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In optics, a pencil or pencil of rays, also known as a pencil beam or narrow beam, is a geometric construct (pencil of half-lines) used to describe a Light beam or portion of a beam of electromagnetic radiation or charged particles, typically in the form of a 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, 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.
In electron optics, scanning electron microscopes use narrow pencil beams to achieve a deep depth of field.
Ionizing radiation used in radiation therapy, whether or , 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 to scan over an object to create an intensity image of the Compton-scattered radiation.
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