The Advantages of Telecentricity | Edmund Optics

Telecentric lenses eliminate the parallax error characteristic of conventional lenses by having a constant, non-angular FOV; at any distance from the lens, a ... HowmayIhelpyou? Engineersareavailabletoassist. Ouroperatorsarenotavailableatthemoment.Pleaseleaveamessageandwewillcontactyousoon Leaveamessage HappyHolidaysfromEdmundOptics!Pleasenoteourspecialopeninghours.      CHAT    EMAIL Country/Region Wehavesetyourcountry/regiontoUnitedStates Youcanchangethisselectionatanytime,butproductsinyourcart,savedlists,orquotemayberemovediftheyareunavailableinthenewshippingcountry/region. 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Welcome! MyAccount AccessDashboard MyProfile UpdateMyProfile Payment&Shipping AddressBook, TaxCertificates WebOrders ViewPastOrders SavedLists SavedItemsforLater LogOut    Products Capabilities KnowledgeCenter Company ContactUs  MyAccount  Cart   hasbeenaddedtoyourcart. OrderNumberhasbeenaddedtoyourcart. Allitemshavebeenaddedtoyourcart. hasbeenaddedtoyourcomparelist. Productaddedtocart KnowledgeCenter /ApplicationNotes /Imaging /TheAdvantagesofTelecentricity TheAdvantagesofTelecentricity http://www.edmundoptics.com/knowledge-center/application-notes/imaging/advantages-of-telecentricity/ EdmundOpticsInc. http://www.edmundoptics.com TheAdvantagesofTelecentricity Authors: GregoryHollows, NicholasJames ThisisSection4.1 oftheImagingResourceGuide. PreviousSectionNextSection Theabilitytoquicklyperformrepeatable,high-accuracymeasurementsiscriticaltomaximizingtheperformanceofmanymachinevisionsystems.Forsuchsystems,atelecentriclensallowsthehighestpossibleaccuracytobeobtained.Thissectiondiscussestheuniqueperformancecharacteristicsoftelecentriclensesandhowtelecentricitycanimpactsystemperformance. ZeroAngularFieldofView:ParallaxErrorElimination Conventionallenseshaveangularfieldsofview(AFOV)suchthatasthedistancebetweenthelensandobjectincreases,themagnificationdecreases.Thisishowhumanvisionbehavesandcontributestoourdepthperception.ThisAFOVresultsinparallax,alsoknownasperspectiveerror,whichdecreasesaccuracy,astheobservedmeasurementofthevisionsystemwillchangeiftheobjectismoved(evenwhenremainingwithinthedepthoffield[DOF])duetothemagnificationchange.Telecentriclenseseliminatetheparallaxerrorcharacteristicofconventionallensesbyhavingaconstant,non-angularFOV;atanydistancefromthelens,atelecentriclenswillalwayshavethesameFOV.SeeFigure1forthedifferencebetweenanon-telecentricandatelecentricFOV. Atelecentriclens’sconstantFOVhasbothbenefitsandconstraintsforgaugingapplications.Theprimaryadvantageofatelecentriclensisthatitsmagnificationdoesnotchangewithrespecttodepth.Figure2showstwodifferentobjectsatdifferentworkingdistances(WD),imagedbothbyafixedfocallength(non-telecentric)lens(center)andatelecentriclens(right).Notethatintheimagetakenwithatelecentriclens,itisimpossibletotellwhichobjectisinfrontoftheother.Withthefixedfocallengthlens,itisquiteobviousthattheobjectthatappearssmallerispositionedfartherfromthelens. WhileFigure2isdrasticintermsofaWDshift,itillustratestheimportanceofminimizingparallaxerror.ManyautomatedinspectiontasksareimagingobjectsthatmovethroughtheFOVofanimagingsystem,andthepositionofpartsisrarelyperfectlyrepeatable.IftheWDisnotidenticalforeachobjectthatthelensisimaging,themeasurementofeachobjectwillvaryduetothemagnificationshift(seeResolutiontolearnmoreaboutmagnificationandhowitisdefined).Amachinevisionsystemthatoutputsdifferentresultsbasedonamagnificationcalibrationerror(whichisunavoidablewithafixedfocallengthlens)isanon-reliablesolutionandcannotbeusedwhenhighprecisionisnecessary.Telecentriclenseseliminatetheconcernaboutmeasurementerrorsthatwouldotherwiseoccurduetofactorssuchasavibratingconveyororinexactpartlocations. Figure1:FOVcomparisonofaconventionalandtelecentriclens.Notetheconventionallens’sAFOVandthetelecentriclens’szeroangleFOV. Figure2:TheAFOVofthefixedfocallengthlenstranslatestoparallaxerrorintheimageandcausesthetwocubestoappeartobedifferentsizes. TelecentricLensesandDepthofField ItisacommonmisconceptionthattelecentriclensesinherentlyhavealargerDOFthanconventionallenses.WhileDOFisstillultimatelygovernedbythewavelengthandf/#ofthelens,itistruethattelecentriclensescanhavealargerusableDOFthanconventionallensesduetothesymmetricalblurringoneithersideofbestfocus.Asthepartunderinspectionshiftstowardorawayfromthelens,itwillfollowtheAFOV(orthechiefray)thatisassociatedwithit.Inanon-telecentriclens,whenanobjectismovedinandoutoffocus,thepartblursasymmetricallyduetoparallaxandthemagnificationchangethatisassociatedwithitsAFOV.Telecentriclenses,however,blursymmetricallysincethereisnoangularcomponenttotheFOV.Inpractice,thismeansthatfeaturessuchasedgesretaintheircenterofmasslocation;anaccuratemeasurementcanstillbemadewhentheobjectisbeyondbestfocusaslongasthecontrastremainshighenoughforthealgorithmbeingusedbythemachinevisionsystemtofunctionproperly. Whileitmayseemcounterintuitive,blurcanbeusedadvantageouslyincertainapplicationswithtelecentriclenses.Forexample,ifamachinevisionsystemneedstofindthecenterlocationofapin,asshowninFigure3a,thetransitionfromwhitetoblackisquitesharpwhenthelensisinfocus.InFigure3b,thesamepinisshownslightlydefocused. Lookingataplotoftheimagegraylevelsfromalineprofiletakenacrosstheedgeofthepart,asinFigure4,theslopeofthelineismuchshallowerfortheslightlydefocusedimage,asthepinedgeisspreadovermorepixels.Duetothesymmetricblurringofthetelecentriclens,thisblurisstillusableasthecentroidhasnotmovedandtheamountofsub-pixelinterpolationneededisdecreased.Thisreducessensitivitytograylevelfluctuationscausedbysensornoiseandallowsthepincenterlocationtobefoundmorereliablyandwithhigherrepeatability. Figure3:Thesamepinimagedbothinandoutoffocus.Notethatthetransitionfromwhitetoblackcoversmanymorepixelswhenthelensisslightlyoutoffocus(b),whichcanbeadvantageous. Figure4:Plotshowingthedifferenceinslopebetweenafocusedanddefocusededge.Thedefocusededgetakesupmanymorepixels;findingtheedgebecomeseasierwithoutrelyingonsub-pixelinterpolation. PreviousSectionNextSection FeaturedResources  ApplicationNote TelecentricDesignTopics  ApplicationNote TypesofMachineVisionLenses  ApplicationNote TelecentricLensSizeControl  TechnicalTool ImagingLensSelector EdmundOptics®Imaging:ManufacturingCapabilities LearnMore   Wasthiscontentusefultoyou? Thankyouforratingthiscontent! RequestanImagingOpticsCatalog Withover800ImagingLensesand40pagesofindepthTechnicalContent,makeEdmundOpticsYourPartnerinImagingOptics. TelecentricLenses Largeformat,compact,in-line,focusadjustment,oririsadjustmentoptionsavailableforyourmachinevisionorinspectionapplication. EOImagingLab Freetrainingvideoswithimagingandilluminationtipsandtricks. 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