Reflection polarized light microscopy and its application to pyrolytic carbon deposits

The methods for the measurement of extinction angles by reflection polarized light microscopy were analyzed with respect to the investigation of pyrolytic carbon deposits. It is demonstrated that measurements of the extinction angle for deposits with circular morphology (e.g., a pyrolytic carbon layer on a fiber) must be strictly distinguished from the standard measurements of optically uniaxial materials with spatially uniform optical properties, such as a single crystal (e.g., graphite or pyrolytic carbon on a planar substrate). On the basis of the mathematical description of the method, an expression of the extinction angle for materials with a circular morphology is derived. This expression differs from the equation describing measurements of the extinction angle for optically uniaxial materials with spatially uniform properties erroneously applied for circular morphology. It is demonstrated on the basis of the developed formalism that no discrepancy exists between measured extinction angles for circular pyrolytic carbon deposits and single crystalline graphite, which was discussed earlier due to the misinterpretation of data measured by two different methods. The physical meaning of extinction angles is discussed and approaches for the measurement of two meaningful ellipsometric parameters instead of one extinction angle are proposed.