thermal emission

Thermal emission (or thermal radiation) is electromagnetic radiation (EMR) emitted by something because of its heat, such as from the activity at the molecular level creating photons. The well-known type of thermal emission, black-body radiation results when the source is in thermodynamic equilibrium, and often the terms thermal emission and thermal radiation are used specifically meaning that case. Thermodynamic equilibrium and implies the source is at a defined, constant temperature, and if a gas, implies the particle velocity is described by the Maxwell-Boltzmann distribution, the Planck function describing the resulting spectrum (black-body spectrum), and the resulting EMR is also termed Planckian.

Though thermal emission is often used to mean specifically the above case, sometimes the term is meant to include analogous emission from a source not in thermodynamic equilibrium, which diverges from the black-body spectrum (thus is non-Planckian, NP). If the source is close to equilibrium, the divergence may be small or may be very specific spectral features, and the black-body spectrum is often taken to be a good approximation. Kirchhoff's laws characterize the results of some situations. Thermal emission from an optically thin plasma can result in thermal bremsstrahlung, another case where the spectrum is non-Planckian.