  We used a multiple-scale homogenization method to derive generalized sheet
transition conditions (GSTCs) for electromagnetic fields at the surface of a
metascreen---a metasurface with a "fishnet" structure. These surfaces are
characterized by periodically-spaced arbitrary-shaped apertures in an otherwise
relatively impenetrable surface. The parameters in these GSTCs are interpreted
as effective surface susceptibilities and surface porosities, which are related
to the geometry of the apertures that constitute the metascreen. Finally, we
emphasize the subtle but important difference between the GSTCs required for
metascreens and those required for metafilms (a metasurface with a "cermet"
structure, i.e., an array of isolated (non-touching) scatterers).
