Fig. 1.

For a non-normally incident source, photons travel an average distance, three-dimensional, at a refracted angle, ${\theta }_t$, before they are scattered. The peak of the diffuse reflectance profile is shifted by some distance, $\mathrm{\Delta }r$, and can be sampled at various distances, $r$, away from the peak, after which Eq. (2) can be fit to the profile.