Effective radius
= Effective radius
{title2=$R_e$}
{wiki}
= Half-light radius
{synonym}
The projected <radius> enclosing half an object's total <luminosity> in a specified <photometric passband>. For circular apertures, the mean linear <surface brightness> inside it is $\mathcal I_e=L/(2\pi R_e^2)$. Elliptical apertures require an axis-ratio factor or a circularized <radius>; that convention must be stated when comparing calibrations.