u ( ν , T ) = 4 ν 3 c 3 e x p ( a ν T ) mit a , b = c o n s t {\displaystyle u(\nu ,T)={\frac {4\nu ^{3}}{c^{3}}}exp\left({\frac {a\nu }{T}}\right)\,{\text{mit}}a,b=const} [1]