Projected surface mass density
= Projected surface mass density
{title2=$\Sigma(\boldsymbol\xi)=\int\rho(\boldsymbol\xi,l)\,dl$}
= Surface mass density
{synonym}
Projected surface mass density is mass per unit projected area, obtained by integrating <mass density> along the line of sight. It determines the enclosed cylindrical mass used by a <thin gravitational lens equation>. It need not equal a physical disk's density, and its aperture mass differs from a spherical three-dimensional enclosed mass.