Editing Retroreflector (section)

==Applications==

===On roads===
{{See also|Raised pavement marker}}
[[File:Bicycle Safety (using retroreflector and Cat-Eye showing day & night difference).gif|thumb|Retroreflector and cat-eye on a [[bicycle]]]]
[[File:CarWithReflectiveStickers.jpg|thumb|Car with reflective stickers]]
Retroreflection (sometimes called retroflection) is used on [[road]] surfaces, [[road sign]]s, [[vehicle]]s, and [[clothing]] (large parts of the surface of special [[safety clothing]], less on regular coats). When the headlights of a car illuminate a retroreflective surface, the reflected light is directed towards the car and its driver (rather than in all directions as with diffuse [[reflection (physics)|reflection]]). However, a [[pedestrian]] can see retroreflective surfaces in the dark only if there is a light source directly between them and the reflector (e.g., via a flashlight they carry) or directly behind them (e.g., via a car approaching from behind). "[[Cat's eye (road)|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]].

Corner reflectors are better at sending the light back to the source over long distances, while spheres are better at sending the light to a receiver somewhat off-axis from the source, as when the light from [[headlight]]s is reflected into the driver's [[Human eye|eye]]s.

Retroreflectors can be embedded in the road (level with the road surface), or they can be raised above the road surface. [[raised pavement marker|Raised reflectors]] are visible for very long distances (typically 0.5–1 [[kilometer]] or more), while sunken reflectors are visible only at very close ranges due to the higher angle required to properly reflect the light. Raised reflectors are generally not used in areas that regularly experience snow during winter, as passing [[snowplow]]s can tear them off the roadways. Stress on roadways caused by cars running over embedded objects also contributes to accelerated wear and [[pothole]] formation.

Retroreflective road paint is thus very popular in [[Canada]] and parts of the United States, as it is not affected by the passage of snowplows and does not affect the interior of the roadway. Where weather permits, embedded or raised retroreflectors are preferred as they last much longer than road paint, which is weathered by the elements, can be obscured by sediment or rain, and is ground away by the passage of vehicles.

===For signs===
[[File:I-90 and I-190 at Cumberland Ave northbound exit (February 2024).jpg|thumb|[[Retroreflective sheeting]] on highway signs and pavement markings]]
For traffic signs and vehicle operators, the light source is a vehicle's headlights, where the light is sent to the traffic sign face and then returned to the vehicle operator. Retroreflective traffic sign faces are manufactured with glass beads or prismatic reflectors embedded in a base sheeting layer so that the face reflects light, therefore making the sign appear more bright and visible to the vehicle operator under darkened conditions. According to the United States [[National Highway Traffic Safety Administration]] (NHTSA), the Traffic Safety Facts 2000 publication states the fatal crash rate is 3-4 times more likely during nighttime crashes than daytime incidents.

A misconception many people have is that retroreflectivity is only important during night-time travel. 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). Rain conditions account for 47% of weather-related accidents. These statistics are based on 14-year averages from 1995 to 2008.

The FHWA's [[MUTCD|Manual on Uniform Traffic Control Devices]] requires that signs be either illuminated or made with retroreflective sheeting materials, and though most signs in the U.S. are made with retroreflective sheeting materials, they degrade over time. Until now, there has been little information available to determine how long the retroreflectivity lasts. The MUTCD now requires that agencies maintain traffic signs to a set of minimum levels but provide a variety of maintenance methods that agencies can use for compliance. The minimum retroreflectivity requirements do not imply that an agency must measure every sign. Rather, the new MUTCD language describes methods that agencies can use to maintain traffic sign retroreflectivity at or above the minimum levels.

