Lights in the Shadows, 3D-modelling knots with MOCASSIN

Most of the enveloppes of Planetary Nebulae (and other objects like novae) are far from beeing homogeneous: clumps, knots and tails are often observed. We present here the first attempt to build a 3D-photoionization model of a knot and the corresponding tail, ionized by diffused radiation issuing from surrounding material.