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A corner reflector is a
retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light or other radiation back to its source with minimum scattering. This works at a wide range of angle of incidence (optics), angle of incidenc ...
consisting of three mutually
perpendicular In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...
, intersecting flat reflective surfaces. It reflects
wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...
s incident from any direction directly towards the source, but translated. The three intersecting surfaces often are triangles (forming a
tetrahedron In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
) or may have square shapes. Radar corner reflectors made of metal are used to reflect radio waves from
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
sets. Optical corner reflectors, called corner cubes or cube corners, made of three-sided glass prisms, are used in
surveying Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...
and laser ranging.


Principle

The incoming ray is reflected three times, once by each surface, which results in a reversal of direction. To see this, the three corresponding normal vectors of the corner's perpendicular sides can be considered to form a basis (a rectangular coordinate system) (''x'', ''y'', ''z'') in which to represent the direction of an arbitrary incoming ray, . When the ray reflects from the first side, say ''x'', the ray's ''x'' component, ''a'', is reversed to −''a'' while the ''y'' and ''z'' components are unchanged, resulting in a direction of . Similarly, when reflected from side ''y'' and finally from side ''z'', the ''b'' and ''c'' components are reversed. Therefore, the ray direction goes from to to to , and it leaves the corner reflector with all three components of direction exactly reversed. The distance travelled, relative to a plane normal to the direction of the rays, is also equal for any ray entering the reflector, regardless of the location where it first reflects.


In radar

Radar corner reflectors are designed to reflect the
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
radio waves Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths ...
emitted by
radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
sets back toward the radar antenna. This causes them to show a strong "return" on radar screens. A simple corner reflector consists of three conducting sheet metal or screen surfaces at 90° angles to each other, attached to one another at the edges, forming a "corner". These reflect radio waves coming from in front of them back parallel to the incoming beam. To create a corner reflector that will reflect radar waves coming from any direction, 8 corner reflectors are placed back-to-back in an
octahedron In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
(diamond) shape. The reflecting surfaces must be larger than several
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s of the radio waves to function. In maritime navigation they are placed on
bridge A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...
abutments,
buoy A buoy (; ) is a buoyancy, floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents. History The ultimate origin of buoys is unknown, but by 1295 a seaman's manual referred to navig ...
s,
ship A ship is a large watercraft, vessel that travels the world's oceans and other Waterway, navigable waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally disti ...
s and, especially, lifeboats, to ensure that these show up strongly on ship radar screens. Corner reflectors are placed on the vessel's masts at a height of at least above sea level (giving them an approximate minimum
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
distance of ).
Marine radar Marine radars are X band or S band radars on ships, used to detect other ships and land hazards, to provide bearing and distance for collision avoidance and navigation at sea. They are electronic navigation instruments that use a rotating a ...
uses
X-band The X band is the designation for a band of frequency, frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is set at approximately 7.0� ...
microwaves with wavelengths of , so small reflectors less than across are used. In aircraft navigation, corner reflectors are installed on rural
runway In aviation, a runway is an elongated, rectangular surface designed for the landing and takeoff of an aircraft. Runways may be a human-made surface (often asphalt concrete, asphalt, concrete, or a mixture of both) or a natural surface (sod, ...
s, to make them show up on aircraft radar. An object that has multiple reflections from smooth surfaces produces a radar return of greater magnitude than might be expected from the physical size of the object. This effect was put to use on the ADM-20 Quail, a small decoy missile which had the same
radar cross section Radar cross-section (RCS), denoted σ, also called radar signature, is a measure of how detectable an object is by radar. A larger RCS indicates that an object is more easily detected. An object reflects a limited amount of radar energy b ...
as a B-52. The corner reflector is not the only efficient radar reflector design; other
retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light or other radiation back to its source with minimum scattering. This works at a wide range of angle of incidence (optics), angle of incidenc ...
designs have also seen use. Luneburg lens, for example, are used on the ADM-141 TALD.


In optics

In
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...
, corner reflectors typically consist of three
mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
s or reflective prism faces which return an incident
light beam A light beam or beam of light is a directional projection of light energy radiating from a light source. Sunlight forms a light beam (a sunbeam) when filtered through media such as clouds, foliage, or windows. To artificially produce a li ...
in the opposite direction. In
surveying Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...
,
retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light or other radiation back to its source with minimum scattering. This works at a wide range of angle of incidence (optics), angle of incidenc ...
prisms are commonly used as targets for long-range electronic distance measurement using a
total station A total station or total station theodolite is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure both vertic ...
. Five arrays of optical corner reflectors have been placed on the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
for use by
Lunar Laser Ranging experiment Lunar Laser Ranging (LLR) is the practice of measuring Lunar distance (astronomy), the distance between the surfaces of the Earth and the Moon using Lidar, laser ranging. The distance can be calculated from the Round-trip delay, round-trip time ...
s observing a
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
's time-of-flight to measure the Moon's orbit more precisely than was possible before. The three largest were placed by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
as part of the
Apollo program The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...
, and the
Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
built two smaller ones into the Lunokhod rovers.
Automobile A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, peopl ...
and
bicycle A bicycle, also called a pedal cycle, bike, push-bike or cycle, is a human-powered transport, human-powered or motorized bicycle, motor-assisted, bicycle pedal, pedal-driven, single-track vehicle, with two bicycle wheel, wheels attached to a ...
tail lights are molded with arrays of small corner reflectors, with different sections oriented for viewing from different angles. Reflective
paint Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are ...
for visibility at night usually contains retroreflective spherical beads. Thin plastic with microscopic corner reflector structures can be used as tape, on signs, or sewn or molded onto
clothing Clothing (also known as clothes, garments, dress, apparel, or attire) is any item worn on a human human body, body. Typically, clothing is made of fabrics or textiles, but over time it has included garments made from animal skin and other thin s ...
.


Other examples

Corner reflectors can also occur accidentally.
Tower block A tower block, high-rise, apartment tower, residential tower, apartment block, block of flats, or office tower is a tall building, as opposed to a low-rise building and is defined differently in terms of height depending on the jurisdiction. ...
s with balconies are often accidental acoustic corner reflectors and return a distinctive echo to an observer making a sharp sound noise, such as a hand clap, nearby.


See also

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References

* * {{DEFAULTSORT:Corner Reflector Mirrors Radar