Diffusion Theory of Light Transport

Abstract

In this chapter light is principally described by particles with energy hv and velocity c. These particles are scattered or absorbed by structures in turbid media such as biological tissues and are reflected at boundaries according to the laws of Fresnel. In this chapter we will consider only monochromatic light, which covers most practical situations in medical laser applications. The formulations are easily extended by polychromatic light as long as scattering events are elastic. The theory becomes more complicated for inelastic scattering, such as the occurrence of fluorescence. However, even the diffusion theory for those cases is a straightforward extension of the discussions given in the following sections. Finally, we neglect polarization and interference. To include polarization one would need four diffusion equations instead of one.1 Polarization of incident light is usually lost in highly scattering media within a millimeter from the surface.2 Therefore, the effort required to extend the diffusion approximation to include polarization is probably not worthwhile, given the limited validity of that approximation, in particular near surfaces and sources of collimated light.