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Pencil (optics)

File:Pencil 1675.png
In 1675, a pencil was interpreted as a double cone of rays, as from an object point, through a lens, to an image point.
File:Pencil beam 2.png
A pencil-beam radar
A pencil-beam radar

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.

In optics, the focusing action of a lens is often described in terms of pencils of rays. In addition to conical and cylindrical pencils, optics deals with astigmatic pencils as well.[1]

In electron optics, scanning electron microscopes use narrow pencil beams to achieve a deep depth of field.[2]

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.[3]

See also


  1. ^ Edward L, Nichols and William S. Franklin (1903). The Elements of Physics: A College Text-book. Macmillan Co. p. 77. 
  2. ^ Nick Johnson (19 May 1983). "The art of seeing the very small". New Scientist 98 (1358): 472. 
  3. ^ Faiz M. Khan (2009). The Physics of Radiation Therapy (4th ed.). Lippincott Williams & Wilkins. p. 521–522. ISBN 978-0-7817-8856-4. 

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