How Invisibility Cloaks Work - Science | HowStuffWorks

These rings steer incoming light waves away from the hidden object, effectively making it invisible. Chinese physicists at Wuhan University have taken this ... HealthScienceHome&GardenAutoTechCultureMoneyLifestyleEntertainmentAdventureAnimalsQuizzesSubmitSearchSignupforourNewsletter!MobileNewsletterbannerclose​HowStuffWorksNewsletterMobileNewsletterchatcloseMobileNewsletterchatdotsMobileNewsletterchatdotsMobileNewsletterchatdotsMobileNewsletterchatavatarMobileNewsletterchatsubscribeHealthScienceHome&GardenAutoTechCultureMoneyLifestyleEntertainmentAdventureAnimalsQuizzes MoreMoreMoneyLifestyleEntertainmentAdventureAnimalsQuizzesAdvertisementHowStuffWorksSciencePhysicalScienceOptics HowInvisibilityCloaksWork By:WilliamHarris&RobertLambShareContentonTwitterShareContentonFacebookShareContentonLinkedInShareContentonFlipboardShareContentonRedditShareContentviaEmail Whatifyoucouldsimplythrowonacloakanddisappearfromsight? Photocourtesy©TachiLaboratory,theUniversityofTokyoAdmitit.You'dlovetoownaninvisibilitycloak.Utteranembarrassingfauxpasataparty?Justthrowonyourmagicalgarmentandvanishfromthesnootygazeofyourfellowpartygoers.Wanttohearwhatyourbossisreallysayingaboutyou?Strollrightintohisorherofficeandgetthegoods.Suchfantasticfashionaccessorieshavebecomeridiculouslystandardintheworldofsciencefictionandfantasy.Everyone,fromboywizardstointergalacticsafarihunters,hasatleastoneinvisibleblouseintheirwardrobe,butwhataboutuspoorsapsintherealworld?AdvertisementWell,Muggles,sciencehassomegoodnewsforyou:Invisibilitycloaksareareality.Thetechnologyisfarfromperfect,butifyou'llstepintoourhigh-techboutiqueofvanishingapparel,we'llguideyouthroughyourinvisibilitycloakoptions.Firstup,we'lllookatsomewonderfulcarbonnanotubefashions--freshfromtheUTDNanoTechInstitutefall2011collection.Thisnewtechnologyisinspiredbythesamenaturalphenomenaresponsiblefordesertmirages.Heatedviaelectricalstimulation,thesharptemperaturegradientbetweenthecloakandthesurroundingareacausesasteeptemperaturegradientthatbendslightawayfromthewearer.Thecatch:Wearersmustlovewaterandbeabletofitinsideapetridish.Orperhapsyou'dprefersomethingmadefrommetamaterials.Thesetinystructuresaresmallerthanthewavelengthoflight.Ifproperlyconstructed,theyguideraysoflightaroundanobject--muchlikearockdivertingwaterinastream.Fornow,however,thetechnologyonlyworksintwodimensionsandonlycomesintheultrapetitesizeof10micrometersacross.Ifyou'remoreintoretrofashion,there'salsotheopticalcamouflagetechnologydevelopedbyscientistsattheUniversityofTokyo.ThisapproachworksonthesameprinciplesofthebluescreenusedbyTVweatherforecastersandHollywoodfilmmakers.Ifyouwantpeopletoseethroughyou,thenwhynotjustfilmwhat'sbehindyouandprojectitontoyourbody?Ifyoutravelwithanentourageofvideographers,thismaybethecloakforyou.Readytotrysomeofthesefashionsonforsize?AdvertisementContentsTheMirageEffect:CarbonNanotubesMetamaterials:BendingLightWavesMetamaterials:InvisibleTanksOpticalCamouflage:AlteredRealityOpticalCamouflage:InvisibilityCloakComponentsOpticalCamouflage:MoreInvisibilityCloakComponentsOpticalCamouflage:TheCompleteInvisibilitySystemOpticalCamouflage:Real-worldInvisibilityApplicationsTheMirageEffect:CarbonNanotubes