Exciton optical spectra are sensitive to even slight changes in the properties of a solid. The spectra of exciton polaritons can be especially informative, although the spectroscopy of these quasiparticles imposes certain restrictions on the measurement temperature and the quality of the material. Exciton parameters such as the resonance frequency ω_T and damping coefficient Γ may vary under the influence of electric fields, defects present in the crystal, or changes in the chemical composition of the film. If the material characteristics vary along the thickness of the film (the z coordinate), so do the resonance frequency and damping coefficient. Here, the spectra of undoped GaAs layers epitaxially grown on GaAs substrates are investigated at T = 1.7 K. Analysis of the spectra at the exciton absorption edge makes it possible to estimate the "dead-layer" depth, the strength of electric fields, and the concentration of impurities.