The material properties determine the particle/surface interaction and are defined in terms of a local spherical coordinate system. Figure 14 defines the cone angle, , and the azimuthal angle, .
Figure 14: Local Material Coordinate System.
Particle/material interactions of five types may occur as shown in Figure 15. Figure 15(a) depicts a ``specular'' transmission, whereby a particle passes straight through a specularly transmitting surface, with no change in direction. Figure 15(b) depicts a ``diffuse'' transmission, where the particle is transmitted, but its direction is ``diffused'' upon transmission. This is akin to a diffuse reflection, but from the ``back'' of the surface.
Figure 15(c) depicts a specular (mirrorlike) reflection where ``the angle of incidence is the angle of reflection,''and Figure 15(d) depicts a diffuse (random) reflection, with an equal probability of reflection in any ``direction.'' (We have exercised editorial license with this statement. In truth, a diffuse reflection has an equal probability of reflection in any solid angle, weighted by projected surface area). Finally, the particle may be absorbed. Note that, if the particle is neither transmitted nor reflected, it is absorbed---so that only 4 of the 5 material properties are required to be stored.
Figure 15: Particle/Material Interactions.