# Corner reflector

Working principle of a corner reflector
Comparison of the effect of corner (1) and spherical (2) retroreflectors on three light rays. Reflective surfaces are drawn in dark blue.

A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves back directly towards the source, but shifted (translated). The three intersecting surfaces often have square shapes. Radar corner reflectors made of metal are used to reflect radio waves from radar sets. Optical corner reflectors, called corner cubes, made of three-sided glass prisms, are used in surveying and laser rangefinding.

The term is also used for a corner reflector antenna, consisting of two flat metal surfaces at a right angle, with a dipole antenna in front of them.

## How it works

The incoming ray is reflected three times, once by each surface, which results in a reversal of direction.[1][2] 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, [a, b, c]. 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 [−a, b, c]. Similarly, when reflected from side y and finally from side z, the b and c components are reversed. So the ray direction goes from [a, b, c] to [−a, b, c] to [−a, −b, c] to [−a, −b, −c] and it leaves the corner reflector with all three components of direction exactly reversed. The distance traveled, 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.[citation needed]

In diagram 1 the green ray is shown as reflecting from only one surface. This is a special case where the incoming ray is exactly normal (perpendicular) to one of the reflective faces. In the same diagram the orange and red rays are shown reflecting off of two surfaces. This is again a special case, the requirement being that the incoming ray is parallel to one of the reflecting planes.

A radar corner reflector (diamond shaped object) on the mast of a yacht. Note that this one is improperly deployed; to best reflect surface radar it should be deployed in the so-called "rain-catching" configuration so as to present an inside corner as shown on the "radar testing" image.
A corner reflector for radar testing
triangular corner reflector as cut from a hollow cube
Buoy in San Diego Harbor. Metal plates near the top form corner reflectors to reflect radar signals. Note that this positioning would be more efficient if the plates were oriented diagonally; but it would result in considerable cost increase in fabrication.

Radar corner reflectors are designed to reflect the microwave radio waves emitted by radar 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 (diamond) shape. The reflecting surfaces must be larger than several wavelengths of the radio waves to function.[3]

In maritime navigation they are placed on bridge abutments, buoys, ships 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 4.6 meters above sea level. Marine radar uses X-band microwaves with wavelengths of 2.5 - 3.75 cm, so small reflectors less than 30 cm across are used. In aircraft navigation, corner reflectors are installed on rural runways, to make them show up on aircraft radar.

## Optical corner reflectors

In optics, corner reflectors typically consist of three mirrors or reflective prism faces which return an incident light beam in the opposite direction. In surveying, retroreflector prisms are commonly used as targets for long-range electronic distance measurement using a total station.

NASA has put several optical corner reflectors made of quartz, known as the Lunar Laser Ranging Experiment, on the Moon for use in laser time-of-flight measurement to measure the Moon’s orbit more precisely than was possible before.

Automobile and bicycle tail lights are molded with arrays of small corner reflectors, with different sections oriented for viewing from different angles. Reflective paint 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.

## Other examples

Corner reflectors can also occur accidentally. Tower blocks with balconies are often accidental corner reflectors for sound and return a distinctive echo to an observer making a sharp noise, such as a hand clap, nearby. Similarly, in radar interpretation, 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 missile which had the same radar cross section as a B-52.