A pencil of points is a set of points that lie on a single line. Optics set of rays forming a rectilinear normal congruence . A homocentric ray pencil is a set of rectilinear rays that all intersect at a common point. A conical ray pencil is a homocentric ray pencil that trace the surface of a right circular cone. A collimated ray pencil is a set of parallel rectilinear rays.
Optics rectilinear normal congruence formed by rays that intersect two nonintersecting focal lines.
The two focal lines need not be mutually perpendicular, but they always lie in mutually perpendicular planes defined by the central ray of the pencil.