传感器相对照度漂移和渐晕.docx

传感器相对照度漂移和渐晕.docx

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传感器相对照度漂移和渐晕

SensorRelativeIllumination,RollOffandVignetting

Inordertoevaluateandunderstandissuesthatcanbeassociatedwithvignetting（theblockingofraysthatpassthroughtheouteredgesofanimaginglens）,roll-offandrelativeillumination,sensorsizes,andformatsneedtobeconsidered.Inadditiontothefollowingoverview,moreinformationonsensorsandformatscanbefoundinUnderstandingCameraSensorsforMachineVisionApplications.

Figure1:

 Roll-offisthedecreaseinrelativeilluminationwithrespecttofieldthatisnotcausedbyvignetting,butbyradiometriclaws.

MATCHINGSENSORSWITHLENSES

Oneissuethatoftenarisesistheabilityofanimaginglenstosupportcertainsensorsizes.Ifthesensoristoolargeforthelensdesign,theresultingimagemayappeartofadeawayanddegradetowardstheedges;thiseffectiscausedbyvignetting.Astheresolutiondemandsofasystemincrease,oneoftwothingsneedstohappen:

pixelsneedtogetsmallerorsensorsneedtogetbigger.Asisdetailedinthesectionson MTF and DiffractionLimit,continuousreductioninpixelsizeproducessignificantissuesregardingtheoptics’abilitytoresolvetruedetail.Thisconcern,combinedwiththesignaltonoiseandsensitivityissuesassociatedwithcurrentsensortechnology,resultsinsensorshavingtogrowinsize;suchgrowthcausestheseissuesregardingvignettingandroll-off,unlesstheproperlensisused.

RELATIVEILLUMINATION

RelativeIlluminationisawayofrepresentingthecombinedeffectofvignettingandroll-offinanimaginglens,andisgenerallygivenasapercentageofilluminationatanypointonthesensornormalizedtothepositioninthefieldwithmaximumillumination.Vignettingandroll-offaretwoseparatecomponentsofrelativeillumination.AnexampleofRIisshowninFigure2.Moredetailsonhowtoreadthiscurvecanbefoundin LensPerformanceCurves

ThecurveinFigure2showsthatatitslowestaperturesetting（f/1.4–theblueline）,thislenshasanRIof57%oftheilluminationleveloftheimagecenteratthecornerofa2/3"sensor.Underthesameconditions,thelenshasanRIof72%inthecornerofa½"sensor.Asthesensorgetssmaller,theRIimproves.AlsonotethattheRIimprovesasthef/#isincreased;thisoccursuntilthereisnomorevignettinginthelens,atwhichpointallhigherf/#settingswillhavethesameilluminationprofile.Increasingthef/#generallydoesnotincreasetheimagecirclesizemuch,inthatalensdesignedforaparticularsensorsizewillnotperformwellonalargersensorevenwiththef/#stoppeddown.

Roll-offwillstilloccurwhenthelensisstoppeddown,asthatisrelatedtotheangleoftheraysandnotthenumberofraysthatpassthroughthelens.Manylenseswillhaveanilluminationprofilethatishighestinthemiddleofthefieldandiseitherflatordeclinestosomelowerpercentageasitapproachestheedge.TherearerarecaseswheretheRIincreasesslightlyacrosstheimagecircle,butthisisrelatedtopupilcompressionandwillnotbecoveredinthistext.

Figure2:

 Arelativeilluminationcurveshowingrelevantimagesensorformatsonthex-axis

VIGNETTINGWITHINALENS 

Vignettingistheresultoflightraysnotmakingitthroughtheentirelenssystemtothesensor,duetobeingblockedbytheedgesofindividuallenselementsormechanicalstops.Thisclippingofrayscanbeintentionalorunintentional,andinsomecaseisunavoidable.Vignettingismostoftenseenatorinlowerf/#s,shortfocallengthlenses,orlenseswherehigherresolutionsneedtobeachievedatalowercost.

Figure3demonstratesclippingasitmayoccurforthesame16mmlensatdifferentf/#s（f/1.8andf/4）.NotetheclippingofraysinFigure3a,asindicatedwithredcircles;theseraysarenotabletopassthroughthealloftheopticsinthelens.Figure3b,ontheotherhand,demonstratesanexamplewithoutvignetting.ThevignettinginFigure3acouldhaveseveralcauses,includingdiameterlimitationsoftheopticsoraneedtoeliminatetheraystoblockstraylight.Vignettingissometimespurposelyincludedinalensdesigntoimproveoveralllensperformanceorreducecost.

Figure3:

 A16mmlensdesignatf/1.8（a）andf/4（b）.Atf/1.8vignettingoccurswherelightraysareclippedbytheedgesofthelens.

VIGNETTINGTOGAINPERFORMANCE（SELECTIVEVIGNETTING） 

Vignettingisoftenusedtomaximizetheresolutionofalensdesignacrosstheentireimagecircle.Sinceitismoredifficulttodirecttheraysthatcreatetheedgeofanimagetothedesiredlocationonasensor,higherresolutionobjectsaregenerallymoredifficulttoreproduceattheedgeoftheimagethanatthecenter.Raysthatenduponthewrongpixelwilldegradetheimageatthatlocation;onewaytomanagethisistoeliminatetheseraysfromthesystem.Iftheundesiredraysdonotmakeittothesensor,theycannotdegradetheimage.Removingthesemis-directedrays,however,reducesrelativeillumination.

