Kirchhoff's law of radiation (G.R. Kirchhoff):

The emissivity of a body is equivalent to its absorptance at the simialr temperature.

Kirchhoff's laws (G.R. Kirchhoff)

Kirchhoff's first law

An incandescent solid or gas under high pressure will generate a continuous spectrum.

Kirchhoff's second law

A low-density gas will radiate an emission-line spectrum along with an underlying emission continuum.

Kirchhoff's third law

Continuous radiation viewed by a low-density gas will generates an absorption-line spectrum.