Telecentric lens - Wikipedia

A telecentric lens is a special optical lens (often an objective lens or a camera lens) that has its entrance or exit pupil, or both, at infinity. Telecentriclens FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Bi-telecentriclenswith208 mmdiameterfrontelementandaC-mountcamerainterface Atelecentriclensisaspecialopticallens(oftenanobjectivelensoracameralens)thathasitsentranceorexitpupil,orboth,atinfinity.Telecentriclensesareoftenusedforprecisionopticaltwo-dimensionalmeasurementsorreproductionandotherapplicationsthataresensitivetotheimagemagnificationortheangleofincidenceoflight. Thesimplestwaytomakealenstelecentricistoputtheaperturestopatoneofthelens'sfocalpoints.Thismakesthechiefrays(lightraysthatpassthroughthecenteroftheaperture)ontheothersideofthelensparalleltotheopticalaxisforanypointinthefieldofview.Commerciallyavailabletelecentriclensesareoftencompoundlensesthatincludemultiplelenselements,forimprovedopticalperformance.Telecentricityisnotapropertyofthelensesinsidethecompoundlensbutisestablishedbythelocationoftheaperturestopinthelens.Theaperturestopselectstheraysthatarepassedthroughthelensandthespecificselectioniswhatmakesalenstelecentric. Ifalensisnottelecentric,itiseitherentocentricorhypercentric.Commonlensesareusuallyentocentric.Inparticular,asinglelenswithoutaseparateaperturestopisentocentric.Forsuchalensthechiefrayoriginatingatanypointoffoftheopticalaxisisneverparalleltotheopticalaxis,neitherinfrontofnorbehindthelens. Anon-telecentriclensexhibitsvaryingmagnificationforobjectsatdifferentdistancesfromthelens.Anentocentriclenshasasmallermagnificationforobjectsfartheraway;objectsofthesamesizeappearsmallerthefarthertheyareaway.Ahypercentriclensproduceslargerimagesthefarthertheobjectisaway. Atelecentriclenscanbeobject-spacetelecentric,image-spacetelecentric,orbi-telecentric(alsodouble-telecentric).Inanobject-spacetelecentriclenstheimagesizedoesnotchangewiththeobjectdistance,andinanimage-spacetelecentriclenstheimagesizedoesnotchangewiththeimage-sidedistancefromthelens. Contents 1Object-spacetelecentriclenses 2Image-spacetelecentriclenses 3Bi-telecentriclenses 4Externallinks 5References Object-spacetelecentriclenses[edit] Object-spacetelecentricimagingwheretheapertureisinthebackfocalplaneoftheobjective. Anobject-spacetelecentriclenshastheentrancepupilatinfinityandprovidesanorthographicprojectioninsteadoftheperspectiveprojectioninanentocentriclens. Object-spacetelecentriclenseshaveaworkingdistance.Objectsatthisdistanceareinfocusandimagedsharplyontotheimagesensoratflangefocaldistanceinthecamera. Anobjectthatiscloserorfartherisoutoffocusandmaybeblurry,butwillbethesamesizeregardlessofdistance. Telecentriclensestendtobelarger,heavier,andmoreexpensivethannormallensesofsimilarfocallengthandf-number.Thisispartlyduetotheextracomponentsneededtoachievetelecentricity,andpartlybecausethefirstelementinanobject-spacetelecentriclensmustbeatleastaslargeasthelargestobjecttobeimaged.Thefrontelementinanobject-spacetelecentriclensisoftenmuchlargerthanthecameramount.Incontrasttoentocentriclenseswherelensesaremadelargertoincreasetheapertureforincreasedcollectionoflightorshallowerdepthoffield,alargerdiameter(butotherwisesimilar)object-spacetelecentriclensisnotfasterthanasmallerlens.Becauseoftheirintendedapplications,telecentriclensesoftenhavehigherresolutionandtransmitmorelightthannormalphotographiclenses. Commercialobject-spacetelecentriclensesareoftencharacterizedbytheirmagnification,workingdistanceandmaximumimagecircleorimagesensorsize.Atrulytelecentriclenshasnofocusringtoadjustthepositionofthefocalplane.Somecommercialtelecentriclenses,however,dofeatureafocusring.Thiscanbeusedtoslightlyadjusttheworkingdistanceandmagnificationwhilelosingalittlebitoftelecentricity.Sometimes,manufacturersspecifyasensorresolutionorpixelsizetodescribetheopticalqualityofthelensandthemaximumopticalresolutionitcanachieveduetothelens'saberrations. Becausetheirimageshaveconstantmagnificationandconstantviewingangleacrossthefieldofview,object-spacetelecentriclensesareusedformetrologyapplications,whereamachinevisionsystemmustdeterminetheprecisesizeandshapeofobjectsindependentlyfromtheirexactdistanceandpositionwithinthefieldofview. Inordertooptimizethetelecentriceffectwhenobjectsareilluminatedfrombehind,anadditionalimage-spacetelecentriclenscanbeusedasatelecentric(orcollimated)illuminator,whichproducesaparallellightflow,oftenfromLEDsources. Image-spacetelecentriclenses[edit] Image-spacetelecentricimagingwheretheapertureisinthefrontfocalplaneoftheobjective. Animage-spacetelecentriclenshastheexitpupilatinfinityandproducesimagesofthesamesizeregardlessofthedistancebetweenthelensandthefilmorimagesensor.Thisallowsthelenstobefocusedtodifferentdistanceswithoutchangingthesizeoftheimage.Animage-spacetelecentriclensisareversedobject-spacetelecentriclens,andviceversa. Sincethechiefraysafteranimage-spacetelecentriclensarealwaysparalleltotheopticalaxis,theselensesareoftenusedinapplicationsthataresensitivetotheangleofincidence.Interference-basedcolor-selectivebeamsplittersorfiltersbutalsoFabry–Pérotinterferometersaretwoexampleswhereimage-spacetelecentricityisused.Anotherexampleisminimizingcrosstalkbetweenpixelsinimagesensorsandmaximizingthequantumefficiencyofasensor.TheFourThirdsSysteminitiallyrequiredimage-spacetelecentriclenses,butwiththeimprovementofsensors,theangleofincidencerequirementhasbeenrelaxed.[1] Sinceeverypixelisilluminatedatthesameanglebyanimage-spacetelecentriclens,theyarealsousedforradiometricandcolormeasurementapplications,whereonewouldneedtheirradiancetobethesameregardlessofthefieldposition. Bi-telecentriclenses[edit] Bi-telecentricimagingwheretheapertureisinthecommonfocalplaneoftwoconfocallenses. Inabi-telecentric(ordouble-telecentric)lens,bothentranceandexitpupilareatinfinity.Themagnificationisconstantdespitevariationsofboththedistanceoftheobjectandtheimagesensorfromthelens,allowingformoreprecisemeasurementsthanwithamono-telecentriclens. Abi-telecentriclensisafocalastheimageofanobjectatinfinityformedbythefirstpartofthelensiscollimatedbythesecondpart. Commercialbi-telecentriclensesareoftenoptimizedforverylowimagedistortionandfieldcurvatureforaccuratemeasurementsacrosstheentirefieldofviewatgreatresolution.Theselensesoftencomprisemorethan10elements. Largeandheavybi-telecentriclenseswithmanyelementsarecommonlyusedinopticallithographycopyingatemplateontosemiconductorwafers.[2] Externallinks[edit] TechnicaldescriptionoftelecentriceffectfromEdmundOptics,amanufactureroftelecentriclenses TelecentriclensestutorialfromOptoEngineering,amanufactureroftelecentriclenses AnothergoodexplanatorypagebyDonaldSimanek AcourseonGaussianopticsoftelecentricsystemsfromSchneiderKreuznachOpticalmeasurementtechniqueswithtelecentriclenses References[edit] ^"MicroFour-ThirdsandTelecentricity". ^Matsuyama,Tomoyuki;Ohmura,Yasuhiro;Williamson,DavidM.(2006)."TheLithographicLens:itshistoryandevolution"(PDF).Proc.ofSPIE.6154.doi:10.1117/12.656163. 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