In this Comment it is shown that the modulation effect in heterodyne detection discussed in a recent paper [H. M. Perry and T. P. Dougherty, Phys. Rev. B 55, 5778 (1997)] is nonexisting since it results from an incorrect description of the propagation of the electromagnetic fields. Hence, the claim of the authors that previous observations of polariton beating result from this modulation effect is incorrect.