Physics:Pencil (optics)

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Short description: Narrow beam of electromagnetic radiation or charged particles
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 beam (conical or cylindrical).

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, and the process is known as beamforming.

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]

In backscatter X-ray imaging a pencil beam of x-ray radiation is used to scan over an object to create an intensity image of the Compton-scattered radiation.

History

In 1675, a pencil was interpreted as a double cone of rays, as from an object point, through a lens, to an image point.
Definitions of ray, pencil, and beam in Henry Coddington's 1829 A System of Optics, Part 1

A 1675 work describes a pencil as "a double cone of rays, joined together at the base."[4] In his 1829 A System of Optics, Henry Coddington defines a pencil as being "a parcel of light proceeding from some one point", whose form is "generally understood to be that of a right cone" and which "becomes cylindrical when the origin is very remote".[5]

See also

References