optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...

, a pencil or pencil of rays is a geometric construct used to describe a beam or portion of a beam of

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...

or charged

particle In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from ...

s, typically in the form of a narrow beam ( conical or cylindrical). Antennas which strongly bundle in

azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematical ...

and

elevation The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Ver ...

are often described as "pencil-beam" antennas. For example, a

phased array antenna In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...

can send out a beam that is extremely thin. Such antennas are used for tracking radar, and the process is known as

beamforming Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in an antenna array in such a way that signals at particular angles e ...

. In

, the focusing action of a

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

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 Electron optics is a mathematical framework for the calculation of electron trajectories along electromagnetic fields. The term ''optics'' is used because magnetic and electrostatic lenses act upon a charged particle beam similarly to optical le ...

, scanning electron microscopes use narrow pencil beams to achieve a deep depth of field. Ionizing radiation used in

radiation therapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radi ...

, whether

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...

s or

charged particle In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary pa ...

s, such as proton therapy and

electron therapy Electron therapy or electron beam therapy (EBT) is a kind of external beam radiotherapy where electrons are directed to a tumor site for medical treatment of cancer. Equipment Electron beam therapy is performed using a medical linear accelerator. ...

machines, is sometimes delivered through the use of pencil beam scanning. In

backscatter X-ray Backscatter X-ray is an advanced X-ray imaging technology. Traditional X-ray machines detect hard and soft materials by the variation in x-ray intensity transmitted through the target. In contrast, backscatter X-ray detects the radiation that ...

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

A 1675 work describes a pencil as "a double cone of rays, joined together at the base." In his 1829 ''A System of Optics'',

Henry Coddington Henry Coddington (1798/9, Oldbridge, County Meath — 3 March 1845, Rome) was an Anglo Irish natural philosopher, fellow and tutor of Trinity College, Cambridge and Church of England clergyman. Life Henry Coddington was the son of Latham Coddi ...

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".

References

Geometrical optics Antennas (radio) Radiation therapy {{optics-stub

History

See also

References