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.
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]
