Pencil (optics)

For example, a phased array antenna can send out a beam that is extremely thin.

In optics, the focusing action of a lens is often described in terms of pencils of rays.

[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".

A pencil-beam radar
A moving or sweeping pencil-beam radar
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