In this paper we consider two methods for the theoretical modelling of the reflection and diffraction spectra from an excitonic diffraction grating made from a quantum well with spatial modulation of the exciton resonance. The modelling was performed in the single-scattering approximation, and by the step-by-step approximations of the exact solution of Maxwell's equations. The expressions are obtained for the modulation of the inhomogeneous broadening of the exciton resonance. Theoretical results are compared with the experimental data for the quantum well grown on an ion irradiated substrate, and for a quantum well irradiated with ions after growth.