Athermanous

We see also why in an athermanous body an increase of thickness does not augment the radiation, —the reason being that, since it is already athermanous, this increase cannot possibly make it absorb more heat, and therefore cannot make it radiate more.

Substances which admit of radiation taking place through them are called Diathermanous. Those which do not allow heat to pass through them without becoming themselves hot are called Athermanous.

Substances which are transparent to visible light waves are not necessarily transparent to radiant heat waves. Thus glass is distinctly athermanous.

A closed hollow solid whose athermanous sides have the same temperature (thermodynamic equilibrium) , is filled with "black-body radiation“.