Excess brightness images used to determine the density excess ρ_e. The upper and lower panels show two different events on 2011 March 7 and 2014 February 25, respectively. Dots in red are the points where the ρ_e measurements have been made. Solid lines refer to the projected shock front determined with the ellipsoid model. Dashed lines represent the ±4% error in the geometry of the shock fronts.