Luminosity density
= Luminosity density
{title2=$\lambda$}
The <luminosity> per unit spatial <volume>. For a spherical source, its enclosed <luminosity> is $L(r)=4\pi\int_0^r\lambda(s)s^2\,ds$, and its projected <surface brightness> is $\mu(R)=2\int_R^\infty\lambda(r)r\,dr/\sqrt{r^2-R^2}$.