Figure 4.

Common geometry used to model tissue. In the semi-infinite geometry, w, h, and d all go to infinity, while in the infinite slab geometry, w and h are infinite but d is finite. Both geometries have azimuthal symmetry about the z-axis, meaning the photon fluence rate only depends on the radial and axial cylindrical coordinates ρ and z. The unit vector n̂ points from inside the tissue to outside. On the left, a single source-detector pair (with separation ρ) in the reflection geometry is shown. Note that for the slab geometry, detectors can also be used for transmission measurements by being placed on the z = d plane. On the right is a cross-section showing that the radiance moving into the turbid medium at the boundary is due to the Fresnel reflection of the radiance incident on the boundary.