Hereweseethemulti-walledcarbonnanotube(MWCT)switchfrominactivetoactive,vanishingfromsightintheprocess. PhotocourtesyAlievAetal.2011NanotechnologyFirst,let'strythiscarbonnanotubeinvisibilitycloakonforsizeandexperiencethewondersofthemirageeffect.You'reprobablymostfamiliarwithmiragesfromtalesofdesertwandererswhoglimpseadistantoasis,onlytodiscoveritwasonlyamirage--nomiraculouslakeofdrinkingwater,onlymorehotsand.AdvertisementThehotsandiskeytothemirageeffect(orphotothermaldeflection),asthestifftemperaturedifferencebetweensandandairbends,orrefracts,lightrays.Therefractionswingsthelightraysuptowardtheviewer’seyesinsteadofbouncingthemoffthesurface.Intheclassicexampleofthedesertmirage,thiseffectcausesa"puddle"ofskytoappearontheground,whichthelogical(andthirsty)braininterpretsasapoolofwater.You'veprobablyseensimilareffectsonhotroadwaysurfaces,withdistantstretchesoftheroadappearingtogleamwithpooledwater.In2011,researchersattheUniversityofTexasatDallasNanoTechInstitutemanagedtocapitalizeonthiseffect.Theyusedsheetsofcarbonnanotubes,sheetsofcarbonwrappedupintocylindricaltubes[source:Alievetal.].Eachpageisbarelyasthickasasinglemolecule,yetisasstrongassteelbecausethecarbonatomsineachtubearebondedincrediblytightly.Thesesheetsarealsoexcellentconductorsofheat,makingthemidealmirage-makers.Intheexperiment,theresearchersheatedthesheetselectrically,whichtransferredtheheattothesurroundingarea(apetridishofwater).Asyoucanseefromthephotographs,thiscausedlighttobendawayfromthecarbonnanotubesheet,effectivelycloakinganythingbehinditwithinvisibility.Needlesstosay,therearen’tmanyplacesyou'dwanttowearatiny,super-heatedoutfitthathastostayimmersedinwater,buttheexperimentdemonstratesthepotentialforsuchmaterials.Intime,theresearchmayenablenotonlyinvisibilitycloaksbutalsootherlight-bendingdevices--allofthemwithahandyon/offswitch.AdvertisementMetamaterials:BendingLightWavesNext,let'sslipintoaninvisibilitycloakmadefrommetamaterials.Metamaterialsofferamorecompellingvisionofinvisibilitytechnology,withouttheneedformultipleprojectorsandcameras.FirstconceptualizedbyRussianphysicistVictorVeselagoin1967,thesetiny,artificialstructuresaresmallerthanthewavelengthoflight(theyhavetobetodivertthem)andexhibitnegativeelectromagneticpropertiesthataffecthowanobjectinteractswithelectromagneticfields.AdvertisementNaturalmaterialsallhaveapositiverefractiveindex,andthisdictateshowlightwavesinteractwiththem.Refractivitystemsinpartfromchemicalcomposition,butinternalstructureplaysanevenmoreimportantrole.Ifwealterthestructureofamaterialonasmallenoughscale,wecanchangethewaytheyrefractincomingwaves--evenforcingaswitchfrompositivetonegativerefraction.Remember,imagesreachusvialightwaves.Soundsreachesusviasoundwaves.Ifyoucanchannelthesewavesaroundanobject,youcaneffectivelyhideitfromvieworsound.Imagineasmallstream.Ifyoustickateabagfullofreddyeintotheflowingwater,itspresencewouldbeapparentdownstream,thankstothewayitalteredthewater'shue,tasteandsmell.Butwhatifyoucoulddivertthewateraroundtheteabag?In2006,DukeUniversity'sDavidSmithtookanearliertheoryposedbyEnglishtheoreticalphysicistJohnPendryandusedittocreateametamaterialcapableofdistortingtheflowofmicrowaves.Smith'smetamaterialfabricconsistedofconcentricringscontainingelectronicmicrowavedistorters.Whenactivated,theysteerfrequency-specificmicrowavesaroundthecentralportionofthematerial.Obviouslyhumansdon'tseeinthemicrowavespectrum,butthetechnologydemonstratedthatenergywavescouldberoutedaroundanobject.Imagineacloakthatcandivertathirdgrader'sstraw-firedspitball,moveitaroundthewearerandallowittocontinueontheothersideasifitstrajectoryhadtakenit,unopposed,straightthroughthepersoninthecloak.Nowhowmuchmoreofastretchwoulditbetodivertarock?Abullet?Smith'smetamaterialsprovedthemethod.Therecipetoinvisibilitylayinadaptingittodifferentwaves.Moreonmetamaterialsnext.TheSmallestFrontierMetamaterials,acreationofscience,don'toccurnaturally.Inordertocreatetheminutestructuresrequiredtoredirectelectromagneticwaves,scientistsemploynanotechnology.ReadHowNanotechnologyWorkstolearnallabouttheworld'ssmallestmachines.AdvertisementMetamaterials:InvisibleTanks