PHOTON MIGRATION AT SHORT-TIME AND DISTANCES AND IN CASES OF STRONG ABSORPTION

Diffusion theory is widely used to describe photon migration in turbid media because it is simple and, in certain cases, can be accurate to within a few percent. However, it neglects the ballistic nature of pho ton propagation between successive scattering events and hence entirel y breaks down for short times and distances, as well as for strong abs orption. Here we generalize on the exact two-stream theory of one-dime nsional photon migration to obtain a telegrapher equation that account s for both diffusive and ballistic aspects of propagation in three-dim ensional media. At long times and distances the standard diffusion the ory is recovered, whereas at short times and distances we find improve d predictions for such phenomena as pulse spreading, diffuse photon-de nsity wave dispersion, transmission through a slab, and pulse reflecti on from a semi-infinite medium. Our theory should be useful for accura tely characterizing turbid media, such as biological tissues, and may also aid in improving the spatial resolution of images made with diffu se light. (C) 1997 Optical Society of America.