In 1992, several independent surface impedance boundary condition (SIBC) formulations were proposed for the FDTD method. Maloney and Smith [109] presented a frequency dispersive formulation for a SIBC which could be used over the spectrum of the incident pulse. In this work, E and H were related by a convolution sum which was subsequently modified using Prony's method. Beggs et al. [110] also presented a frequency dispersive SIBC formulation, although their SIBC is limited to the case of high conductivities. Yee et al. [111] presented two SIBC algorithms, one for an inductive SIBC and one for a capacitive SIBC, that can be used for monochromatic excitation. Kashiwa et al. [112] also presented a constant SIBC algorithm, though their algorithm is limited to problems which can be expressed by an equivalent circuit. In 1993, Kellali et al. [113,114] presented a frequency dispersive SIBC formulation which, unlike previous formulations which assumed normal incidence, is valid for any single angle of incidence. Lee et al. [115] have also presented a SIBC for use with the FDTD method in which they relate the tangential fields to their normal derivatives by a partial differential equation. Wang [116,117] has extended this alternate approach to both parallel and perpendicularly polarized 2-D electromagnetic waves.