In [[Canada]], [[aerodrome]] lighting can be replaced by appropriately colored retroreflectors, the most important of which are the white retroreflectors that delineate the runway edges, and must be seen by aircraft equipped with landing lights up to 2 nautical miles away.<ref>{{cite web|url=http://www.tc.gc.ca/eng/civilaviation/regserv/cars/part3-301-155.htm#301_07|title=Transport Canada CARs 301.07|website=tc.gc.ca|access-date=6 April 2018}}</ref>

===Ships, boats, emergency gear===
Retroflective tape is recognized and recommended by the International Convention for the Safety of Life at Sea ([[International Convention for the Safety of Life at Sea|SOLAS]]) because of its high reflectivity of both light and [[radar]] signals. Application to [[Lifeboat (shipboard)|life rafts]], personal flotation devices, and other safety gear makes it easy to locate people and objects in the water at night. When applied to boat surfaces it creates a larger [[Radar cross section|radar signature]]—particularly for fiberglass boats, which produce very little radar reflection on their own. It conforms to International Maritime Organization regulation, IMO Res. A.658 (16) and meets U.S. Coast Guard specification 46 CFR Part 164, Subpart 164.018/5/0. Examples of commercially available products are 3M part numbers 3150A and 6750I, and Orafol Oralite FD1403.

===Surveying===
[[File:Prism Target used with survey equipment (cropped).jpg|thumb|upright=0.7|A typical surveying prism with back target]]

In [[surveying]], a retroreflector—usually referred to as a '''''prism'''''—is normally attached on a [[Pole (surveying)|surveying pole]] and is used as a target for [[distance measurement]], for example, a [[total station]]. The instrument operator or robot aims a [[laser]] beam at the retroreflector. The instrument measures the propagation time of the light and converts it to a distance. Prisms are used with survey and 3D point monitoring systems to measure changes in horizontal and vertical position of a point.
Two prisms may also serve as targets for [[angle measurement]]s, using total stations or simpler [[theodolite]]s; this usage, reminiscent of the [[heliotrope (instrument)|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.

===In space===
====On the Moon====
{{Main|List of retroreflectors on the Moon|Lunar Laser Ranging Experiment}}
[[Image:Apollo 11 Lunar Laser Ranging Experiment.jpg|thumb|200px|The Apollo 11 Lunar Laser Ranging Experiment]]

[[Astronaut]]s on the [[Apollo 11]], [[Apollo 14|14]], and [[Apollo 15|15]] missions left retroreflectors on the [[Moon]] as part of the [[Lunar Laser Ranging Experiment]]. The [[Soviet Union|Soviet]] ''[[Lunokhod 1]]'' and ''[[Lunokhod 2]]'' rovers also carried smaller arrays. Reflected signals were initially received from ''Lunokhod 1'', but no return signals were detected from 1971 until 2010, at least in part due to some uncertainty in its location on the Moon. In 2010, it was found in [[Lunar Reconnaissance Orbiter]] photographs and the retroreflectors have been used again. ''Lunokhod 2's'' array continues to return signals to Earth.<ref>[http://ilrs.gsfc.nasa.gov/docs/williams_lw13.pdf NASA.gov]</ref> Even under good viewing conditions, only a single reflected photon is received every few seconds. This makes the job of filtering laser-generated photons from naturally occurring photons challenging.<ref>{{cite web|url=http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html |title=NASA - Accuracy of Eclipse Predictions |publisher=eclipse.gsfc.nasa.gov |access-date=2015-08-15}}</ref>

[[Chandrayaan-3#Vikram lander|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.<ref name=":12">{{Cite news |date=2024-01-19 |title=NASA spacecraft pings India's Chandrayaan-3 lander on the moon |url=https://www.thehindu.com/sci-tech/science/nasa-spacecraft-pings-indias-chandrayaan-3-lander-on-the-moon/article67755589.ece |access-date=2024-01-22 |work=The Hindu |language=en-IN |issn=0971-751X}}</ref>

====On Mars====
A similar device, the [[Laser retroreflector array|Laser Retroreflector Array]] (LaRA), has been incorporated in the Mars [[Perseverance (rover)|''Perseverance'' rover]]. The retroreflector was designed by the [[National Institute for Nuclear Physics]] of Italy, which built the instrument on behalf of the [[Italian Space Agency]].
[[File:PIA24097-MarsPerseveranceRover-LaRA-20200928.jpg|thumb|right|200px|Mars [[Perseverance (rover)|''Perseverance'' rover]] - LaRA - (artwork)]]

