23.

A meaning similar to that of the volume density of the total radiation $u$ is attached to the volume density of radiation of a definite frequency ${𝗎}_{\nu }$. Summing up for all parts of the spectrum we get: $u={\int }_0^{\infty }{𝗎}_{\nu }d\nu .\mathrm{\backslash Label}[eqn](22)\text{upshape (22)}$

Further by combining equations [eqn:(9)] (9) and [eqn:(20)] (20) we have: ${𝗎}_{\nu }=\frac{1}{q}\int ({𝖪}_{\nu }+{𝖪}_{\nu }^{\prime })d\Omega ,\mathrm{\backslash Label}[eqn](23)\text{upshape (23)}$ and finally for unpolarized radiation uniformly distributed in all directions: ${𝗎}_{\nu }=\frac{8\pi {𝖪}_{\nu }}{q}.\mathrm{\backslash Label}[eqn](24)\text{upshape (24)}$

[Radiation at Thermodynamic Equilibrium]IIRadiation at Thermodynamic Equilibrium. Kirchhoff's Law. Black Radiation