EffectsofVignettingatthePixelLevel:

LargePixels

Figure4Illustrateslightraysincidentonapixelinthecornerofasensoratf/1.4（a）andf/2（b）.InFigure4a,somelightspillsontotheadjacentpixelcreatingimageandcontrastdegradation.Increasingthef/#（Figure4b）essentiallycreatesvignettingwhichclipsthoseextraneousrays.Figure5illustratesthissamevignettingeffectatthecenterofthesensor.However,withtheselargepixels,thechangeinf/#haslittleeffectontheoverallimagequality.

Vignettingcanalsobepurposefullydesignedintolensesinscenariosinwhichtheeffectsfrommanufacturingtolerancesadverselyaffectthecontrolofrays,causingimagedegradation.Thelooserthetolerancesonthelens,themoreadversethesedegradingeffectscanbecome,andtighteningthetolerancesisoftennotpracticalduetotheincreaseinmanufacturingcost.Often,abalancemustbestruckbetweenreducingmanufacturingcostandmaintainingimagequality.Incaseswherecostisaprimaryfactor,vignettingmustbeutilizedinanattemptmaintainresolutionacrossthefieldofview.Doingsuchwillhaveanadverseeffectontheilluminationprofile.Designingvignettingintoalenscanbeaccomplishedinacoupledifferentways:

bypurposefullydesigningtheclearaperturesoftheindividuallenselementssuchthattheyvignetteseverelyoff-axisrays,orbyintroducingmechanicalaperturestoblockaberratedrays,asshowninFigure8a.

EffectsofVignettingatthePixelLevel:

SmallPixels

InFigures6and7,thepixelshavebeenreducedtohalfthesize,yieldinga4Xresolutionincrease.Inthisexample,vignettingbyincreasingf/#greatly,improvesperformanceacrosstheentiresensor,asopposedtothefirstexamplewhichonlyslightlyimprovedtheimagingperformanceinthecorneroftheimage.AlloftheseFigures（4-7）exhibitnominaldesigncapabilitiesanddonotaccountforreducedperformancethatresultsfrommanufacturingtolerances.Withtolerancesincluded,theneedforvignettingcanbeevenmorepronounced,especiallyincaseswherecostisadrivingfactor.

Figure4:

 Lightraysincidentonpixelsinthecornerofasensoratf/1.4（a）andf/2（b）.Increasingthef/#createsvignettingwhichclipstheextraneousraysspillingontotheadjacentpixelin4a.

Figure5:

 Lightraysincidentonpixelsinthecenterofanimageatf/1.4（a）andf/2（b）.Increasingthef/#hasnoeffectsignificantonimagequalityasallrayswerecontainedinthedesiredpixelforeachexample.

Figure6:

 Lightraysincidentonpixelsinthecornerofasensoratf/1.4（a）andf/2（b）.Increasingthef/#createsvignettingwhichclipstheextraneousraysspillingadjacentpixels.

Figure7:

 Lightraysincidentonpixelsinthecenterofanimageatf/1.4（a）andf/2（b）.Increasingthef/#createsvignettingwhichpreventstheextraneousraysfromspillingontonearbypixels.

DemonstrationofVignettinginDifferentLensDesigns 

Figure8andassociatedMTFcurvefeaturesastandard12mmlensdesignandrelativeilluminationcurve.Notethesizedifferenceoftheraybundlesin8aatthecenter（bluelines）andcorner（greenlines）;thesizedifferencedemonstratesalargeamountofselectivevignetting.Thevignettingleadstolowerilluminationattheedgesoftheimagethaninthecenter（8b）.Thisisdoneinanefforttominimizethecostsassociatedwithmaterialsandmanufacturingtolerances,whilemaintainingreasonableperformanceatalowerprice.

ThelensinFigure9,anultra-highresolution12mmlensdesign,hasamuchmoreevenlysizedraybundlesacrossthefield（9a）duetoalowlevelofvignetting.Thistranslatestomuchmoreevenrelativeilluminationacrosstheentiresensor（9b）.Thelensinthisexampleisdesignedusingmorecostlymaterialsathighertolerances,whichallowsittomaintainhighlevelsofperformanceacrosstheimage,withouttheneedtointroducevignettingtoimproveitsperformance.Thetradeoffinusingsuchalensisthattheultra-highresolutionlensismoreexpensivethanthestandarddesign.

Figure8:

 Astandard12mmlensraypath（a）,relativeilluminationcurve（b）,andMTFcurve（c）.

Figure9:

 Aultra-highresolution12mmlensraypath（a）,relativeilluminationcurve（b）,andMTFcurve（c）.

ILLUMINATIONROLL-OFF 

Initssimplestform,themaximumbrightnessofalensforsomegivenimagecirclewithnovignettingislimitedbythefourthpowerofthecosineofthechiefrayangleinimagespace.Thisisknownascos4qrolloff.Figure10showsthechiefraysforthecenterandthecorneroftheimage（highlightedinred）.

Figure10:

 Imaginglenslayout,highlightingthechiefraysoftheraybundlesbelongingtothecenter（bluelines）andthecorner（greenlines）