Corner reflector

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Working principle of a corner reflector

A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects electromagnetic waves back towards the source. The three intersecting surfaces often have square shapes. This is also known as a corner cube.

Buoy in San Diego Harbor. Note metal plates near the top configured at right angles to reflect radar signals.

Such devices are often used as radar targets or markers and are often employed on ships and, especially, lifeboats; Francis Rogallo invented a target kite incorporating corner reflectors. These normally consist of three conducting metallic surfaces or screens perpendicular to one another. In optics, corner reflectors typically consist of three mirrors or reflective prisms which return an incident light beam in the opposite direction. Arrays of such retroreflectors are used in bicycle reflectors, automobile tail lights and as targets for laser range finding. Microscopic corner reflector structures can be incorporated into reflective paint for increased visibility at night, although retroreflective spherical beads are more common for this purpose.

A directional antenna using two mutually intersecting conducting flat surfaces is also considered to be a type of corner reflector.

triangular corner reflector as cut of a cube

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.

NASA has put several corner reflectors on the Moon, for use in laser interferometry to measure the Moon's orbit more precisely than was possible before. The reflector was hand machined by Silicon Valley engineering manager Wayne Rosing, who also grinds telescope mirrors.