Pencil (optics)

In optics, a pencil or pencil of rays is a geometric construct used to describe a beam or portion of a beam of electromagnetic radiation or charged particles, typically in the form of a narrow cone or cylinder.

Antennas which strongly bundle in azimuth and elevation are often described as "pencil-beam" antennas. For example a phased array antenna can send out a beam that is extremely thin. Such antennas are used for tracking radar. See Beamforming for further details.