====In satellites{{anchor|In Earth orbit}}====
{{Further|Satellite laser ranging}}

Many [[artificial satellites]] carry retroreflectors so they can be tracked from [[ground stations]]. Some satellites were built solely for laser ranging.
[[LAGEOS]], or Laser Geodynamics Satellites, are a series of scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth.<ref>{{cite journal |last1=Pearlman |first1=M. |last2=Arnold |first2=D. |last3=Davis |first3=M. |last4=Barlier |first4=F. |last5=Biancale |first5=R. |last6=Vasiliev |first6=V. |last7=Ciufolini |first7=I. |last8=Paolozzi |first8=A. |last9=Pavlis |first9=E. C. |last10=Sośnica |first10=K. |last11=Bloßfeld |first11=M. |title=Laser geodetic satellites: a high-accuracy scientific tool |journal=Journal of Geodesy |date=November 2019 |volume=93 |issue=11 |pages=2181–2194 |doi=10.1007/s00190-019-01228-y|bibcode=2019JGeod..93.2181P |s2cid=127408940 }}</ref> There are two LAGEOS spacecraft: LAGEOS-1<ref>[https://ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/current_missions/lag1_general.html NASA.gov]</ref> (launched in 1976), and LAGEOS-2 (launched in 1992). They use cube-corner retroreflectors made of fused silica glass. As of 2020, both LAGEOS spacecraft are still in service.<ref>{{cite journal |last1=Zajdel |first1=R. |last2=Sośnica |first2=K. |last3=Drożdżewski |first3=M. |last4=Bury |first4=G. |last5=Strugarek |first5=D. |title=Impact of network constraining on the terrestrial reference frame realization based on SLR observations to LAGEOS |journal=Journal of Geodesy |date=November 2019 |volume=93 |issue=11 |pages=2293–2313 |doi=10.1007/s00190-019-01307-0|bibcode=2019JGeod..93.2293Z |doi-access=free }}</ref> Three [[Starshine (satellite)|STARSHINE]] satellites equipped with retroreflectors were launched beginning in 1999. The [[LARES (satellite)|LARES]] satellite was launched on February 13, 2012. (See also: [[List of laser ranging satellites]].)

