Specific intensity is power per projected area, solid angle, and frequency interval. In vacuum it is conserved along a ray.
In local thermodynamic equilibrium, matter populations are thermal at the local temperature and the source function is the Planck function, .
When the source function varies approximately linearly with optical depth, the emergent intensity obeys .
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Radiative transfer is the process by which energy, in the form of electromagnetic radiation, is transmitted through a medium. This process is governed by the interaction of radiation with matter, which can absorb, emit, and scatter the radiation. Radiative transfer is fundamental in various fields, including astrophysics, climate science, meteorology, remote sensing, and optical engineering.