I am seeking clarification of the following terms:
- Black body radiation
- Thermal radiation
- Thermal light source
At the first glance, the first two are the same thing, and they are the radiation emitted by the third. However, once we define them more rigorously, they do not seem identical anymore:
- Black body radiation is an equilibrium state of a photon gas, from where Planck's law readily follows.
- Thermal radiation is the radiation emitted by thermal light sources.
- Thermal light sources are light sources in a state of thermal equilibrium.
My point is that, although black body radiation is an excellent first approximation for the light emitted by thermal light sources, it is only an approximation. Indeed, the radiation emitted by an incandescent lamp, heated object or a star is not in equilibrium - otherwise, the source would be absorbing as much radiation as it emits, i.e., it would be not a light source but a black body. Furthermore, the term thermal light source arguably refers not to the properties of the radiation, but to the mechanism of the emission: the material of the source is in the state of thermal (quasi)equilibrium, but the source is not in equilibrium with the surrounding radiation. In fact, this is manifestly the case for most of the light sources that are conventionally referred to as thermal: a heated object cools down, incandescent lamp does not give light, if a current doesn't continuously heat it, and the stars keep radiating only because of the constant supply of energy due to nuclear reactions.
To summarize: there seems to be much ambiguity about what is called thermal in this context. I will appreciate clear definitions, preferably supported by references.
Black-body radiation is the thermal electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body).
An example of a light source in thermal (quasi)equilibrium, but not in thermal equilibrium with the radiation is a gas lamp, which emits radiation corresponding to a specific atomic transition.