Concave and Spherical Retroreflectors - Used in Space, but do they have any Earth-bound Applications?

Retroreflectors

Retroreflectors are reflectors with a particular property that they reflect the light straight back at the source no matter from which angle the light is thrown and how the reflector is placed with minimum scattering. A plain mirror has to be perfectly perpendicular to the line of incidence of the incoming beam to reflect it back. This property of retroreflectors make them very useful for certain applications like search and rescue, calculating the distance to the moon, cautions to motorists, etc.

Retroreflectors are insensitive to motion and tilt and are basically self-compensating mirrors. The incident light is reflected three times on mirrors placed 90 degrees to each other and return to the source at exactly the same angle at which it entered the reflector. Apollo 11 placed retroreflectors on the surface of the moon which is still used by scientists worldwide to calculate the distance of the moon from the earth.

Retroreflector on Lifeboat

Type of retroreflectors

Retroreflectors are of two types:

• Solid retroreflectors
• Hollow retroreflectors.

Solid Reflectors are basically triple prisms and works on the basic principle of total reflection. They can also be 90 degree corner cubes or high index of refraction (2) spheres, transparent with reflective coating. The solid retroreflectors are called by many names like cat’s eye.

Hollow Retroreflectors utilize three mirrors at 90 degrees to each other to return the light to the source. A CCR, also known as a Concave Corner Retroreflector, is an assembly of three concave mirrors at right angles to each other which reflect back the light back to its source. There are different types of concave retroreflectors like spherical, paraboloidal, ellipsoidal, and toroidal reflectors.

Retroreflector on Boilersuit

Application of Retroreflectors

Because of their peculiar properties they have found various applications which have been listed below:

• Retroreflective tapes used for search and rescue, in marine ships, for motorists etc.
• Retroreflective tapes used in life boats which make it easier for detection of the life boats in Radars.
• Retroreflectors used in remote tachometers to measure the speed of a shaft which has a marking of retroreflective tape.
• Space applications like measuring the distance to the moon and of various satellites.
• In war environment as fire control technology of Apache helicopters.
• Precision positioning technologies which are used in space and battlefield.
• For personal safety clothing.
• On the road for signs and marks.
• Used in space for geodynamical studies of the earth which have been put in a geostationary orbit.
• Potential use for making invisibility shields and cloaks used in battlefields.
• Modulated retroreflectors to be used in free space optical communication network.
• Retroreflective tapes has been recognized for maritime use by SOLAS because of its high reflectivity of both lights and radar signals.
• In land surveying where a laser light is fired at retroreflectors held by survey man and the time of propagation can be calibrated as distance.
• In digital cameras the sensory system is retroreflective.
• Automobile reflectors and also used in bicycles.

Retroreflector on Lifebuoy

Retroreflectors and Solar Power

Solar power is a renewable source of energy and can be employed in conjunction with retroreflectors for communication in inaccessible places where electricity is unavailable. One application is the use in modulating retroreflectors in communication engineering which are solar powered.

However the use of retroreflector for the concentration and collection of solar power is limited due to its property of reflecting back to source. In the collection and collection of solar power we use flat and concave reflectors, while flat reflectors merely change direction of the incident radiation, the concave reflectors also concentrate the solar radiation and increasing the efficiency of the solar plant.

The different types of concentrating collectors are parabolic trough reflector, mirror strip reflector, Fresnel lens reflector, paraboloidal type, etc.

Retroreflectors on Oil Skins

References

Physics: A World View By Larry D. Kirkpatrick, Gregory E. Francis

https://www.jc-solarhomes.com/fair/parabola20.htm

https://www.physics.ucsd.edu/~tmurphy/apollo/lrrr.html

https://www-ee.stanford.edu/~jmk/pubs/ccr.jmems.pdf

Image Credits

All images by Mohit Sanguri, Chief Engineer. Images taken on the Panamax bulk carrier M.V. Maha Roos owned by Five Stars Bulk Carriers Ltd.