Light transport in tissue.

The propagation of light in tissue may be calculated by exact transport theory, or the approximate diffusion theory, provided the optical properties are known at the source wavelength. Optical properties for the exact methods are the absorption coefficient, scattering coefficient, and angular distribution of scattering. Appropriate properties for diffusion theory are the diffusion length and diffusion coefficient (corrected for anisotropic scattering). Computer programs and analytical solutions (for some simple geometries) exist, but the optical properties have to be determined experimentally and are not well defined as yet. The radiant energy fluence rate and the diffuse transmittance and reflectance have been measured in several tissues and in a few geometries, but there are gaps in the data as a function of wavelength. Calculations and measurements reveal that very large errors can result if the optical properties (for example, the diffusion length) are inaccurate, if anisotropic scattering is neglected, or if the finite size of the irradiating light beam is not taken into account. Furthermore, the radiant energy fluence and transmittance are perturbed by local regions of lesser or greater absorption, although recovery of the fluence and transmittance occurs beyond some three diffusion lengths.