Brightness in synthetic images

The brightness in each pixel (300*300) is proportional (because of the instrument sensitivity function) to the intensity of the Thomson-scattered solar white light from the electrons of the solar corona plasma, integrated over the line-of-sight.

The limb darkening (black disk in the center of the image) eliminates the contribution from the direct (not scattered) solar white light.

It is not a direct proxy of density as long as the scattered intensity depends both on the distance from the Sun and from the angle of scattering.

The change in brightness witnesses that the density distribution along the line of sight grows up. Quantitatively, the difference in brightness and visibility depends on many factors, in addition to the quantitative increase in density.