  We present a numerical study of electromagnetic reflection and cavity modes
of 1D-sub-wavelength rectangular metallic gratings exposed to TM-polarized
light. Computations are made using the modal development. In particular we
study the influence of the choice of boundary conditions on the metallic
surfaces on the determination of modes, on specular reflectance and cavity mode
amplitudes. Our full real-metal approach shows some advantages when compared to
former results since it is in better accordance with experimental results.
