Lorentz-invariant specific intensity
= Lorentz-invariant specific intensity
{c}
{title2=$I_\nu/\nu^3$}
The <photon> phase-space occupation is a <Lorentz scalar>. For <radiation> propagating through vacuum, this makes <specific intensity> divided by cubed <frequency> invariant under a change of inertial observer. With <Doppler factor> $D$, $I_\nu(\nu)=D^3I'_{\nu'}(\nu/D)$. This describes corresponding ray directions; it does not assert that an angularly integrated <radiation> field is isotropic in both frames.