Fallback approach – generic models

This PhD aims to provide models that are capable of capturing light scattering phenomena at a more fundamental physical level compared to geometrical optics. The implemented physical optics models mostly rely on fundamental physical quantities, particularly on the complex-valued index of refraction. Even though these quantities are accessible to measurement, they might not be readily available for many materials. In order to still be able to use the method with insufficient data, a fallback approach is pursued. The implementation of fallback cases allows a gradual downgrade of the models, finally ending at the level of common geometrical optics. In the case of missing material information, an inversion approach can be used to derive the information based on available empirical data (see section 4.12). If, however, the required spectrally resolved data is unavailable, the implementation of the fallback approach allows a downgrade to simpler, generic models. For example, if a material’s spectral refractive index function is unavailable, a reasonable constant value for the global radiation range can still be applied to achieve spectrally resolved results. If this approach seems inappropriate, the refractive index function alternatively can be set to unity. In this case, the subsurface reflectance function can be used to model ideal diffuse reflection. Again, any available spectrally resolved information can be used in this model, either highly resolved or integrated for specific wavelength ranges (e.g. visible, UV, IR). If even these spectrally resolved reflectance values are unavailable, a constant diffuse reflectance value can be assumed for the entire spectrum. This final and most simple generic model corresponds to the geometrical optics model that most standard approaches are based on.