Minimum Opaque Covers for Polygonal Regions
  The Opaque Cover Problem (OCP), also known as the Beam Detector Problem, is the problem of finding, for a set S in Euclidean space, the minimum-length set F which intersects every straight line passing through S. In spite of its simplicity, the problem remains remarkably intractable. The aim of this paper is to establish a framework and fundamental results for minimum opaque covers where S is a polygonal region in two-dimensional space. We begin by giving some general results about opaque covers, and describe the close connection that the OCP has with the Point Goalie Problem. We then consider properties of graphical solutions to the OCP when S is a convex polygonal region in the plane. 
J. Scott Provan, Marcus Brazil, Doreen Thomas, Jia F. Weng
cs.CG