ThisopticalimageshowstheUniversityofMarylandmetamaterialsinaction,steeringlightwavesawayfromtheeachcentralcircle.Thearrowsindicatethedirectionofthelightwaves. ImagecourtesyoftheUniversityofMarylandDepartmentofElectricalandComputerEngineeringIn2007,theUniversityofMaryland'sIgorSmolyaninovledhisteamevenfartherdowntheroadtoinvisibility.IncorporatingearliertheoriesproposedbyPurdueUniversity'sVladimirShaleav,Smolyaninovconstructedametamaterialcapableofbendingvisiblelightaroundanobject.Amere10micrometerswide,thePurduecloakusesconcentricgoldringsinjectedwithpolarizedcyanlight.Theseringssteerincominglightwavesawayfromthehiddenobject,effectivelymakingitinvisible.ChinesephysicistsatWuhanUniversityhavetakenthisconceptintotheaudiblerange,proposingthecreationofanacousticinvisibilitycloakcapableofdivertingsoundwavesaroundanobject.AdvertisementForthetimebeing,metamaterialinvisibilitycloaksaresomewhatlimited.They'renotonlysmall;they'relimitedtotwodimensions--hardlywhatyou'dneedtovanishintothesceneryofa3-Dwarzone.Plus,theresultingcloakwouldweighmorethanevenafull-grownwizardcouldhopetolugaround.Asaresult,thetechnologymightbebettersuitedtoapplicationssuchashidingstationarybuildingsorvehicles,suchasatank.OpticalCamouflage:AlteredReality Opticalcamouflagetechnologywon'tmakeyouinvisibletomulti-eyedBeholdermonsters--oreventostraycatsandsquirrels. Photocourtesy©TachiLaboratory,theUniversityofTokyoReadytoslipintosomeold-schoolopticalcamouflagefashions?Thistechnologytakesadvantageofsomethingcalledaugmented-realitytechnology--atypeoftechnologyfirstpioneeredinthe1960sbyIvanSutherlandandhisstudentsatHarvardUniversityandtheUniversityofUtah.AdvertisementOpticalcamouflagedeliversasimilarexperiencetoHarryPotter'sinvisibilitycloak,butusingitrequiresaslightlycomplicatedarrangement.First,thepersonwhowantstobeinvisible(let'scallhimHarry)donsagarmentthatresemblesahoodedraincoat.Thegarmentismadeofaspecialmaterialthatwe'llexaminemorecloselyinamoment.Next,anobserver(let'scallhimProfessorSnape)standsbeforeHarryataspecificlocation.Atthatlocation,insteadofseeingHarrywearingahoodedraincoat,Snapeseesrightthroughthecloak,makingHarryappeartobeinvisible.TheabovephotographshowsyouwhatSnapewouldsee.AndifSnapesteppedtothesideandviewedHarryfromaslightlydifferentlocation?Why,he'dsimplyseetheboywizardwearingasilvergarment.Scowlsanddetentionswouldlikelyfollow.LuckyforHarry,hisfictionalcloakaffords360-degreeprotection.Opticalcamouflagedoesn'tworkbywayofmagic.Itworksbytakingadvantageofsomethingcalledaugmented-realitytechnology--atypeoftechnologyfirstpioneeredinthe1960sbyIvanSutherlandandhisstudentsatHarvardUniversityandtheUniversityofUtah.YoucanreadmoreaboutaugmentedrealityinHowAugmentedRealityWorks,butaquickrecapwillbehelpfulhere.Augmented-realitysystemsaddcomputer-generatedinformationtoauser'ssensoryperceptions.Imagine,forexample,thatyou'rewalkingdownacitystreet.Asyougazeatsitesalongtheway,additionalinformationappearstoenhanceandenrichyournormalview.Perhapsit'stheday'sspecialsatarestaurantortheshowtimesatatheaterorthebusscheduleatthestation.What'scriticaltounderstandisthataugmentedrealityisnotthesameasvirtualreality.Whilevirtualrealityaimstoreplacetheworld,augmentedrealitymerelytriestosupplementitwithadditional,helpfulcontent.Thinkofitasaheads-updisplay(HUD)foreverydaylife.Mostaugmented-realitysystemsrequireausertolookthroughaspecialviewingapparatustoseeareal-worldsceneenhancedwithsynthesizedgraphics.Theyalsocallforapowerfulcomputer.Opticalcamouflagerequiresthesethingsaswell,butitalsonecessitatesseveralothercomponents.Here'severythingneededtomakeapersonappearinvisible:agarmentmadefromhighlyreflectivematerialadigitalvideocameraacomputeraprojectoraspecial,half-silveredmirrorcalledacombinerOnthenextpage,we'lllookateachofthesecomponentsingreaterdetail.AdvertisementOpticalCamouflage:InvisibilityCloakComponents Opticalcamouflageworksbytakingadvantageofsomethingcalledaugmented-realitytechnology.Learnhowitworksandfindoutwhatgoesintothecloak. Allright,soyouhaveyourvideocamera,computer,projector,combinerandwondrousreflectiveraincoat.Justhowdoesaugmented-realitytechnologyturnthisoddshoppinglistintoarecipeforinvisibility?First,let'stakeacloserlookattheraincoat:It'smadefromretro-reflectivematerial.Thishigh-techfabriciscoveredwiththousandsandthousandsofsmallbeads.Whenlightstrikesoneofthesebeads,thelightraysbouncebackexactlyinthesamedirectionfromwhichtheycame.AdvertisementTounderstandwhythisisunique,lookathowlightreflectsoffothertypesofsurfaces.Aroughsurfacecreatesadiffusedreflectionbecausetheincident(incoming)lightraysscattersinmanydifferentdirections.Aperfectlysmoothsurface,likethatofamirror,createswhatisknownasaspecularreflection--areflectioninwhichincidentlightraysandreflectedlightraysformtheexactsameanglewiththemirrorsurface.Inretro-reflection,theglassbeadsactlikeprisms,bendingthelightraysbyrefraction.Thiscausesthereflectedlightraystotravelbackalongthesamepathastheincidentlightrays.Theresult:Anobserversituatedatthelightsourcereceivesmoreofthereflectedlightandthereforeseesabrighterreflection.Retro-reflectivematerialsareactuallyquitecommon.Trafficsigns,roadmarkersandbicyclereflectorsalltakeadvantageofretro-reflectiontobemorevisibletopeopledrivingatnight.Themoviescreensfoundinmostmoderncommercialtheatersalsotakeadvantageofthismaterialbecauseitallowsforhighbrillianceunderdarkconditions.Inopticalcamouflage,theuseofretro-reflectivematerialiscriticalbecauseitcanbeseenfromfarawayandoutsideinbrightsunlight--tworequirementsfortheillusionofinvisibility.AdvertisementOpticalCamouflage:MoreInvisibilityCloakComponents Asyoucanseeinthisimage,theexperiencecloselyresembleswalkingdirectlyinfrontofamovieprojectionscreen,onlywitharealbackground. APPhoto/ShizuoKambayashi Fortherestofthesetup,thevideocameraneedstobepositionedbehindthesubjecttocapturethebackground.Thecomputertakesthecapturedimagefromthevideocamera,calculatestheappropriateperspectiveandtransformsthecapturedimageintothepicturethatwillbeprojectedontotheretro-reflectivematerial.Theprojectorthenshinesthemodifiedimageonthegarment,byshiningalightbeamthroughanopeningcontrolledbyadevicecalledanirisdiaphragm.Thisdiaphragmismadeofthin,opaqueplates,andturningaringchangesthediameterofthecentralopening.Foropticalcamouflagetoworkproperly,thisopeningmustbethesizeofapinhole.Why?Thisensuresalargerdepthoffieldsothatthescreen(inthiscasethecloak)canbelocatedanydistancefromtheprojector.AdvertisementFinally,theoverallsystemrequiresaspecialmirrortobothreflecttheprojectedimagetowardthecloakandtoletlightraysbouncingoffthecloakreturntotheuser'seye.Thisspecialmirroriscalledabeamsplitter,oracombiner--ahalf-silveredmirrorthatbothreflectslight(thesilveredhalf)andtransmitslight(thetransparenthalf).Ifproperlypositionedinfrontoftheuser'seye,thecombinerallowstheusertoperceiveboththeimageenhancedbythecomputerandlightfromthesurroundingworld.Thisiscriticalbecausethecomputer-generatedimageandthereal-worldscenemustbeintegratedfullyfortheillusionofinvisibilitytoseemrealistic.Theuserhastolookthroughapeepholeinthismirrortoseetheaugmentedreality.Onthenextpage,we'lllookathowthiswholesystemcomestogether.AdvertisementOpticalCamouflage:TheCompleteInvisibilitySystem Onewaytomakeapersonappeartransparent Nowlet'sputallofthesecomponentstogethertoseehowtheinvisibilitycloakappearstomakeapersontransparent.Thediagrambelowshowsthetypicalarrangementofallthevariousdevicesandpiecesofequipment.Onceapersonputsonthecloakmadewiththeretro-reflectivematerial,here'sthesequenceofevents:AdvertisementAdigitalvideocameracapturesthescenebehindthepersonwearingthecloak.Thecomputerprocessesthecapturedimageandmakesthecalculationsnecessarytoadjustthestillimageorvideosoitwilllookrealisticwhenitisprojected.Theprojectorreceivestheenhancedimagefromthecomputerandshinestheimagethroughapinhole-sizedopeningontothecombiner.Thesilveredhalfofthemirror,whichiscompletelyreflective,bouncestheprojectedimagetowardthepersonwearingthecloak.Thecloakactslikeamoviescreen,reflectinglightdirectlybacktothesource,whichinthiscaseisthemirror.Lightraysbouncingoffthecloakpassthroughthetransparentpartofthemirrorandfallontheuser'seyes.Rememberthatthelightraysbouncingoffthecloakcontaintheimageofthescenethatexistsbehindthepersonwearingthecloak.Thepersonwearingthecloakappearsinvisiblebecausethebackgroundsceneisbeingdisplayedontotheretro-reflectivematerial.Atthesametime,lightraysfromtherestoftheworldareallowedtoreachtheuser'seye,makingitseemasifaninvisiblepersonexistsinanotherwisenormal-lookingworld.AdvertisementOpticalCamouflage:Real-worldInvisibilityApplications Whileaninvisibilitycloakisaninterestingapplicationofopticalcamouflage,it'sprobablynotthemostusefulone.Learnaboutsomereal-worldapplications. Thewords"invisibilitycloak"tendstosummonimagesoffantasticadventure,magicalespionageandotherworldlydeception.Theactualapplicationsforopticalcamouflage,however,arefarlessoutthere.YoucanforgethidingyourRomulanstarshiporhangingoutintheladywizards'dormitory,butthatdoesn'tmeantherearen'tanumberofviableusesforthetechnology.Forinstance,pilotslandingaplanecouldusethistechnologytomakecockpitfloorstransparent.Thiswouldenablethemtoseetherunwayandthelandinggearsimplybyglancingdownatthefloor(whichwoulddisplaytheviewfromtheoutsideofthefuselage)Similarly,driverswouldn'thavetodealwithmirrorsandblindspots.Instead,theycouldjust"lookthrough"theentirerearofthevehicle.Thetechnologyevenboastspotentialapplicationsinthemedicalfield,assurgeonscoulduseopticalcamouflagetoseethroughtheirhandsandinstrumentsforanunobstructedviewoftheunderlyingtissue.AdvertisementInterestinglyenough,onepossibleapplicationofthistechnologyactuallyrevolvesaroundmakingobjectsmorevisible.Theconceptiscalledmutualtelexistenceandessentiallyinvolvesprojectingaremoteuser'sappearanceontoarobotcoatedinretro-reflectivematerial.Sayasurgeonwereoperatingonapatientviaremotecontrolroboticsurgery.Mutualtelexistencewouldprovidethehumandoctorsassistingtheprocedurewiththeperceptionthatthey'reworkingwithanotherhumaninsteadofamachine.Rightnow,mutualtelexistenceissciencefiction,butscientistscontinuetopushtheboundariesofthetechnology.Forexample,pervasivegamingisalreadybecomingareality.Pervasivegamingextendsgamingexperiencesoutintotherealworld,whetheroncitystreetsorinremotewilderness.Playerswithmobiledisplaysmovethroughtheworldwhilesensorscaptureinformationabouttheirenvironment,includingtheirlocation.Thisinformationdeliversagamingexperiencethatchangesaccordingtowhereusersareandwhattheyaredoing.Don'tdisappearonus.Wehavelotsmorelinksforyoutoexplorenext.AdvertisementLotsMoreInformationRelatedArticlesHowHarryPotter'sWandWorksHowtheBatsuitWorksHowAugmentedRealityWillWorkIsthearmytestinganinvisibletank?Howcanyoumakewaterinvisible?SuperheroQuizHow3-DGlassesWorkHowLightWorksHowNanotechnologyWorksSoundMoreGreatLinksAugmentedRealityPage-JimVallino,DepartmentofSoftwareEngineering,RochesterInstituteofTechnologyTachiLaboratoryattheUniversityofTokyoElectromagneticcloakinginthevisiblefrequencyrange(PDF)SourcesAdler,Robert."Acoustic'superlens'couldmeanfinerultrasoundscans."NewScientist.January2008.(Oct.13,2009)http://www.newscientist.com/article/dn13156-acoustic-superlens-could-mean-finer-ultrasound-scans.htmlAliev,AliE.etal."Mirageeffectfromthermallymodulatedtransparentcarbonnanotubesheets."Nanotechnology22.2011.(Oct.13,2011)http://iopscience.iop.org/0957-4484/22/43/435704/pdf/0957-4484_22_43_435704.pdfBarras,Colin."Goldringscreatefirsttrueinvisibilitycloak."Oct.2.2007.(Oct.13,2009)http://www.newscientist.com/article/dn12722-gold-rings-create-first-true-invisibility-cloak.htmlBBCNews."Inventorplans'invisiblewalls'."BBCNews.June14,2004.http://news.bbc.co.uk/2/hi/technology/3791795.stmBland,Eric."InvisibilityCloakCloserThanEvertoReality."DiscoveryNews.Jan.15,2009.(Oct.13,2009)http://dsc.discovery.com/news/2009/01/15/invisibility-cloak.htmlBrown,Mark."Watch:‘InvisibilityCloak’UsesMiragestoMakeObjectsVanish."Wired.Oct.4,2011.(Oct.13,2011)http://www.wired.com/dangerroom/2011/10/invisibility-cloak-mirage/Feiner,StevenK."Augmentedreality:Anewwayofseeing,"ScientificAmerican.April2002.Inami,Masahikoetal."Visuo-HapticDisplayUsingHead-MountedProjector."http://projects.star.t.u-tokyo.ac.jp/projects/MEDIA/xv/oc.htmlInami,Masahikoetal."OpticalCamouflageUsingRetro-reflectiveProjectionTechnology,"ProceedingsoftheSecondIEEEandACMInternationalSymposiumonMixedandAugmentedReality(ISMAR03).http://projects.star.t.u-tokyo.ac.jp/projects/MEDIA/xv/oc.html"'InvisibilityCloaks'CouldBreakSoundBarriers."DukeEngineering.Jan.9,2008.(Oct.13,2009)http://www.pratt.duke.edu/news/?id=1193McCarthy,Wil."BeingInvisible."Wired.November2008.(Oct.13,2009)http://www.wired.com/wired/archive/11.08/pwr_invisible_pr.htmlMullins,Justin."Workinginvisibilitycloakcreatedatlast."Oct.19,2006.(Oct.13,2009)http://www.newscientist.com/article/dn10334-working-invisibility-cloak-created-at-last.htmlPendry,John."Metamaterials."NewScientist.(Oct.21,2011)http://www.newscientist.com/data/doc/article/dn19554/instant_expert_7_-_metamaterials.pdfSmolyaninov,Igoretal."Electromagneticcloakinginthevisiblefrequencyrange."UniversityofMarylandDepartmentofElectricalandComputerEngineering.Dec.10,2007.(Oct.13,2009)http://arxiv.org/ftp/arxiv/papers/0709/0709.2862.pdfTachi,Susumu."TelexistenceandRetro-reflectiveProjectionTechnology(RPT),"Proceedingsofthe5thVirtualRealityInternationalConference(VRIC2003),pp.69/1-69/9.http://projects.star.t.u-tokyo.ac.jp/projects/MEDIA/xv/oc.html​ CiteThis! 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