We present a magneto-optical study of the energy spectrum of two-dimensional electrons subjected to a perpendicular magnetic field in the presence of a square lattice of antidots. Oscillations of the luminescence intensity, measured at the Fermi energy, reproduce structure observed previously in transport experiments ascribed to commensurate cyclotron orbits that fit around a specific number of antidots. We demonstrate that commensurability features in the luminescence intensity can be observed below the Fermi energy, leading to a quadratic dependence of their spectral positions on the applied magnetic field. The possible origin of this novel effect will be discussed by comparing it with model calculations.