In this thesis the reflectance of ultra-sharp periodic groove arrays in a gold surface is studied for a general direction of light incidence. This includes the case of incident light propagating along the grooves. These structures are interesting due to their tunable absorption selective properties, which make them promising candidates in various methods of renewable energy production such as thermophotovoltaics and concentrated solar power. Two efficient numerical modelling approaches are presented, namely a simple and approximate stack matrix method that uses the mode-index of gap-plasmon polaritons (G-SPP's) as an effective index, and a rigorous Green's Function Surface Integral Equation Method (GFSIEM). The results of the highly simple stack matrix method show remarkable similarity to the exact results obtained with the rigorous GFSIEM, which reinforces the idea that the physics of light absorption in such structures is dominated by the coupling of light into plasmons.