Actually, this is a two-step process which, for convenience, we ``lump'' into one subheading. The two steps are:

- Trace particles to intersection points, and
*Determine the particle's interaction with the surface of interest.*

These steps are displayed pictorially in Figure 19. Note that, the particles are ``translated'' along their initial direction vectors until a surface of intersection is determined (this completely specifies the geometry of intersection, in particular the angle of incidence which determines the material properties). At these angles of incidence, and for the specific materials of the surfaces of intersection, we ``look up'' the material properties. We have the following possibilities, so indicated on Figure 19: absorbed (), diffusely reflected () or specularly reflected () (here, we disallow transmissions for clarity). For those particles which are absorbed, we count them in the absorption matrix , where:

Thus, after this event:

Figure 19 Step-2 Particle Disposition. View Figure