We propose a simple and reliable method for extracting the optical constants of homogeneous dielectrics which can be pure or impure, and polar or nonpolar. The extraction is made from fringing reflectance spectra of slab samples with and without metal on the backside. The method is demonstrated to work well for polar semiconductors GaAs and InP in the infrared regime. The extracted extinction coefficient spectra exhibit plenty of features which correspond well to those in absorption and Raman spectra. (C) 2014 AIP Publishing LLC.