Characterisation of layered scattering media using polarized light
This thesis investigates the properties of backscattered polarized light from layered scattering media with a view to application in the imaging of in-vivo skin for medical application. The research includes investigation of numerically simulated samples, tissue phantoms and in-vivo tissue. The aim of the early research is to identify the differences in behaviour between initially linearly and circularly polarized illumination concerning the rates of depolarization with scattering. Initial examination is made through Monte Carlo simulations. The analysis yields results which indicate, for forward scattering media, circular polarizations maintain their initial state to greater depths within a scattering medium than linearly polarized light. This result is exploited to show sensitivity of the different polarizations to different layers within a medium and indicates the potential to achieve coarse optical sectioning. These fundamental properties are extended to a full field imaging arrangement, using both simulated and experimental results to illustrate polarization gating to perform subsurface object imaging in a medium composed of uniform scatterers. The concepts are extended to imaging an in-vivo sample. The removal of multiple scatter and surface reflections is performed using a combination of linearly and circularly polarized illumination coupled with image subtraction, to provide a sub-surface, localised tissue image. This provides an improvement on currently applied techniques, which use linearly polarized light and the application of a flat glass plate and matching fluid. To exploit the spectral properties of tissue, the development and construction of a fully automated, multiple wavelength, polarization imaging system, suitable for testing in a clinical setting is presented. Theoretical investigation into the potential to perform the determination of the optical properties of a two layered sample, using simulated results in-keeping with the system's detection regime, is performed. It is illustrated that the variation of top layer thickness, bottom layer scattering, top and bottom layer absorption coefficient provides well-conditioned data when combining polarization and spectral information.