  The image of planetary nebulae is made by three different physical processes.
The first process is the expansion of the shell that can be modeled by the
canonical laws of motion in the spherical case and by the momentum conservation
when gradients of density are present in the interstellar medium. The second
process is the diffusion of particles that radiate from the advancing layer.
The 3D diffusion from a sphere as well as the 1D diffusion with drift are
analyzed. The third process is the composition of the image through an integral
operation along the line of sight. The developed framework is applied to A39,
to the Ring nebula and to the etched hourglass nebula MyCn 18.
