vision strip
n.— «Indeed, during the 1970s, many districts, including San Diego Unified, began to construct new schools with no windows. Educators argued that windows prevented students from concentrating; architects of these schools created “vision strips,” as they called them, thin slots at the tops of the walls that allowed some light to come into the room, but nothing else.» —“Going beyond No Child Left Behind” by Richard Louv San Diego Union-Tribune (California) Jan. 31, 2006. (source: Double-Tongued Dictionary)