Study of light in Diffuse Media

Abstract

In materials where both absorption and scattering are present, measurement of absorption 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). These relations are valid only in the diffusive light transport regime where scattering dominates over absorption. They also fail near boundaries where incident beams are not diffusive. In this thesis, we have developed an integrating sphere based diffuse optical measurement system. Using the system, we measured total transmittance and total reflectance of samples with varying optical parameters and obtained empirical relations between K-M coefficients and the radiative transport coefficients which are valid both in the diffusive and non-diffusive regimes. Our empirical relations show that the K-M scattering coefficients depend only on reduced scattering coefficient (s’) while the K-M absorption coefficient depends both on absorption (a) and reduced scattering (s’) coefficients of radiative transfer theory. We have shown that these empirical relations can predict total reflectance within an error of 10%. They also can be used to solve the inverse problem of obtaining multiple optical parameters such as chromophores concentration and sample thickness from the measured reflectance spectra with a maximum accuracy of 90-95%. We have also used our method to decouple the absorption and scattering properties of micron sized iron oxide particles which is not possible with standard absorption spectroscopy techniques. Our method is capable of measuring the specific absorption of iron oxide within 5- 10% error. This method along with the derived empirical relations can be further extended to UV regimes to study nanoparticles which are of relevance in various photonic applications.