The range of validity and accuracy of the GO/PO method for the analysis of ILAs has been determined by comparing the GO/PO results to reference data obtained using BoR-FDTD. The internal reflections have been modeled in both cases. Six lens configurations have been considered: two lens diameters (3lambda0 and 6lambda0), and three lens materials (Rexolite, Macor, and Silicon). Our results have shown that GO/PO can be used to study ILAs whose size is typically larger than 4~5lambda0. Additional results obtained with lenses coated with quarter wavelength matching layers will be provided during the conference. In that case, we will show that GO/PO is applicable for slightly smaller lenses.