In general, the exciton eigen energies consist of a series of bound states that emerge energetically well below the fundamental bandgap energy and a continuum of unbound states that appear for energies above the bandgap. Therefore, a typical semiconductor's low-density absorption spectrum shows a series of exciton resonances and then a continuum-absorption tail. For realistic situations, γ γ λ λ increases more rapidly than the exciton-state spacing so that one typically resolves only few lowest exciton resonances in actual experiments.