Other satellites include retroreflectors for orbit calibration<ref>{{cite journal |last1=Kazmierski |first1=Kamil |last2=Sośnica |first2=Krzysztof |last3=Hadas |first3=Tomasz |date=2017-11-06 |title=Quality assessment of multi-GNSS orbits and clocks for real-time precise point positioning |journal=GPS Solutions |volume=22 |issue=1 |pages=11 |bibcode=2018GPSS...22...11K |doi=10.1007/s10291-017-0678-6 |doi-access=free}}</ref> and orbit determination,<ref>{{cite journal |last1=Bury |first1=Grzegorz |last2=Sośnica |first2=Krzysztof |last3=Zajdel |first3=Radosław |date=2018-04-19 |title=Multi-GNSS orbit determination using satellite laser ranging |journal=Journal of Geodesy |volume=93 |issue=12 |pages=2447–2463 |bibcode=2019JGeod..93.2447B |doi=10.1007/s00190-018-1143-1 |doi-access=free}}</ref> such as in [[satellite navigation]] (e.g., all [[Galileo satellites]],<ref>{{cite journal |last1=Sośnica |first1=Krzysztof |last2=Prange |first2=Lars |last3=Kaźmierski |first3=Kamil |last4=Bury |first4=Grzegorz |last5=Drożdżewski |first5=Mateusz |last6=Zajdel |first6=Radosław |last7=Hadas |first7=Tomasz |title=Validation of Galileo orbits using SLR with a focus on satellites launched into incorrect orbital planes |journal=Journal of Geodesy |date=February 2018 |volume=92 |issue=2 |pages=131–148 |doi=10.1007/s00190-017-1050-x|bibcode=2018JGeod..92..131S |doi-access=free }}</ref> most [[GLONASS satellites]],<ref>{{cite journal |last1=Zajdel |first1=Radosław |title=A New Online Service for the Validation of Multi-GNSS Orbits Using SLR |journal=Remote Sensing |date=14 October 2017 |volume=9 |issue=10 |pages=1049 |doi=10.3390/rs9101049|bibcode=2017RemS....9.1049Z |doi-access=free }}</ref> [[Indian Regional Navigation Satellite System|IRNSS satellites]],<ref>{{Cite web|url=https://ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/current_missions/irns_reflector.html|title=IRNSS: Reflector Information|website=ilrs.cddis.eosdis.nasa.gov|archive-url=https://web.archive.org/web/20190325175546/https://ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/current_missions/irns_reflector.html|archive-date=2019-03-25|access-date=2019-03-25}}</ref> [[BeiDou]],<ref>{{cite journal |last1=Sośnica |first1=Krzysztof |last2=Zajdel |first2=Radosław |last3=Bury |first3=Grzegorz |last4=Bosy |first4=Jarosław |last5=Moore |first5=Michael |last6=Masoumi |first6=Salim |title=Quality assessment of experimental IGS multi-GNSS combined orbits |journal=GPS Solutions |date=April 2020 |volume=24 |issue=2 |pages=54 |doi=10.1007/s10291-020-0965-5|doi-access=free |bibcode=2020GPSS...24...54S }}</ref> [[QZSS]],<ref>{{cite journal |last1=Sośnica |first1=K. |last2=Bury |first2=G. |last3=Zajdel |first3=R. |last4=Strugarek |first4=D. |last5=Drożdżewski |first5=M. |last6=Kazmierski |first6=K. |title=Estimating global geodetic parameters using SLR observations to Galileo, GLONASS, BeiDou, GPS, and QZSS |journal=Earth, Planets and Space |date=December 2019 |volume=71 |issue=1 |pages=20 |doi=10.1186/s40623-019-1000-3|bibcode=2019EP&S...71...20S |doi-access=free }}</ref> and two [[GPS satellites]]<ref>{{cite journal |last1=Sośnica |first1=Krzysztof |last2=Thaller |first2=Daniela |last3=Dach |first3=Rolf |last4=Steigenberger |first4=Peter |last5=Beutler |first5=Gerhard |last6=Arnold |first6=Daniel |last7=Jäggi |first7=Adrian |title=Satellite laser ranging to GPS and GLONASS |journal=Journal of Geodesy |date=July 2015 |volume=89 |issue=7 |pages=725–743 |doi=10.1007/s00190-015-0810-8|bibcode=2015JGeod..89..725S |doi-access=free }}</ref>) as well as in [[satellite gravimetry]] ([[GOCE]]<ref>{{cite journal |last1=Strugarek |first1=Dariusz |last2=Sośnica |first2=Krzysztof |last3=Jäggi |first3=Adrian |title=Characteristics of GOCE orbits based on Satellite Laser Ranging |journal=Advances in Space Research |date=January 2019 |volume=63 |issue=1 |pages=417–431 |doi=10.1016/j.asr.2018.08.033|bibcode=2019AdSpR..63..417S |s2cid=125791718 }}</ref>) [[satellite altimetry]] (e.g., [[TOPEX/Poseidon]], [[Sentinel-3]]<ref>{{cite journal |last1=Strugarek |first1=Dariusz |last2=Sośnica |first2=Krzysztof |last3=Arnold |first3=Daniel |last4=Jäggi |first4=Adrian |last5=Zajdel |first5=Radosław |last6=Bury |first6=Grzegorz |last7=Drożdżewski |first7=Mateusz |title=Determination of Global Geodetic Parameters Using Satellite Laser Ranging Measurements to Sentinel-3 Satellites |journal=Remote Sensing |date=30 September 2019 |volume=11 |issue=19 |pages=2282 |doi=10.3390/rs11192282|bibcode=2019RemS...11.2282S |doi-access=free }}</ref>).
Retroreflectors can also be used for inter-satellite laser ranging instead of ground-tracking (e.g., [[GRACE-FO]]).<ref>{{cite web |last=Schwarz |first=Oliver |date=2016-01-21 |title=GRACE FO Laser Ranging Interferometer |url=http://spacetech-i.com/products/optical-instruments/grace-fo-laser-ranging-interferometer |url-status=dead |archive-url=https://web.archive.org/web/20191206085105/https://spacetech-i.com/products/optical-instruments/grace-fo-laser-ranging-interferometer |archive-date=2019-12-06 |access-date=2018-04-06 |website=SpaceTech GmbH |language=en}}</ref>

