Reflectance spectroscopy is an experimental technique for acquisition and analysis of light backscattered from a turbid medium. In the field of biomedical optics, reflectance spectroscopy is most commonly used for estimation of turbid medium optical properties, which carry information about the structural and chemical composition of a biological tissue. As such, reflectance spectroscopy has the potential to provide insight into various diseases and malformations that affect the biological tissue and could thus aid to better understanding and evaluation of a disease. The main advantage of the reflectance spectroscopy lies in the convenient experimental setting that allows illumination and light detection to be performed on the same side of the turbid medium. A broadband light source in the visible and near-infrared spectral range is usually used to illuminate the turbid medium. The incident light is then either reflected from the turbid medium or refracted into the turbid medium, where it is subjected to physical processes such as absorption and scattering. Due to scattering in the turbid medium, the propagation direction of light changes. Consequently, some light is backscattered from the turbid medium, where it can be detected by optical fiber probes, integrating spheres or by using hyperspectral imaging systems. The detected backscattered light forms the so-called reflectance spectrum. The optical properties of the turbid medium can be estimated by modeling the reflectance spectra using a light propagation model and subsequently computing the free parameters of the model by minimizing the difference between the modeled and measured reflectance spectra.
The estimation of optical properties utilizing reflectance spectroscopy is complex and often requires the use of simplified light propagation models. However, such simplifications may limit the applicability of the models. Namely, many studies tend to substantially simplify the geometrical and structural description of the experimental setup. This especially holds for the optical fiber probes, where the probe-medium interface is often assumed to be a
simple laterally uniform boundary between two media with mismatched refractive indices. Additionally, proper description of the scattering phase function, which defines the angular properties of scattering, is often overlooked in the models that are used to estimate optical properties. The role of scattering phase function becomes increasingly important with decreasing distance between the light source and detector. In this case, the acquired reflectance, in addition to the absorption and scattering coefficients, significantly depends on the scattering phase function. Although incorporating the scattering phase function into the estimation of optical properties presents a difficult task, the shape of the scattering phase function carries important information about the turbid medium microstructure. As such, estimation of any additional information associated to the scattering phase function could yield information about biological tissues on a cellular level.
This thesis focuses on accurate estimation of optical properties from reflectance spectra acquired with optical fiber probes. Firstly, we quantitatively evaluate the impact of commonly used simplified geometrical and structural properties of the optical fiber probes in the simulation of light propagation in turbid media utilizing the Monte Carlo method. We find that in order to accurately estimate the optical properties, the materials that form the optical fiber probe tip and their arrangement have to be taken into account. Subsequently, we objectively evaluate the existing models for estimation of optical properties from reflectance spectra. We prove that the scattering phase function plays an important role in the estimation procedure. By accounting for some of the scattering phase function information, we show that the models can be substantially improved and thus yield more accurate estimates of the optical properties such as the absorption and reduced scattering coefficients and the additional scattering phase function related parameters.
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