Black Body

A black body was first defined by Gustav R. Kirchhoff (1824–87) in 1859 as an object that absorbs all radiation falling upon it. Such a conception of an ideal black body was crucial for understanding heat radiation and its laws. Since a completely black body does not exist in nature, it had to be constructed. Kirchhoff had already suggested that a black body was technically feasible in his famous paper formulating his radiation law: “If a volume is enclosed by bodies of the same temperature and rays cannot penetrate those bodies, then each bundle of rays inside this volume has the same quality and intensity it would have had if it had come from a completely black body of the same temperature and is therefore independent of the constitution and the shape of these bodies and is determined by the temperature alone.”

Although Kirchhoff as well as Ludwig Boltzmann (1844–1906) had already experimented with the design of a black body using a heated cavity, most of the first experimentalists trying to verify the radiation laws did not take up Kirchhoff's idea. Instead they made do with metal sheets with specially prepared surfaces or metals — through oxidizing, a layering of lamp black, roughening, etc. — to achieve a maximum of blackness. For instance the Danish physicist Christian Christiansen (1843–1917) had carried out such experiments around 1880. He tested the optical behavior of such powders as soot. He also made the observation, that conical tubes radiate with an emissivity of about 1, which means that they act as “small black spots”. All these arrangements had shown that it was possible to make a black body effective for a limited range of wavelengths and temperatures, but a totally black body remained a distant hope.