Page "Black-body radiation" Paragraph 14

The concept of the black body is an idealization, as perfect black bodies do not exist in nature.

Graphite and lamp black, with emissivities greater than 0. 95, however, are good approximations to a black material.

Experimentally, black-body radiation may be established best as the ultimately stable steady state equilibrium radiation in a cavity in a rigid body, at a uniform temperature, that is entirely opaque and is only partly reflective.

A closed box of graphite walls at a constant temperature with a small hole on one side produces a good approximation to ideal black-body radiation emanating from the opening.