Kirchhoff's Law refers to the fact that all bodies own the quality of emission and absorption. At a given temperature (T), in conditions of thermodynamic equilibrium, the ratio of capacity of energy and capacity of absorption of a certain body is a constant value, equal to the capacity of emission of the absolute black body, both, for radiation with a specific wavelength and for the total radiation with different wavelengths.
Kirchhoff's law leads to the following conclusions: a body that is a good absorber is also a good radiant.
The perfect black body absorbs all of the amount of radiant energy that falls on its surface. His capacity of absorption is equal to 1.