Fig. 2.

The geometry of a slab of tissue is shown (a) with orthogonal axes $(x,y,z)$ and radiant emission $R(z=0)$ out of the top surface, in the $-z$ direction. The geometry to estimate internal fluence with the extrapolated boundary method is shown in (b) with the assumption that at some distance above the surface, the fluence drops to zero. $z_0$ represents the depth of the apparent source, $S(r)$.