The [[BLITS|BLITS (Ball Lens In The Space) spherical retroreflector satellite]] was placed into orbit as part of a September 2009 Soyuz launch<ref>{{Cite web |last1=Zak |first1=Anatoly |last2=Günes |first2=S. |date=2007-04-25 |title=Space exploration in 2009 |url=http://www.russianspaceweb.com/2009.html |url-status=live |archive-url=https://web.archive.org/web/20240115053452/https://www.russianspaceweb.com/2009.html |archive-date=2024-01-15 |access-date=2024-01-15 |website=RussianSpaceWeb.com |language=en}}</ref> by the [[Federal Space Agency of the Russian Federation|Federal Space Agency of Russia]] with the assistance of the [[International Laser Ranging Service]], an independent body originally organized by the [[International Association of Geodesy]], the [[International Astronomical Union]], and international committees.<ref>{{cite web |last=Tyahla |first=Lori J. |date=2013-02-20 |title=ILRS Missions: BLITS |url=http://ilrs.gsfc.nasa.gov/missions/satellite_missions/current_missions/blit_general.html |url-status=dead |archive-url=https://web.archive.org/web/20130220215607/http://ilrs.gsfc.nasa.gov/missions/satellite_missions/current_missions/blit_general.html |archive-date=2013-02-20 |access-date=2013-02-20 |website=International Laser Ranging Service |language=en}}</ref> The ILRS central bureau is located at the United States' [[Goddard Space Flight Center]].
The reflector, a type of [[Luneburg lens]], was developed and manufactured by the Institute for Precision Instrument Engineering (IPIE) in Moscow. The mission was interrupted in 2013 after a collision with [[space debris]].<ref name="EOportal">{{cite web|title=BLITS (Ball Lens In The Space)
 |url=https://directory.eoportal.org/web/eoportal/satellite-missions/b/blits
 |publisher=[[ESA]], Earth Observation portal}}</ref><ref>{{cite web |last=Blau |first=Patrick |date=2013-03-09 |title=Russian BLITS Satellite hit by Space Debris |url=http://www.spaceflight101.net/blits-satellite-collision-january-2013.html |url-status=usurped |archive-url=https://archive.today/20161005043844/http://www.spaceflight101.net/blits-satellite-collision-january-2013.html |archive-date=2016-10-05 |access-date=2020-04-16 |website=Spaceflight101: Space News and Beyond |language=en}}</ref>{{cbignore|bot=InternetArchiveBot}}

===Free-space optical communication===
Modulated retroreflectors, in which the reflectance is changed over time by some means, are the subject of research and development for [[free-space optical communication]]s networks. The basic concept of such systems is that a low-power remote system, such as a sensor mote, can receive an optical signal from a base station and reflect the modulated signal back to the base station. Since the base station supplies the optical power, this allows the remote system to communicate without excessive power consumption. Modulated retroreflectors also exist in the form of modulated phase-conjugate mirrors (PCMs). In the latter case, a "time-reversed" wave is generated by the PCM with temporal encoding of the phase-conjugate wave (see, e.g., SciAm, Oct. 1990, "The Photorefractive Effect," David M. Pepper, ''et al.'').

Inexpensive corner-aiming retroreflectors are used in user-controlled technology as optical datalink devices. Aiming is done at night, and the necessary retroreflector area depends on aiming distance and ambient lighting from street lamps. The optical receiver itself behaves as a weak retroreflector because it contains a large, precisely focused [[lens (optics)|lens]] that detects illuminated objects in its focal plane. This allows aiming without a retroreflector for short ranges.

