Radiation energy density
= Radiation energy density
{title2=$U/V=\int_0^\infty u(\omega)\,d\omega$}
The spectral radiation <energy density> $u(\omega)$ is defined per unit <angular frequency> and physical volume. Thermal <Planck law> convention excludes the vacuum zero-point term. A density per ordinary frequency instead obeys $u_\omega=u_\nu/(2\pi)$, evaluated at $\nu=\omega/(2\pi)$; <Einstein coefficients> multiplying these densities must be converted correspondingly.