a positive scalar

R Finite-element model of the region R

R Finite-element model of the boundary aR

Rs, s=1,2,..S Finite-element models of the complementary

regular subsurfaces aRs, s=1,2,...S

x=(x1,x2,x3) A point in R having coordinates x1, x2,

and x3 with respect to an orthogonal coordinate system

x=(x,y,z) A point in R having coordinates x,y and z with respect to a

Cartesian coordinate system

p Unit outward normal of the boundary aR at a point x e aR

zo Thickness of half thickness

s Optical distance

so Optical thickness or optical half thickness