We carried out measurements of optical transmission through a Frequency Selective Surface (FSS) on a silicon substrate perforated periodically with square cavities of 1 mm². The substrate is covered on one side with a thin film (1 μm thick) of silicon-nitride, thus forming a membrane for each cavity. The measurements were taken using a Martin-Puplett Interferometer over a spectral range from 100 to 650 GHz, providing a maximum transmission value of around 40% at 480 GHz. Analytical and computed results are also presented for comparison purposes.