then I can know EM wave distribution.

Thank you for your response