Projected surface mass density ($\Sigma (\mathbf{\xi })=\int \rho (\mathbf{\xi },l)dl$)

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.