Quantitative imaging birefringence in optical anisotropic media

Abstract

Optical anisotropy is a property found in many natural and synthetic materials, such as minerals, biological tissues, and liquid crystals. It manifests as birefringence (due to refractive index anisotropy) or, more rarely in the visible range, as diattenuation (due to absorption anisotropy). Imaging birefringence is essential in various fields, including geology (for mineral identification and characterization), engineering (to reveal stress distribution through photoelasticity), biology (e.g., tissues diagnostics), and heritage science (e.g., parchment conservation). This thesis presents the development of an original polarizing microscopy system aimed at generating quantitative maps of optical retardance and optical axis orientation in anisotropic materials.
The system probes the sample in transmission using elliptically polarized light generated by a polarization state generator. The birefringent sample modifies the different input polarization states, which are then analyzed. Numerical processing of the recorded intensity images enables reconstruction of the two-dimensional spatial distributions of retardance and optical axis orientation. This thesis focuses on two key challenges: intrinsic mathematical ambiguities in the birefringence analysis and the impact of input ellipticity on the measurement accuracy. The results show that measurement accuracy increases with higher retardance values (e.g., negligible at $\Delta=10^\circ$, significant at $\Delta=90^\circ$) and is exacerbated by the use of low ellipticity.

Experiments were conducted to characterize both in-plane and out-of-plane optical axis orientations, notably using a geological thin section composed mainly of quartz. Additionally, by rotating a polypropylene film around three axes (one normal to the sample surface and the two others lying in the sample surface), it was possible to determine the nature of the sample anisotropy (uniaxial or biaxial). The influence of diattenuation was also assessed using a dichroic mineral, tourmaline. While diattenuation was found negligible for retardance measurement in thin sections, it significantly affected the optical axis orientation measurement. Finally, the set-up was used to probe collagen fiber orientations in parchments (optical axis along fiber's axis). Results revealed that transparent areas on parchments are not necessarily degraded (gelatinized) but arise due to fiber compression, which reduces light scattering and makes the parchment translucent.

The set-up developed in the thesis was shown to be useful for the study of both uniaxial and biaxial anisotropic transparent samples such as quartz crystal and polypropylene film, as well as parchment, which is a translucent and fibrous material. The limitations of the technique arise from the reduced signal-to-noise ratio and the depolarization, whenever the sample strongly absorbs light or is thick, or scatters light. However, it was shown that despite these limitations, it is possible to measure the birefringence of a fibrous material that scatters light as far as the thickness is low enough.

The experimental set-up could be upgraded to enable the measurement of diattenuation in addition to birefringence, and to determine the optical character and sign of anisotropic materials, i.e., whether the material is uniaxial or biaxial, and exhibits positive or negative birefringence.

Date of Award	5 Sept 2025
Original language	English
Awarding Institution	University of Namur
Supervisor	Olivier Deparis (Supervisor), Johan Yans (President), Augustin Dekoninck (Jury), Michel Voué (Jury), Michael Shribak (Jury) & Marc Wuilpart (Jury)