An elegant experimental  proof of Kirchhoff's law that a good absorber is a good emitter is the classic experiment by Ritchie .  I've seen a better presentation of it than the image below but have not been able to find it again . This image is extracted from Demonstration Experiments in Physics by Richard Manliffe Sutton , 1938 .  I'd appreciate a better source if someone finds one .

Classic experimental proof of Kirchhoff's law