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.
Ionizing radiation used in radiation therapy, whether photons or charged particles, such as proton therapy and electron therapy machines, is sometimes delivered through the use of pencil beam scanning.
In Backscatter X-ray imaging a pencil beam of x-ray radiation is used the scan over an object to create an intensity image of the Compton-scattered radiation.
- Edward L. Nichols & William S. Franklin (1903). The Elements of Physics: A College Text-book. Macmillan Co. p. 77.
- Nick Johnson (19 May 1983). "The art of seeing the very small". New Scientist. 98 (1358): 472.
- Faiz M. Khan (2009). The Physics of Radiation Therapy (4th ed.). Lippincott Williams & Wilkins. pp. 521–522. ISBN 978-0-7817-8856-4.