Study of Light Transport in Multilayered Turbid Media

Abstract

Many materials observed in nature exhibit multilayer optical structures. Few examples include skin of humans, epithelial cells, leaf in plants etc. The optical properties of these materials are of great interest to scientists for variety of reasons including disease diagnosis, chlorophyll estimation etc. However most of these materials exhibit strong scattering and measurement of absorption properties of these materials becomes difficult since it is not possible to decouple absorption from scattering using standard absorption spectroscopy techniques. Various authors have tried to use diffuse optical spectroscopy to address this. However these measurements involve computer intensive calculations to obtain optical parameters from the diffuse optical spectra. Kubelka-Munk (K-M) theory is a phenomenological light transport theory that provides analytical expressions for reflectance and transmittance of diffusive substrates. Many authors have derived relations between coefficients of K-M theory and that of the more fundamental radiative transfer equations (RTE). In this thesis, we have modified an empirical model developed earlier by Roy et al to relate the K-M and RTE coefficients and improved its accuracy.

We have validated the feasibility of using these empirical relations to decouple the absorption and scattering properties of a turbid medium. We find that in presence of absorption, the scattering properties of a scattering material decreases. We have developed an empirical equation to obtain the reduced scattering coefficient of the pure scattering material from the scattering properties of the same material in the presence of absorption which can predict the reduced scattering coefficient very accurately.

We have built a double layer optical model using the K-M theory. In order to validate the model we developed optical phantoms using a mix of PDMS and commercially available iron oxide particles. Using the empirical relations and the double layer model we can extract the optical properties of the double layer optical phantom system within an error of 10% establishing the feasibility that this model can be used to study the optical properties real systems such as skin tissues and plant leafs.