A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects radiation (usually light) back to its source with minimum scattering.

There are several ways to obtain retroreflection:[1] A set of three mutually perpendicular reflective surfaces, placed to form the internal corner of a cube, work as a retroreflector.

The three corresponding normal vectors of the corner's sides form a basis (x, y, z) in which to represent the direction of an arbitrary incoming ray, [a, b, c].

A large relatively thin retroreflector can be formed by combining many small corner reflectors, using the standard hexagonal tiling.

For one, it is sometimes preferable to have an imperfect, slightly divergent retroreflection, as in the case of road signs, where the illumination and observation angles are different.

Due to spherical aberration, there also exists a radius from the centerline at which incident rays are focused at the center of the rear surface of the sphere.

An alternative form of the cat's eye retroreflector uses a normal lens focused onto a curved mirror rather than a transparent sphere, though this type is much more limited in the range of incident angles that it retroreflects.

Traffic engineers use an observation angle of 0.2 degrees to simulate a reflector target about 800 feet in front of a passenger automobile.

"Cat's eyes" are a particular type of retroreflector embedded in the road surface and are used mostly in the UK and parts of the United States.

Raised reflectors are generally not used in areas that regularly experience snow during winter, as passing snowplows can tear them off the roadways.

Stress on roadways caused by cars running over embedded objects also contributes to accelerated wear and pothole formation.

However, in recent years, more states and agencies require that headlights be turned on in inclement weather such as rain and snow.

According to the United States Federal Highway Administration (FHWA): Approximately 24% of all vehicle accidents occur during adverse weather (rain, sleet, snow and fog).

Rather, the new MUTCD language describes methods that agencies can use to maintain traffic sign retroreflectivity at or above the minimum levels.

[4] Retroflective tape is recognized and recommended by the International Convention for the Safety of Life at Sea (SOLAS) because of its high reflectivity of both light and radar signals.

Application to life rafts, personal flotation devices, and other safety gear makes it easy to locate people and objects in the water at night.

Two prisms may also serve as targets for angle measurements, using total stations or simpler theodolites; this usage, reminiscent of the heliotrope, does not involve retroreflection per se, it only requires visibility by means of any source of illumination (such as the sun) for direct sighting to the center of the target prism as seen from the optical instrument.

Astronauts on the Apollo 11, 14, and 15 missions left retroreflectors on the Moon as part of the Lunar Laser Ranging Experiment.

[6] Vikram lander of Chandrayaan-3 left Laser Retroreflector Array (LRA) instrument supplied by NASA's Goddard Space Flight Center as part of international collaboration with ISRO.

On 12 December 2023, Lunar Reconnaissance Orbiter was successfully able to detect transmitted laser pulses from Vikram lander.

The retroreflector was designed by the National Institute for Nuclear Physics of Italy, which built the instrument on behalf of the Italian Space Agency.

The reflector, a type of Luneburg lens, was developed and manufactured by the Institute for Precision Instrument Engineering (IPIE) in Moscow.

[24][25] Modulated retroreflectors, in which the reflectance is changed over time by some means, are the subject of research and development for free-space optical communications networks.

The optical receiver itself behaves as a weak retroreflector because it contains a large, precisely focused lens that detects illuminated objects in its focal plane.

Inspired by the natural world, the inventor of road 'cat's eyes' was Percy Shaw of Boothtown, Halifax, West Yorkshire, England.

When the tram-lines were removed in the nearby suburb of Ambler Thorn, he realised that he had been using the polished steel rails to navigate at night.

[36] The retroreflecting lens had been invented six years earlier for use in advertising signs by Richard Hollins Murray, an accountant from Herefordshire[37][38] and, as Shaw acknowledged, they had contributed to his idea.