Biomedical and atmospheric applications of optical spectroscopy in scattering media

Sammanfattning: Spectroscopic analysis of scattering media is difficult because the effective path length of the light is non-trivial to predict when photons are scattered many times. The main area of research for such conditions is biological tissues, which scatter light because of variations of the refractive index on the cellular level. In order to analyze tissues to diagnose diseases, or predict doses during, for example, laser treatment, it is necessary to be able to model light propagation in the tissue, as well as quantify the scattering and absorption properties. Problems of this type occur in many other areas as well, for example in material science, and atmospheric and ocean-water optics. This thesis deals with light propagation models in scattering media, primarily based on radiative transport theory. Special attention has been directed to the Monte Carlo model to solve the Boltzmann radiative transport equation, and to develop faster and more efficient computer methods. A Monte Carlo model was applied to solve a spectroscopic problem in monitoring the emission of gases in smoke plumes. An important theme in the thesis deals with measurement of the optical properties, with emphasis on biomedical applications. Several different measurement techniques based on a wide range of instruments have been developed or improved upon, and the strengths and weaknesses of these methods have been evaluated. The measurement techniques have been applied to analyze the scattering and absorption properties of some biological tissues. Much devotion has been directed to optical characterization of blood, which is an important tissue from a health-care perspective. At present, the complex scattering properties of blood prevents detailed optical analysis of whole blood. The work presented here is also aimed at acquiring a better understanding of the fundamental scattering processes at a cellular level.