===Other uses===
Retroreflectors are used in the following example applications:
*In common (non-SLR) digital cameras, the sensor system is often retroreflective. Researchers have used this property to demonstrate a system to prevent unauthorized photographs by detecting digital cameras and beaming a highly focused beam of light into the lens.<ref>{{cite web |last=Eng |first=Paul |date=2005-09-19 |title=Device Seeks to Jam Covert Digital Photographers |url=https://abcnews.go.com/Technology/FutureTech/story?id=1139800&page=1 |url-status=live |archive-url=https://web.archive.org/web/20240120175219/https://abcnews.go.com/Technology/FutureTech/story?id=1139800&page=1 |archive-date=2024-01-20 |access-date=2018-04-06 |website=ABC News |language=en}}</ref>
*In movie screens to allow for high brilliance under dark conditions.<ref>{{cite web |last1=Harris |first1=William |last2=Lamb |first2=Robert |date=2005-07-20 |title=How Does an Invisibility Cloak Work? |url=https://science.howstuffworks.com/invisibility-cloak.htm#pt1 |url-status=live |archive-url=https://web.archive.org/web/20230921232249/https://science.howstuffworks.com/invisibility-cloak.htm#pt1 |archive-date=2023-09-21 |access-date=2018-04-06 |website=HowStuffWorks |language=en}}</ref>
* [[Digital compositing]] programs and [[chroma key]] environments use retroreflection to replace traditional lit backdrops in composite work as they provide a more solid color without requiring that the backdrop be lit separately.<ref>{{cite web |last1=Thomas |first1=Graham |date=2009-12-06 |title=Making things Vanish - The Truematte Technology |url=https://www.bbc.co.uk/blogs/legacy/researchanddevelopment/2009/12/making-things-vanish-the-truem.shtml |url-status=dead |archive-url=https://web.archive.org/web/20170705092201/http://www.bbc.co.uk/blogs/researchanddevelopment/2009/12/making-things-vanish-the-truem.shtml |archive-date=2017-07-05 |access-date=2014-10-25 |website=BBC |language=en}}</ref>
* In Longpath-[[Differential optical absorption spectroscopy|DOAS]] systems retroreflectors are used to reflect the light emitted from a lightsource back into a telescope. It is then spectrally analyzed to obtain information about the trace gas content of the air between the telescope and the retro reflector.
* [[Barcode]] labels can be printed on retroreflective material to increase the range of scanning up to 50 feet.<ref>{{cite web |last=Thermal |first=Timmy |date=2014-07-08 |title=Retroreflective Labels |url=https://midcomdata.com/reflective-labels/ |url-status=live |archive-url=https://web.archive.org/web/20230925015638/https://midcomdata.com/reflective-labels/ |archive-date=2023-09-25 |access-date=2014-07-16 |publisher=Midcom Data Technologies, Inc. |language=en}}</ref>
*In a form of [[3D display]]; where a [[Retroreflective sheeting|retro-reflective sheeting]] and a set of projectors is used to project stereoscopic images back to user's eye. The use of [[pico projector|mobile projectors]] and [[Inertial measurement unit|positional tracking]] mounted on user's spectacles frame allows the illusion of a hologram to be created for [[Computer-generated imagery|computer generated imagery]].<ref>{{cite web|url=http://www.creol.ucf.edu/Research/Publications/1467.PDF |title=Design of an ultra-light head-mounted projective display (HMPD) and its applications in augmented collaborative environments |publisher=Proceedings of SPIE |year=2002}}</ref><ref>{{cite web|url=https://patents.google.com/patent/US20160277725 |title=Retroreflective light field display |publisher= US Patent & Trademark Office |date=2016-09-22 }}</ref><ref>{{cite web|url=https://www.youtube.com/watch?v=H3HGrclGkIE  |archive-url=https://ghostarchive.org/varchive/youtube/20211211/H3HGrclGkIE| archive-date=2021-12-11 |url-status=live|title=castAR Test Footage - Flight Simulator  |publisher=YouTube |date=2013-09-26 }}{{cbignore}}</ref>
*Flashlight fish of the family [[Anomalopidae]] have natural retroreflectors. See [[tapetum lucidum]].