For a thermal black body, the total amount of radiated energy E* in Watts per square
meter is proportional to the fourth power of the temperature of the body according the
relation E* = εσT^4. This formula had been deduced already in 1884 and
is called the Stefan-Boltzmann law. In this formula, T is the temperature of the body in
Kelvin and σ is the so-called Stefan-Boltzmann constant being 5,67 x 10^-8
W/m^2.K^4. The radiation factor ε has no dimension and it indicates to which
degree a certain body is a perfect thermally black body. The radiation factor therefore
is always smaller than one and for a non-perfect black body the curve of the
emission spectrum is proportionally lower while the relative distribution of the
wavelengths of the radiation roughly stays the same.