Reflection*

You can either perform a Kramers-Kronig Analysis for which you would ideally need a spectrum of the material over a very large range (which is strictly speaking possible only for amorphous materials or in case of cubic materials or if the spectrum is taken with the polarisation along the crystal axes for uniaxial and orthorhombic and the b-axis for monoclinic crystals) or you can perform dispersion analysis (with similar restrictions like the Kramers-Kronig analysis) from which you can directly obtain the oscillator frequencies (which is much better than using the maximas of the absorbance which are equal to the oscillator frequencies only for weak absorptions). Oh, by the way, you have to analyze always the complete spectrum, a simple way to just convert reflection maximas to absorbance maximas is in general not possible!