Extension of Planck's law of thermal radiation to systems with a steady heat flux

Planck's law of thermal radiation is limited to equilibrium systems that have a definite temperature and do not carry any heat flux. Here we extend it to steady‐state systems with a constant heat flux. The obtained formulas explicitly describe the spectrum of thermal radiation in every direction and provide a sound basis for the self‐consistent analysis of radiative heat transport across interfaces, gaps, layered and other important structures.