Frequency selective surface

A frequency-selective surface (FSS) is a thin, repetitive surface (such as the screen on a microwave oven) designed to reflect, transmit or absorb electromagnetic fields based on the frequency of the field. In this sense, an FSS is a type of optical filter or metal-mesh optical filter in which the filtering is accomplished by virtue of the regular, periodic (usually metallic, but sometimes dielectric) pattern on the surface of the FSS. Though not explicitly mentioned in the name, FSSs also have properties which vary with incidence angle and polarization as well; these are unavoidable consequences of the way in which FSSs are constructed.

Source: Wikipedia — Frequency selective surface (CC BY-SA 4.0)