The black body radiator was a concept from the middle of the 19th century, introduced by Kirchhoff in 1859.
Kirchhoff discussed the thermal
equilibrium between the radiation (heat) within an arbitrarily shaped container and
the walls of the container (black bodies) that completely absorb all incident radiation.
The walls also emit radiation since otherwise there would be no radiation
within the container at all (which contradicts experiments).
The radiation inside such a cavity is called **black body radiation**.

From thermodynamics (second law) one
can show that the **spectral energy density** of
black body radiation, i.e. the radiation energy per volume and per frequency
interval, is only a function of the frequency and the temperature of the walls, and does not depend,
e.g., on the shape of the container:

Note: is the radiation energy within the frequency interval .

Kirchhoff had already pointed out the importance of determining the explicit form
function . Its importance should lie in the fact that it was
*universal*, i.e. independent of any details of the geometry of the container. Such a universal function
could be expected to contain deep physical insights about thermodynamics and radiation.

In fact, it took 40 years and the efforts of many physicists to finally find the explicit form of . After Kirchhoff's introduction of the `black body', people where wondering how to realise this theoretical concept experimentally. They first tried to blacken metallic (platinum) plates without much success. The success came by Otto Lummer and Wilhelm Wien (`Physikalisch-Technische-Reichsanstalt', the PTR in Berlin, a precessor of the nowadays PTB, the German national Bureau of standards). They went back to the original definition (thermal equilibrium with the walls of the container) of the black body and argued that one should use a cavity with a small hole inside to get the black body radiation out of it, without disturbing it too much.

At that time, there was already the theoretical prediction by **Wien**
who had found in 1893 a *scaling law* for
, stating

with an (unknown) `scaling function' of only

Wien even made a suggestion for the explicit form of in analogy to Maxwell's velocity distribution in a gas (

where is the speed of light. Wien's law was compatibel with the experimental results until the middle of the year 1900. Lummer and his coworker Kurlbaum had developed a very precise bolometer, based on the bolometer by Samuel P. Langley used in astrophysics from 1880. Furthermore, Lummer and his coworker Pringsheim developed black body radiators that could operate in a very large temperature range between -188C and 1200C, later up to temperatures of 1600C.

It turned out that Wien's law (1.4) was quite a good description of the experimental data but there were small deviations at large temperatures. Lummer and Pringsheim again improved their experiment into the range of up to wavelengths of m and K, and the deviations became even stronger. The story became even more confusing in the autumn of 1900 when Friedrich Paschen in Hannover claimed good agreement of his data with (1.4), and Max Planck also had `proven' it by thermodynamic considerations.

The bomb came with new measurements by a guest scientist at the PTR (Heinrich Rubens) which extended
up to
m. The deviations from Wien's law could not discussed away any longer. Rather, in the
extreme long wave-length limit, Rubens found good agreement with another radiation law that had previously
set up by Lord Rayleigh (**Rayleigh-Jeans-law**),

where is the Boltzmann constant. Rayleigh's law followed from the

Rubens told Planck of his observations over afternoon tea,
and the same evening Planck, in a desperate attempt to
`improve' Wien's law, suggested an interpolation formula between (1.4) and (1.5),
**Planck's law**

where the new constant is the

Planck's law turned out to give excellent agreement with all the experimental data. He solved this puzzle by the hypothesis