A body that is hit by radiation and absorbes it all, is called a black radiator.
This body has no losses due to reflection or transmission and emits the maximum possible energy in all directions equally.
For a defined waveband, the emitted radiation depends only on the temperature of the black radiator.
In reality one can not find a body that can emit 100% of a radiation.
The best possible reproduction is achieved with a cavity with matte black walls all having the same temperature.
A beam falling through a hole into the cavity is reflected and absorbed several times at the inner wall.
The radiation coming out of the hole corresponds to the temperature of the body.
This effect is used, e.g. in the calibration of IR measuring instruments.